National Company, Inc.  -  NC-173, NC-183 & NC-183D
 

History and Circuit Description

NC-173 Announcement Ad - QST March 1947
The receiver would be at dealers in April '47

The NC-183 was introduced in the December 1947 issue of QST. Double preselection was needed for image rejection on frequencies higher than about 15mc. The two TRF amplifier stages did require a complete extra tuning stage with variable capacitors and coils for each band along with the increased number of segments for the band switch. The result was the NC-183 chassis is three inches deeper than the NC-173 chassis. High Fidelity audio was just beginning to become popular with better recordings on newer mediums that favored wide-audio frequency range reproduction. National thought that combining SWL, Hi-Fi and Ham use into one receiver would appeal to several different types of potential buyers. "A Receiver designed to combine performance and versatility "plus" for the critical user" was National's marketing claim for the NC-183. Even the manual anticipates the variety of potential users by providing one set-up section for neophyte radio operators and another set-up section for experienced users. As would be expected, the resulting sales favored the less-sophisticated NC-173 out selling the NC-183 by a large margin due to its much lower price and that low cost seemed to be more important to the ham buyers than Hi-Fi audio and double preselection.

Today's vintage ham gear enthusiasts appreciate National's combination of robust Hi-Fi V-T (vacuum tube) push-pull audio and double preselection resulting in great reception and audio reproduction possibilities making the NC-183 and, even more so, the later NC-183D, very popular choices for vintage ham station receivers.

NC-183 Announcement Ad - QST Dec 1947
Note the vertically-oriented toggle switches

NC-183 Receiver   SN: 241 0262   ca: 1949
Shows the light-gray, slightly silvery color used on all NC-173 receivers and on the NC-183 receivers during the first two-thirds of production (up to about 1951.) The NC-183 was a fabulous receiver even though its design and performance was definitely from the early post-WWII era with many of the circuits and parts obviously from WWII design engineering.

The NC-173 and NC-183 - Both of these receivers were single conversion superheterodynes. The NC-173 provided single-preselection, two stages of 455kc IF amplification and a single 6V6 providing about 2 watts of audio output power. The NC-183 offered double-preselection (two TRF amplification stages,) two 455kc IF amplifier stages and featured a high fidelity audio system using push-pull 6V6 tubes for 8 watts of low-distortion, high-quality audio output power. A matching loudspeaker was included with the receivers. NC-173 and NC-183 advertising stated that a Crystal Calibrator could be installed in the accessory socket but this wasn't exactly the case (unless a homebrew or kit was used.) The accessory socket was for either the National NBFM adapter (Narrowband FM) or for the National Select-O-Ject. The NC-173 and the NC-183 tube compliment consisted of mostly metal octal tubes (rectifier, regulator and audio tubes were glass octals.) Frequency coverage for both versions was from .54mc up to 31mc and additional tuning of 47mc to 55mc for 6M amateur band coverage using only the Band Spread tuning. The Band Spread tuning dial featured calibrated scales for 80M, 40M, 20M, 10M and 6M along with a 0 to 200 logging scale on each dial. The NC-173 used 13 tubes and sold for $199 in 1950 (most dealers sold the NC-173 for $179 to $189.) The NC-183 used 16 tubes and sold for $269 in 1950 (again, lower priced from dealers like Newark, Walter Ashe, Fort Orange, Radio Shack of Boston and other dealers of the day. However, usually these lower dealer prices didn't include the loudspeaker,...that was priced at $10 extra.)

The NC-183 and the NC-173 had a unique method of compensating for LO thermal drift by using a bi-metallic strip that changed the position of the brass part of the strip's proximity to the Band Spread LO tuning section stator contact. It was essential that this assembly be properly mounted and that the .020" gap be maintained for correct operation (this device wasn't used on the NC-183D receivers.) For alignment stability, all of the LO trimmers were ceramic variables, not compression trimmers (the NC-183D changed just the AM-BC band LO adjustments to a compression trimmers.) The NC-183 was also available in a rack mount version that featured a black or dark gray wrinkle finish front panel. The rack mount versions used a "R" suffix in the model number. Sometimes in advertising and in manuals, a "T" suffix was used for the standard table cabinet versions (but "T" never appeared on the receiver itself.)

The NC-173 was available from April 1947 through 1951. The NC-183 was available from December 1947 up through most of 1952 when it was replaced with the thoroughly updated and improved NC-183D.  By the early fifties, evolution in tube types available and ham user demands for a more advanced receiver motivated National to perform a major upgrade on the NC-183 receiver with many significant circuit design changes along with many mechanical improvements that resulted in the new NC-183D. The NC-183D was announced in December 1952.

Besides the use of mostly newer miniature tubes, new dials and new S-meter, there were other major circuit changes such as double-conversion on the three highest frequency bands using a 1720kc Converter and a 455kc Converter and the addition of a third IF amplifier stage. The second 455kc IF amplifier used dual IF transformers that were tertiary-interconnected to provide steep sidewalls and a "flat nose" to the passband. On Bands A, B and C, there were four tuned circuits in the 1720kc to 455kc conversion process plus 12 tuned circuits in the 455kc IF for a total of 16 tuned IF circuits. On Bands D and E, single conversion was used with just the 12 tuned 455kc IF circuits being used. This resulted in an IF passband of about 3.5kc at -6db, narrow selectivity for the crowded ham bands of the fifties and sixties. There were many other subtle electronic and several obvious mechanical upgrades in the NC-183D. The 6J5 Phase Inverter used in the earliest of the NC-183D receivers was replaced with a dual triode 6SN7 with one triode acting as the Phase Inverter and the other triode used as a S-meter amplifier circuit. The mounting of the pinch wheel drive for the tuning dials was modified to be much more robust working against a metal rim base plate on the dials instead of the rim-edge of an all-plastic dial (as in the 173/183.)

NC-173 Receiver   SN: 173 2821   ca: 1948
The front panel size and control layout for the NC-173 is nearly the same as the NC-183 but notice that on the NC-173 panel the embossed area around the dials only surrounds the dials while the embossed area on the NC-183 surrounds the entire top half of the front panel. The NC-173 had less dimensional depth at only 12" deep - the NC-183 was 15" deep.

NC-183D Top of Chassis   SN: 430 0135
The first "three in a row" IF cans are for the 1720kc IF. The first five of the "six in a row" IF cans are for the 455kc IF. The rear-most can is for the Amplified AVC. Late production NC-183D receivers will have a potted power transformer identified as S461 as this receiver has.

NC-183 Top of Chassis    SN: 241 0262
Behind the Crystal Filter are the two 455kc IF cans. The rear-most LC (in can) is for the amplified AVC. Note the numerous WWII style metal-octal tubes. The power transformer is the SA3901 used in the NC-183 and in early NC-183D receivers.

NC-173 Top of Chassis    SN: 173 2821
Note the different style power transformer that was used on all NC-173 receivers. The pattern metal venting on the back panel that was added by late-1947. Note how short the tuning condenser cover is compared to the NC-183.

The Physical - The cabinets on early examples of the NC-183 and NC-173 were painted a light-gray color that had a slightly silver undertone. This was a semi-gloss, smooth-finish paint that was very durable but had a tendency to show any nicks and scratches quite well. The knobs used on these receivers had gray plastic knob grips with bright silver skirts and the tuning knobs had bright silver "knob-brights." The earliest NC-173 and NC-183 builds had have vertically oriented toggles switches as shown in early advertising. Also, the earliest NC-173 didn't have an accessory socket (the accessory socket was probably added by late-1947 when the NBFM adapter became available.) Early examples of the NC-183 had a punched pattern metal screen spot-welded in place on the rear panel (see photo right.) The earliest NC-173s (about the first 1000 receivers) had a solid back cabinet that didn't have had any rear venting at all. Later NC-173 cabinets have the patterned metal screen venting. By 1949, the NC-183 rear panels had horizontal-rectangular vents that were punched into the sheet metal before cabinet assembly. Probably around 1951 or so, the cabinet paint on the NC-183 was changed to a dark gun metal silver-gray and the knob grips were changed to black. During the last of the NC-183 production, in 1952, other minor upgrades were beginning to be incorporated into the receiver. The gunmetal silver-gray color and black knobs were carried over to the NC-183D cabinet introduced in late-1952. Late versions of the NC-183D replaced the standard SA3901 power transformer with a potted power transformer identified as S461. The older style filter choke was replaced with a potted filter choke identified as S-669.

This is an Early NC-183 showing early style cabinet rear venting and the lack of a "punch-out" hole for the Select-O-Ject accessory. To the left of the fuse holder cap is the three-pin speaker socket. Serial number is 202 0442 (built while the HRO-7 was still being produced.) Interestingly, SN 202 1318 does have the "punch-out" hole.
 

photo from eBay

NC-183R Rack Mount Receiver Version

The rack mount version used a standard 19" front panel that wasn't an integral part of the cabinet like the table models used. The flat front panel allowed for engraved nomenclature that was cut into the panel after the wrinkle finish paint was applied. In this method, the nomenclature will show the base metal, aluminum, through the paint as "bright silver nomenclature" - well, when it was new anyway. Though missing on this receiver, normally the rack mount versions had a dust cover that protected the chassis. Receiver pictured is ca: 1951-52

The NC-173 was a physically smaller receiver compared to its bigger brothers measuring 20"W by 10"H by 12"D and it also weighed at least ten pounds less. The NC-183 and the "D" were 20"x10"x15" with a weight of approximately 65 lbs.

Other Versions - Like its predecessors, the NC-183D was also available in a rack mount version designated the NC-183DR. The earlier NC-183 had used a dark gray, almost black wrinkle finish, panel but the NC-183DR used a gunmetal gray smooth finish front panel. There was a military version NC-183D, the NC-183MR that also had the military designation R-651/URR-39. This receiver was installed in a 30" tall table rack that included a matching rack mount speaker. The complete military-version receiver was designated as the AN/URR-39 and that consisted of the R-651/URR-39 receiver, the LS-228/U rack speaker and the MT-1529/U table rack (with shock mounts.) This version eliminated the 6M coverage and provided a LF band (50kc to 150kc) instead. The band spread wasn't calibrated for ham bands and just had a logging scale.

Artwork for the NC-183D showing the yellow scale S-meter that probably was never used in production

 
AN/URR-39 Receiver
photo from navy-radio.com

For more details and a critical analysis of the quirky multiple IF sections and the multitude of mechanical issues that plagued the NC-400 receiver you can go to the "National NC-400" article on this website. Use the Home/Index link at the bottom of this article for navigation.

NC-400  SN: 543 0006  from 1959

Summary of NC-173, NC-183 and NC-183D Receiver Features

NC-173

Produced from 1947 to 1951    Orig. Price - $199

Circuit - 1 RF Amplifier, 2 IF Amplifiers, 1 Audio Output Amplifier

Tubes used: RF-6SG7, Mix-6SA7, LO-6J5, IF-6SG7 (2), Det-6H6, AVC Amp-6AC7, NL-6H6, BFO-6SJ7,1stAF-6SJ7, Audio Output-6V6, Rect-5Y3, Reg-0D3

Audio Output Z - 8 ohms and 500 ohms from three-pin mini-socket

Tunes .54-31mc and 48-56mc

Antenna Input - Three screw terminal strip, 300Z nom.

AM Sensitivity - <2.0uv 6db s/n ratio (1948 catalog spec)

     Selectivity - ~8kc at -10db w/o Crystal Filter
 

NC-183

Produced from 1948 to 1952    Orig. Price - $269

Circuit - 2 RF Amplifiers, 2 IF Amplifiers, P-P Audio Output

Tubes Used: RF-6SG7 (2), Mix-6SA7, LO-6J5, IF-6SG7 (2), Det-6H6, AVC Amp-6AC7, NL-6H6, BFO-6SJ7, 1stAF-6SJ7, Phase Inv-6J5, Push-Pull Audio Output-6V6 (2), Rect-5U4, Reg-0D3

Audio Output Z - 8 ohms and 500 ohms from three-pin mini-socket

Tunes - .54-31mc and 48-56mc

Antenna Input - Three screw terminal strip, 300Z nom.

AM Sensitivity - <1.5uv 6db s/n ratio (1948 catalog spec) 

Selectivity - ~8kc at -10db w/o Crystal Filter
 

NC-183D

Produced from 1952 to 1958    Orig. Price - $369.50

Circuit - 2 RF Amplifiers, 3 IF Amplifiers, Double-Conversion, P-P Audio Output

Tubes Used: RF-6BA6(2), 1720kc Conv-6BE6, 455kc Conv-6BE6, IF-6BA6 (3), Det-6AL5, AVC Amp-6AH6, NL-6AL5, BFO-6SJ7, 1stAF-6SJ7, Phase Inv and S-meter Amp-6SN7, Push-Pull Audio Output-6V6 (2), Rect-5U4, Reg-0B2

Audio Output Z - 8 ohms and 500 ohms from a three screw terminal strip

Tunes - .54-31mc and 48-56mc

Antenna Input - Three screw terminal strip, 50 to 300Z nom.

AM Sensitivity - <3.5uv at 300Ω 10db s/n ratio (1956 ARRL HB Ad spec)  

Selectivity - 3.5kc at -6db w/o Crystal Filter
 

NC-173, NC-183 and NC-183D Serial Number Log

Serial numbers are very helpful for determining when a particular receiver was built. The NC-173, NC-183 and the NC-183D all use the standard post-WWII National format of a seven digit serial number. It's fortunate that National incorporated the specific "production run number" as the first three digits of the serial number. The production run numbers were assigned chronologically. Each run produced a certain quantity of receivers that were assigned a four digit number that included "leading zeros" to identify specific receivers. For example, HRO-5A1 serial number 184 0009 would be run #184 and this receiver was the 9th off the line. The serial numbers collected from later post-war HROs and NC-2-40D receivers have already provided several examples of the chronological issuing of many of the run numbers. However, I really don't have anything specific to the NC-173, NC-183 or the NC-183D. So, I'll start collecting serial numbers on those types of receivers and see what can be determined. UPDATE: So far, it looks like ALL NC-173 receivers used #173 as the production run identification and the four digit serial number provides the chronological build-date information.

SN Locations: The NC-173 serial number is usually stamped into the metal chassis at the top-rear-center of the chassis just above and between the filter choke and the filter capacitor. The NC-183 serial number and the NC-183D serial number are located above the accessory socket and have "SER. NO." stamped into the metal chassis (or silk-screened) proceeding the actual seven digit number that is stamped into the metal chassis. The three photos to the right show the locations of the serial numbers for each receiver type.

Report Your SN: If you have examples of these receivers, please e-mail the serial number of each receiver. Be sure to mention if your receiver is original and has any significant differences from what's described in this article or anything unique (and factory original) about the receiver such as cabinet paint color used, cabinet rear vent type, power transformer type on 183D, factory circuit changes, etc. I'll add the serial numbers to the log and after a while the log should produce some interesting information. The variations tied to serial numbers will help date when the changes happened. Here's an e-mail link below or just use the e-mail link on website Home Page (both links are the same e-mail address.)

Send your NC-173, NC-183 or NC-183D serial number to: Serial Numbers 173-183, National, WHRM

NC-173:  173 0212(vots,nas,SBC),  173 0912(vots, nas,SBC),  173 2155,  173 2821(pmrv),  173 4204(pmrv),

NC-183:  202 0442(eB,pmrv), 202 1318(pmrv),  241 0128,  241 0262,  241 0377,  309 0159(R),  309 0368(R)  

NC-183D:  328 0584(ST,sM),  357 0271(NOT), 357 0668(ST,PS-J),  372 0538(PT),   372 0574(PT),  396 0086(PT,PS-J),
  430 0135(PT)

R= Rack Mount
PS-J = Parts Set that was Junked
ST= standard pwr transformer SA3901 (on NC-183D)
PT = potted pwr transformer S461 (on NC-183D)
NOT = Non-original pwr transformer  
eB = seen on eBay
sM = seen at swap meet
pmrv = pattern metal rear vent
vots = vertically oriented toggle switches
nas =  no accessory socket
SBC = solid back cabinet (NC-173)
If no notations are shown then just the serial number was reported
 

Receiver General Information

NC-173 and NC-183 dials are a plastic disk that is riveted to the metal hub. This type of material will darken as it ages and with its long-term exposure to light and heat. The NC-173 and the NC-183 used the exact same dials.

The NC-183D addressed this dial material problem by using copper metal-backing plates that had a Lucite dial riveted to the front of the metal plate (Lucite is a high quality-type of crystal-clear Acrylic that won't yellow or crack and is very strong but lightweight.) The Lucite was reverse-side silk-screened with the dial nomenclature. This gave the dial scale the appearance of "depth." These dials had to be illuminated from the perimeter edge with the lamplight going through the Lucite which makes the silk-screened numbers very bright and visible (it requires the position of the lamp be as far forward as possible for optimum illumination.) Since Lucite was used the NC-183D dials, they are almost always in excellent condition (something that, unfortunately, can't be said for the NC-173 and NC-183 dials.) Dirt, debris and staining can get behind the Lucite if the dial is subjected to a very humid or wet environment. Extreme moisture behind the dial will deteriorate the silk-screened nomenclature causing it to crack and flake resulting in partially missing numbers and tiny flecks of white silk-screen paint.

NOTE: As a restoration hint,...the dials from the NC-173 and the NC-183 are identical with the same part numbers so they can be interchanged if necessary.

NOTE 2: It's interesting that while the NC-173 and NC-183 Band Spread dials don't have the 15 meter band, the Main Tuning dial does have 21.0mc to 21.5mc highlighted as an amateur band. Although the IARU had approved a 15M band and it was known that a ham band was going to be created in and around 21mc starting around 1946, it wasn't an officially usable ham band until 1950. Notice that the NC-183D Band Spread dial shown to the right does have the 15M band calibration (actually, 19.8mc to 21.56mc.)

NC-183D dial showing the metal rim of the copper backing plate and the Lucite front dial. Lucite was used for airplane cockpit covers and aircraft windows because of its strength and it will not yellow or crack with exposure to UV or weather.

This Marion Electric meter is actually for a National NC-100 Series receiver but it's the same phenolic material that was used in NC-173 and NC-183 receiver dials.

Original NC-173/NC-183 Dial Color - The S-meter shown to the left is a NOS example that has been kept inside a box and has never been exposed to sunlight, illumination lamps or heat. It shows the correct color that the acetate-based phenolic dials were when new. The red numbers and letters show how vibrant the red was when new. 

Reproduction Dials and S-meter Scale - Radio Daze, the major supplier of reproduction plastic dials, in their online catalog show that they offer reproduction dials for the NC-173/NC-183 (same dials) and also for the S-meter scale. But, before ordering, I would contact them to be sure that the material they are using to make the dials is the light yellow color shown in their online depictions. A few years ago, I ordered a RCA ACR-136 repro dial that was shown as the correct light yellow material online but what was delivered was a dark reddish-brown material that was as dark as the darkened dial I was trying to replace. According to Radio Daze, the reason for the dark color was that was the only phenolic material available (at that time,...2021.) Since the cost for the NC-173/183 repros is over $70 (the complete set of two repro dials is $47 and meter scale is $24) I'd check with Radio Daze prior to ordering to confirm what color material is going to be used. Be sure to also realize that you'll only receive the plastic repro dials. You have to remove the old dial from the hub and mount the new repro dial on the old hub. You don't need to use rivet mounting (as original) since there's a lot of clearance for screw and nut mounting. Also note, Radio Daze has a six week lead time for delivering repro dials (they blamed COVID in 2021, I don't know what the problem is now.)

Receiver Accessories

There's Only One Accessory Socket - Only one accessory socket was provided for either the optional NBFM adapter or for the optional National Select-O-Ject (not both, and not for a National Co. plug-in Crystal Calibrator.) The earliest versions of the NC-173 didn't have any accessory socket. The accessory socket was probably added when the NBFM adapters became available around August 1947.

The Select-O-Ject - SOJ-2, SOJ-3 - This accessory was connected to the NC-173, NC-183 or NC-183D accessory socket via a cable with plug connector on the end from the Select-O-Ject for power and routing of the signal lines. The SOJ-2 was the version that had to be used with the NC-173, NC-183. There was a "punch-out" hole in the rear cabinet panel to provide access to the accessory socket for the SOJ-2 cable (the "punch-out" wasn't on the earliest 1947 NC-183 models, but was added by SN 202 1318, and the NC-173 didn't need the "punch-out" because it had the accessory socket on the rear chassis apron.) The NC-183 accessory socket had to be rewired to work with the SOJ-2 (per the National instructions but one would think it would be more prudent and easier to rewire the SOJ-2 plug to work with the NC-183 accessory socket) and the Radio/Phono switch on the receiver was used to place the SOJ-2 into operation (the NC-173 had to have a "dummy" phone plug inserted into the front panel PHONO jack since it didn't have a Radio/Phono switch on the front panel.) The NC-183D used the SOJ-3 that didn't require the accessory socket be rewired. The NC-173 also didn't require accessory socket modification to work with the SOJ-2. The Select-O-Ject was a tunable audio frequency peaking or nulling circuit usable mostly for CW but able to enhance other types of signals also. Nowadays, Select-O-Jects seem to be in the "seldom-seen" category of National accessories but, perhaps that's because the current owners just don't want to part with a versatile and usable device like the Select-O-Ject.
 

NBFM Adapter - NFM-73, NFM-83 - Starting in the late-1940s and going through most of the 1950s, Narrowband FM was thought of as a practical solution to the ever-growing problems of AM TVI and also for RFI-QRN reception problems in some locations. Since FM eliminated the amplitude variations of the signal that caused most of the TVI issues and replaced the amplitude variations with frequency variations that seemed invisible to TV watchers, NBFM seemed perfect for Voice communications for hams. The problem was that NBFM took up just about the same bandwidth, maybe a little more depending on the modulation f-deviation, as the standard AM signal. So, although TVI and QRN issues might be gone, there was the possibility that the HF ham bands couldn't accommodate a lot of NBFM users in competition with the AM users. As a result, the FCC decided to gather some data on NBFM in the HF amateur bands. For a period of one year starting in late-1947, NBFM was allowed anywhere in the normal HF amateur phone bands. All of the manufacturers knew this and responded by introducing adapters and NBFM capabilities in their gear produced in 1947 and 1948. At the end of one year (about the end of 1948,) the FCC determined that NBFM was going to have negative impacts on ham phone operation on 160M, 80M, 40M and 20M and operation of NBFM was not going to be allowed on those bands. The FCC allowed for limited the operation of NBFM to frequencies above 27mc, relegating NBFM to 11M and 10M ham bands below 30mc. Though several manufacturers offered a NBFM function on their receivers, it never became a popular mode of transmission below 30mc. This was partially because of the transmitting frequency limitations imposed by FCC regulations that prevented wide-spread use of NBFM in the HF region of the spectrum. However later on, NBFM did become the popular voice mode on VHF, especially on the 2M band. Currently, NBFM on HF is authorized for 29.2mc to 29.3mc in the 10 meter band and this small portion of the spectrum is the only HF authorization for NBFM communications. When the NFM-83 adapter is installed into the Accessory socket, the adapter receives its operational voltages from the receiver. Two tubes are used in the adapter, a 6SK7 IF amplifier and a 6H6 discriminator/detector. The signal input is from the IF output and AVC line via the Accessory socket and the NFM-83 output is via the Accessory socket to the receiver's audio input. The NFM-83 is essentially "in operation" whenever the receiver is turned on but the output utilizes the PHONO-RADIO switch to route the NFM-83 output to the receiver's audio system. When installed, PHONO is used for NBFM operation and RADIO is used for normal receiver operation. The NFM-73 has a different configuration that allows it to be plugged into the NC-173 accessory socket that's on the rear chassis apron on the back of the receiver. The NC-173 didn't have a PHONO-RADIO switch so a dummy phone plug was inserted into the PHONO input on the front panel to actuate the jack-switch which then disconnected the AM detector and routed the NFM-73 output to the audio amplifier stages. Withdrawing the dummy plug returned the NC-173 to AM-CW reception. No modifications to the receiver were required for using the NFM-73 adapter. The NFM-73 and NFM-83 are fairly easy to find. Either of the adapters are usually cheap and they should be since they are practically useless devices nowadays.

NFM-83 Narrowband FM Adapter Is it really a useless device? Since NBFM on HF is only authorized on 29.2-29.3mc, finding any HF-FM activity today would be a challenge.

Kit-type 100kc Crystal Calibrator - I don't know of any reason that National didn't provide for using their onboard Crystal Calibrator other than economics. Certainly, had National included their typical 1000kc/100kc plug-in calibrator, that would have been the most useful of the commonly available National plug-in accessories but it would have required additional wiring for an additional accessory socket and front panel switching to operate the calibrator switching from the front panel of the receiver. That's why the HRO-50/60 receivers have two accessory sockets - to allow using a Crystal Calibrator with front panel switching plus either the NBFM adapter or the Select-O-Ject that were operated with a separate rotary mode switch for FM and the panel switch for the Select-O-Ject for its operation.

Since regulated +150vdc and 6.3vac are available at the NC-183 or NC-183D (or even the NC-173) accessory socket, any 100kc crystal calibrator kit could be built to use the accessory socket of the receiver. Generally these kits will have a toggle switch on the calibrator to turn off the B+. The output can usually just be electrostatically coupled to the antenna input and provide sufficient signal level. The hassle is that the lid of the receiver has to be lifted to turn the calibrator on or off or, for the NC-173, one would have to reach around behind the receiver to turn the calibrator on or off. National, along with most of the other receiver manufacturers, certainly thought that any ham would have had either a surplus Heterodyne Frequency Meter or a ham Frequency Standard (an external, self-contained crystal oscillator providing 1000kc, 100kc and 10kc markers) for accurate frequency measurement of a received signal or for determining ham band edges so an "onboard" 100kc crystal calibrator wasn't considered a necessity. Additionally, the logging scale provided on each dial is 0-200 and that provided a lot of resolution for accurate resetting to known "logged" frequencies. Using any sort of calibrated signal source (a borrowed Frequency Meter, for instance) it would have been possible for a ham to "log" all of the ham band edges and any specific ham schedule frequencies using the Main Tuning Log setting and the Band Spread Log setting. This was an easy and convenient method for accurate frequency reset that didn't require an on-board Crystal Calibrator. As an example, on my NC-183 receiver, the Nevada Vintage Mil-Rad Net on 3.974mc is tuned by setting the Main Tuning to Log 168.0 and the Band Spread to Log 170.5. It's a quick setting of the two dials and the receiver is "on frequency." 
 

Phono Input - Although the Phono circuitry was used via the accessory socket wiring (pin out for audio varies with model) for the Select-O-Ject output or the NBFM adapter output, the Phono jack input could be used for any other sort of audio amplification as long as the input levels and the impedances matched. The SOJ-2 had to be disconnected to use the Phono input for a phonograph cartridge input. The National NBFM adapter instructions don't indicate that the adapter had to be removed from the accessory socket to use the Phono input, but it did.

Battery Operation - The octal socket on the rear chassis apron allows access to the power supply voltages, remote standby functions and can allow a hook-up for battery operation of the receiver. Remote standby was later routed to its own three screw terminal strip on the NC-183D. As for battery operation, one famous use of a NC-173 operated entirely on dry cell batteries was the receiver used on the Kon-tiki Expedition in 1947. National advertising promoted this trans-Pacific experimental raft sailing from South America to Polynesia and its use of the NC-173 in several ads at the time. Several battery and crank-generator small transmitters were also used on the Kon-tiki. National advertising in the September 1947 QST issue implies (but doesn't specifically state) that the Kon-tiki also carried a National HRO-7 receiver that was also dry-cell battery operated, however the official write-up in Dec. 1947 QST states that the NC-173 was the main receiver and the HRO-7 isn't even mentioned. It seems unlikely that the HRO-7 was aboard since it wasn't introduced until August 1947.
 

NC-173TS 6" diameter PM Loudspeaker for the NC-173
Although the assumption would be that an 8" loudspeaker would be installed into this size cabinet, I've only seen 6" diameter loudspeakers installed. The advertising doesn't specify the speaker diameter on these smaller versions. The cabinet color is the light grey with a slightly silver tint.

Cosmetic Issues

Cleaning Phenolic Dials on the NC-173 and NC-183 - As mentioned, the acetate-based phenolic plastic dials used on the NC-173 and NC-183 are always much darker nowadays than they were when new. Most of the time, the section of dial that was exposed to light through the front panel dial opening will be much darker than the unexposed areas. This darkening is inside the plastic material and is impossible to correct. Sometimes a slight improvement can result with damp-cleaning the back-side of the dial. Any cleaning of the front-side of the dial should only be attempted after a test-cleaning of the part number that's located near the hub of the dial. If the part number isn't affected by damp-cleaning then the entire dial front can be cleaned by that method. I've found that many plastic dials made after WWII will have very durable ink that can be damp-cleaned, BUT TEST TO BE SURE. The NC-183/173 dials used very durable ink on the dials and they can be cleaned directly using a Glass Plus dampened paper towel. DON'T EVER USE WINDEX, the ammonia is a harsh cleaner, use only Glass Plus, don't scrub, don't use a lot of pressure and watch the ink for any thinning. I didn't have any ink degrading with Glass Plus when damp-cleaning either the NC-183 dials or the NC-173 dials yet years of dirt and tobacco staining were removed. After the cleaning, the dial will have a "flat" look but lightly polishing with a dry soft flannel cloth will impart a sheen to the plastic dial without doing any damage. This dial cleaning won't change the darkening that has happened because of sunlight. That discoloration is deep inside the plastic material. What the cleaning does is remove other types of dirt on the surface of the dials and that will generally brighten the dial somewhat and increase the contrast between the dial color and the nomenclature. Cigarette smoke contamination on the phenolic-acetate dials is the most common "dirt" that can usually be easily removed on post-WWII dials, but be sure to test the silk-screened ink on a non-visible area first.

Cleaning Lucite Dials on NC-183D - Lucite can be cleaned using Glass Plus or a plastic foam cleaner. Don't use WINDEX. I've used Glass Plus with paper towels and had no scratching occur but some plastic cleaners will suggest using a soft cotton cloth to avoid scratching. It probably depends on the paper towel brand used. Avoid the cheap paper towels that are coarse and stiff feeling. The most important part of cleaning is to NOT use WET paper towels or cloths. The excess wetness will wick down the rivet holes and get behind the Lucite. Normally, this will dry and not cause any problems but if there is any dust or a film coating on the inside of the Lucite dial, then it's possible that some residual pattern will show in the dust after the wetness has dried. It's not staining but you can't get rid of it unless you disassemble the dial which means drilling out the rivets. That's a major pain that's best avoided by only using slightly damp paper towels or cloths. Other than avoiding excessively wet cleaning, the Lucite dials are very forgiving and easy to clean. NOTE: If a little moisture did wick down the rivet holes usually the "stain" won't show when the entire receiver is assembled since the rivets are hidden by the shape of the dial openings in the front panel.

Dial Covers - The dial cover on the NC-173 and the NC-183 is a single, thin plastic, rectangular piece that has the band indicators for both dials applied on the backside and then covered with gray paint for color and protection. This piece can and easily does get very dirty. That adversely affects the dial visibility. The dial cover plastic is mounted with two aluminum strip spacers, one on each side. The aluminum strips are only used on the NC-183 to provide the proper spacing so the dial cover is the correct height behind the front panel dial openings. There are two screws, lock washers and nuts on each side to mount the plastic piece. The screw heads are on the outside of the front panel. On the inside is the aluminum strip, then the plastic, then the lock washer and nut. The nuts aren't overly tight or the plastic could possibly crack over time. Once the dial cover is dismounted, it can be easily cleaned with Glass Plus. Both sides need to be clean. Usually the gray paint that covers the nomenclature is in good condition. If it has scratches or missing paint areas and the lettering isn't affected then carefully match the paint and apply on the backside. Remount using the original spacers and hardware. The NC-173 dial cover mounts the same way but without any aluminum spacers. Since the front panel's embossed area is just around the dials and the mounting screws for the dial cover are in the area of the panel outside the embossed area, the dial cover ends up being flush with the inside of the dial openings with no spacers.

The NC-183D uses two Lucite dial windows that are approximately 0.125" thick and have a section that has a black background paint applied. The index line is an engraved line on the interior side filled with white paint. The mounting edge of each piece is slotted so the mounting screws only have to be loosened. The Lucite windows both mount just on one side with an aluminum spacer and another aluminum strip that has tapped holes. The side nearest the S-meter isn't supported except for a metal support guide that is mounted by way of the Trimmer shaft bearing bracket. The slotted mounting on one side only will allow for slight position adjustments. The band indicators are on the exterior side of the Lucite with engraved nomenclature (actually, I think the nomenclature was stamped in some way) with white fill paint. Clean the Lucite pieces with Glass Plus. Be careful of the band indicator engraving and fill paint because it's extremely delicate and can easily be damaged or entirely removed. Putting the band indicators on the outside of the dial covers was a "BIG MISTAKE" by National. If it had been done like the NC-183 or NC-173 with the indicator nomenclature on the interior side protected with a coat of paint then the following cosmetic problem would have been avoided,...

NC-183D Band Indicators - An Insolvable Problem - Another cosmetic problem involves the Lucite dial covers and the white paint fill on the band identification markers used on just the NC-183D. The engraving is very, very,...well,...extremely shallow and the thin white fill paint can be easily destroyed by careless cleaning efforts, for example, using Windex to clean the dial covers,...a common mistake. The shallow nature of the engraving prevents re-doing the white fill paint in a normal manner. Any attempt to remove excess fill-paint is impossible because the "wiping action" tends to "pull" the fill-paint out of the extremely shallow engraving. Worn original white fill-paint seems to be a very common cosmetic problem that seems impossible to restore, at least, I've never found a successful method to accomplish it. National must have had some special process to accomplish the white fill of the nomenclature on these NC-183D band indicators since the debossed nomenclature doesn't look like typical engraving. I suspect that National somehow "hot stamped" the nomenclature and the color simultaneously and that's why the debossed lettering and numbers are so, so shallow (not at all like true engraving.) Since this was some sort of "special process" National used that certainly required specialty tools, today it has become impossible for restorers to duplicate the end results.

Some Things I've Tried - That Don't Work:  Rubber squeegee - didn't work. Lacquer Stiks - didn't work. White Testor's Enamel painted just on the engraving with as little over-spread as possible. Let this dry and see if I can remove the excess carefully with a new razor blade. This was very close to working with the razor "rolling" the paint off nicely but the razor blade is too aggressive for the plastic leaving very fine but noticeable scratch marks. Next, I'm going to try a Plastic business card - didn't work. The trick seems to be letting the paint dry some and then it will "roll" off leaving just a little bit of paint to be removed later when it's dry enough that it won't be pulled out of the engraving. How to remove the remaining paint residue is a problem and thinner seems to easily remove the fill paint, even though it's fairly dry. I'm thinking about going back to Artist's Acrylic and using this plastic card procedure. With acrylic paint, clean up would be much easier using Glass Plus that doesn't react with the plastic.  Well, that didn't work either...the acrylic paint won't "roll off" like Testor's does and the plastic business card won't shave off the acrylic paint but just smears it around. Testor's and Plastic Card - when Testor's was tried again, the plastic business card isn't sharp enough to make the paint "roll off." To sum it up,...Testor's and a new razor blade comes the closest to working. But, it's far from perfect. If the original band indicators are faded but can be seen and read, I wouldn't do anything to them because anything you do will only make things worse. If the band indicators are completely gone (like these were that I experimented on,...from a "parts set") then the Testor's and a razor will provide some fill to the nomenclature but test your razor technique on a scrap piece of Lucite or plexiglass first and see how much scratching you get,...it probably will change your mind. Maybe if there was a plastic razor blade,...maybe that would work. Or, a very sharp edge using a piece of plexiglass. It might work,...Tried it,...nope, it doesn't work very well. It's probably the only option that sort of works and doesn't damage the Lucite. Then there's this solution,... The Best Option - Actually, I've come to the conclusion that finding a "parts set" that has good dial band indicator/covers is probably the best solution. Although, unless the "parts set" can be found locally to eliminate the exorbitant cost of shipping a heavy receiver that's only needed for parts, it might end up being a very expensive method to fix this seemly insolvable restoration problem. Hopefully, there might be other "needed" parts that can be also be salvaged from a "parts set purchase + shipping" expenditure. 

All of these special and difficult to solve cosmetic issues seem to justify the high prices that a "near mint" NC-173, NC-183 or especially the NC-183D can garner today.

General Information on Rebuilds

Is rebuilding really necessary? If you intend to use any of these receivers as a vintage ham receiver in a ham radio station environment where the receiver will be turned on regularly, used for a couple of hours and be depended upon to provide reliable operation, then the answer is yes. If you want to experience the tremendous audio capabilities of the NC-183 or NC-183D then a rebuild will absolutely be required. Most of these receivers, when in "as-found" condition, have been stored away and haven't had power applied to their circuitry in years,...perhaps decades. Defective components are a certainty and if the receiver is powered up in this condition it's quite possible to damage other components in the receiver. The attention to detail necessary for accomplishing the rebuild will allow you to examine every part of the radio and this will result in you discovering many other somewhat latent problems and to take care of any mechanical issues found. With a complete, thorough rebuild, the end result will be a receiver that can be left turned on for several hours without overheating components, it will operate to original specifications and it will be a pleasure to use. With the NC-183 and NC-183D, the audio produced by these receivers will be impressive when everything is correct.

Typical "As Found" Condition -  Today's vintage ham gear enthusiasts appreciate National's combination of robust push-pull V-T audio with great reception possibilities making the NC-183 and, even more so, the NC-183D very popular choices for vintage ham station receivers. Additionally, these were very well-built receivers that used top-quality (for the time) components. The chassis is high-quality cadmium plated and many of the steel parts are nickel-plated. Other chassis components have a matte-aluminum finish. The NC-183D chassis is often found in beautiful condition, though it does depend a lot on how well-cared-for the receiver was during its active period and then ultimately where the receiver was stored afterward. It does seem that finding an excellent condition, all original NC-183D isn't as difficult as finding the NC-173 or NC-183 in a similar pristine condition. The earlier receivers seem to have been used for a much longer time, so they might have ended up becoming victims of amateur repairs and seemingly modified more often than the NC-183D versions. The expense of the "D" might have been another reason for its better preservation. Also, the NC-173 and NC-183 were good usable receivers for a much longer period of time than the NC-183D. When the last of the NC-183Ds were produced, in 1958, the ham radio market was beginning to change to smaller and lighter-weight ham receivers. The desire to keep a large, heavy receiver around perhaps didn't last too much longer. The NC-183Ds were retired with less total time in active use, so many were "put away" in excellent condition having been used a for just a relatively short time. But, just because any of these receivers might have been retired and put away in good shape doesn't mean they stayed in that condition. Poor storage conditions and endless time spent in a humid, rodent infested environment has irreparably destroyed many of these receivers. Some retired receivers were traded-in on new equipment to be then sold by dealers as "second-hand" receivers. These receivers didn't seem to benefit as much from "new owner appreciation" and being older gear, many second-hand receivers were soon hacked for repairs, modified in attempts to modernize the performance, abused, thrown around, scratched-up and finally ended up becoming part of a vertical pile of collected gear,...usually rusting away in a leaky shed or damp basement. The upshot is,...any NC-173, NC-183 or NC-183D found in excellent physical condition today is a rarity. But, you're more likely to find a really nice NC-183D than either of its predecessors (although I've seen a lot of really poor condition NC-183D receivers too.)

NC-183D - Many as-found condition NC-183D receivers will have a variety of operational problems nowadays generally due to poor storage, hamster rework in the form of "junk box components" used for repairs that are "hook-spliced" into the circuit. Sometimes unnecessary and destructive modifications are incorporated into the circuit. In addition to those problems, some original leaky paper-dielectric capacitors can potentially cause heat-related failure of the power transformer or the filter choke if the receiver is operated for long-hours without a proper rebuild (power transformer failure seems to be a moderately common occurrence judging by how often replacement power transformers are encountered in these receivers.) A full rebuild and complete alignment is normally required to obtain the "top performance" that the NC-183D is capable of providing. If you're planning a NC-183D rebuild, there are 16 (up to 19 in later versions) molded-plastic-tubular, paper-dielectric capacitors to replace but, luckily, a large number (all but five) of the .01uf capacitors are ceramic disks that won't require replacement. There are 5 electrolytic capacitors that will probably need replacement. Reform and test at full operating DC voltage if you plan on using the original electrolytics. All components are easy to access. Check the carbon resistors for being out-of-tolerance since any leaky bypass capacitors can easily over-heat associated load resistors. Many of the 470K resistors are in parallel with other components in the circuit and will not measure their actual value "in the circuit" and will need one lead "lifted" for accurate measurement (if you feel it's necessary - like if you're having trouble with the circuit.) On replacement parts, use only new capacitors. Polyfilm "Yellow Jackets" will work fine but the end result will look like the cheapest parts available were used for the rebuild. CDE 715P Orange Drops are polypropylene dielectric and high quality capacitors that will make the rebuild look much more professional. Replacement resistors should be NOS JAN CC types (10% tolerance or better) if possible, and their value must be verified with an accurate resistance measuring device (some brands of carbon resistors will drift in value even if they have never been used. Allen-Bradley JAN types are the best for holding their value.) Replace any tubes that don't exceed minimum acceptable transconductance by a significant percentage (NOS tubes are best.) Finish with a complete IF and RF tracking alignment and your NC-183D should become an easy-to-use station receiver that provides excellent sensitivity, the necessary selectivity if you use the Crystal Filter and very high-quality audio reproduction if you use the original National NC-183DTS 10" table loudspeaker or perhaps even a better speaker set-up.

NC-173 and NC-183 - These earlier receivers have wax-covered cardboard shell, paper-dielectric capacitors installed (and there are a lot of 'em - 26 paper-dielectric wax-coated types to be exact,...plus four rectangular bakelite molded paper caps - 30 paper-dielectric capacitors total for the NC-183. The NC-173 has a total of 26 paper-dielectric capacitors and two bakelite molded paper capacitors.) In the NC-183, the four rectangular-shaped, brown-molded plastic capacitors look like over-size micas but are actually .05uf 600wvdc paper-dielectric capacitors and these should also be replaced (these are WWII surplus components that many manufacturers used post-WWII.) In the NC-173, the molded bakelite capacitors are .005uf 600wvdc paper-dielectric types (only two of these are used.) Check electrolytic capacitors for value and leakage current. The power supply filter electrolytics will almost certainly need replacing. Sometimes the filter capacitor might have an excellent seal and it might function after reforming but be sure to reform and test at full working DC voltage (but I still don't know if I'd trust it for long hours of operation. After cutting apart a few of these Aerovox filters and seeing what's inside,...I'd replace or rebuilt any original multi-section,...it's a failure waiting to happen!) Also, many times the cathode bypass electrolytics are still good (low operating voltage, connected in parallel with a low value resistor and the small size provided excellent sealing preventing most of them from drying-out. Testing and reforming will be required - but replacement is certainly a better option for top performance of the hi-fi audio section.) Resistors should be checked for values versus tolerance. Basically, it's the same procedure as with the "D" model but with a lot more paper-dielectric capacitors to replace.

Expected Performance

NC-183D - Vintage AM ham users favor the NC-183D primarily because of its double-conversion, its tertiary component IF system that also provided an extra stage of IF amplification and its superb high fidelity audio reproduction. The receiver was supplied with the NC-183DTS, a 10" PM loudspeaker in a matching housing but these are rarely found with the receiver anymore. The original National 10" PM speaker actually sounds very good, but these large 10" versions are becoming more and more difficult to find along with its ever-increasing expense to purchase them when they are in good condition (not to mention that most of them will have to be shipped to you adding to the overall expense.) However, if a larger speaker system is available, the NC-183D can produce really fabulous audio on AM signals. Lots of bass is available and the 3.5kc IF passband (at -6db) is sufficient for a fairly wide-sounding, bass-laden audio reproduction (at one time, I used a 15" Jensen coaxial speaker housed in a Jensen KM bass-reflex box - described further down.) The audio bandwidth specs are a surprise for a communications receiver and feature a -5db 20hz low end, flat from 50hz to 7khz and an expected drop off beyond that to -5db down at 12Khz and there's 8 watts of V-T audio power available and 11 watts peak. Though it might be thought that by tuning to one sideband or the other, higher audio frequencies could be recovered, the shape of the NC-183D IF passband is such that as one tunes "off carrier center" more and more signal attenuation occurs and that results in the best audio reproduction actually happening when the signal is tuned "carrier center." The TONE control can be used to maximize upper audio response, if that's what's desired. It's too bad that AM-BC programming is so dismal (around here it's nothing but sports-talk, poli-talk and the aurally torturous country-western stations on AM ) and quality SW-BC is also a rarity these days. Well, there's always low power DIY BC-ing. Or, for better signal quality, you can use a lab-type RF signal generator and feed your CD-player (showing my age,...sorry,...your MP3 device although maybe that's obsolete too) output into the EXT. MOD input, adjusting the sig-gen modulation gain level for a quality waveform. Then connect the RF sig-gen to the NC-183D antenna input and decide on a frequency to use. It's like having whatever programming you want delivered via cable to the receiver. Of course, audio quality is highly dependent on the type of external equipment used for this "hook-up" and the resulting "quality" is certainly a subjective judgment.

The NC-183D is also sensitive enough and provides adequate selectivity to deal with most actual ham band reception issues. I've used the NC-183D as a station receiver on 75M and it's easily able to cope with all of the QRM and QSB issues along with having a good ability to provide Q5 copy of very weak signals in the AM mode. It's also convenient that remote standby is very easy to set up and use. When examining the specifications one will notice that the NC-183D has almost all of the features of the HRO-60 but minus the headache of dealing with plug-in coil sets that have to be manually extracted from the receiver to change bands or to be switched between general coverage and ham band spread coverage; or the expense of purchasing additional coil sets if coverage other than 1.5mc to 30mc was desired; or the problem of how to store unused coil sets or dealing with a PW-D micrometer dial that was pretty much useless by the HRO-60 production (the PW-D had to be installed, otherwise it wouldn't have been a HRO.) Maybe you can't use the plug-in National Crystal Calibrator on the NC-183D but there are other methods for externally determining received frequency (using a heterodyne frequency meter was popular in the 1950s but nowadays it's much easier to use a digital frequency counter, or just use the logging scales for accurate frequency resetting.) If you want the performance of the HRO-60 but with less accessories and in a more convenient-to-use form with features like simultaneous band spread and general coverage operation, tuning range changes via a front panel band switch or high fidelity audio, the NC-183D is that receiver. Likewise, the earlier NC-183 was intended to be competitive with the HRO-7 first and later, the HRO-50.

NC-183 - The earlier NC-183 is obviously lacking some of the more sophisticated design features of the NC-183D, but the NC-183 was from where the "D" evolved. Even though it's the early version, the NC-183 was considered a "hot receiver" when it was introduced in 1947,...at least according to National advertising that stated the NC-183 was the "deluxe receiver for optimum reception under all conditions!" Nowadays, when the NC-183 is fully rebuilt and aligned, it will perform more or less like the "D" on lower frequencies but the NC-183D's double conversion and more modern tubes definitely benefit its reception capabilities above 20mc. Since the NC-183 only uses two stages of IF amplification using standard IF transformers, the result will be a broader IF passband that will tend to favor the hi-fi audio capabilities of the receiver when listening to quality AM broadcasts (if they exist.) The Crystal Filter can narrow the passband as needed and will probably only be needed occasionally. Crystal Filters can easily be used for AM reception and are excellent for reducing adjacent frequency SSB QRM. The single-conversion design really only adversely affects performance on the highest frequencies. Up to about 15 meters, the NC-183 does a great job. Above 25mc, the NC-183 begins to show its WWII design roots as the sensitivity falls off somewhat (the 6M coverage requires strong signals,...and some actual activity on the band.) Mechanically, the tuning on the NC-183 can have problems since the plastic dials are rim-driven with a pinch-wheel system. If the plastic dials are damaged along the rim, they will never work correctly (and these dials can be damaged easily by careless removal of the chassis from the cabinet.) The odd-ball three pin loudspeaker socket is a hassle to find a mating plug for but using just the correct size metal pin to fit the one large socket receptacle (chassis ground) and to fit a smaller pin for either 8Z or 500Z outputs will work fine and eliminates trying to find the almost unique plug. The NC-183 is difficult to find in excellent cosmetic condition (I've never seen an excellent condition original NC-183) and almost any example will always need some mechanical rework and definitely will need electronic rebuilding to get it performing up to its original capability. I've also used the NC-183 (fully rebuilt) as a station receiver on 75 meters and have found it easily able to cope with all of the normal QRM issues. Actually, 75 meters isn't much of a challenge for almost any receiver and selectivity becomes the most important facet of performance when operating on this band. The NC-183 Crystal Filter deals with QRM very well. Because of its wider "bell-shaped" IF passband that's very different from the "steep sides and flat nose" passband of the NC-183D, I think the NC-183 actually produces a slightly better sounding audio reproduction of a good quality AM broadcast signal if compared to the same type of signal received through the "QRM-fighting" 3.5kc IF passband of the NC-183D. And, with the NC-183, if more selectivity is required,...there's always the Crystal Filter.

NC-173 - The NC-173 is a slightly smaller receiver not having the depth that was required by the NC-183 chassis. Also, the weight is noticeably less than the NC-183. The Phono input is on the front panel and includes an internal phone plug-operated switch that disconnects the receiver circuitry from the audio section when the Phono jack is used. The accessory socket is on the back panel. The smaller 6" loudspeaker (NC-173TS) was supplied with the NC-173 and a standard three screw-terminal strip was used for the 8Z and 500Z audio outputs. The National NC-173 advertising always showed the smaller loudspeaker in a housing that's the same height as the receiver cabinet where the NC-183TS 10" speaker housing is noticeably taller than the receiver. The NC-173 receiver used a different style power transformer (lower voltage B+ winding since less power required for the single audio output tube is used) and the audio output transformer was also different since it was for a single output tube. The audio output power isn't shown in the manual specifications but given that the 6V6 plate voltage is around +180vdc with an average of about -18vdc grid bias, the power output is about 2 watts. This isn't implying that the NC-173 has anemic audio,...it doesn't. The receiver will really sound very good when it's rebuilt and is used with a decent quality loudspeaker (the 10" NC-183DTS sounds great when driven by a NC-173 but even the smaller six-inch speaker sounds pretty good.) NC-173 reception might have problems with images starting around 14mc. To a certain extent, actually hearing images will be related to the incoming signal strength so a very large broadband antenna or a long untuned end-fed wire might actually allow more images to be heard than a simple resonant antenna system would. As a test, using a ten foot long wire antenna, I absolutely could not hear the 15mc WWV image on 14.090mc even though the 15mc signal was S-9+. But the 20mc WWV image on 19.090mc was as strong as the actual signal. Switching to the Collinear Array antenna and the 15mc WWV image was easy to find but even with this large antenna I couldn't hear the 10mc WWV image. One does have to consider (as maybe the original purchasers did) that the majority of ham operations would have been on 80 or 40 meters where images wouldn't have been a problem and the receiver's selectivity was the important performance requirement. If frequencies higher than 14mc were of interest and it was thought that images might cause a problem it was always possible to add an external preselector to improve image rejection. On 80M, no problems would be encountered using the NC-173 as the vintage station receiver. Likewise, 40M operation wouldn't present a challenge. The NC-173 uses the same Crystal Filter as the NC-183, so it provides the selectivity needed to cope with QRM. The NC-173 is rarely found in pristine condition although I've seen a couple over the years.

NOTE: An observation on using WWV as an "image test" signal - No HF signal is more recognizable than WWV. This station almost always provides a powerful signal but, no matter how weak the signal is, one can always recognize its unique characteristics. The typically powerful WWV signal with easy recognition skews the "image test" to a certain extent. A normal ham signal image probably wouldn't even be heard or noticed. In fact, the only ham images I've ever heard when using single-preselection receivers on 20M have been from high power ham stations using high gain directional beam antennas and excellent propagation resulting in extremely strong signals. And, the only reason these images were even noticed was because the ham signal appeared to be way outside the 20M ham band (so it was obviously an image.) So, even though 15mc or 20mc WWV images can be received, I doubt that any normal ham signal would be. Even SW-BC stations in the 19M band are rarely heard as images. So, although I mention the WWV image test, remember that this is the absolutely worst case image test if one is using actual received over the air signals.

No Receiver is Perfect

Though the NC-183D, the NC-183 and the NC-173 can perform very well when fully rebuilt and aligned, providing fabulous audio and excellent sensitivity, these receivers were designed during the post-WWII era. In fact, the early receivers will have some WWII surplus components used and the metal octal tubes are mostly WWII design improvements over the late-thirties metal octal tube designs. The early receivers operate as would be expected for the time period but, they can still do a fine job on 75M,...almost any receiver can. The later NC-183D has several circuit design improvements, more modern tubes and some nice mechanical improvements. But, it still operates like an early-1950s receiver but with the ability of better higher frequency performance.

Antennas are very important for top performance. A random length, untuned wire will not allow any of these receivers to perform to their maximum specifications. A matched antenna or a resonant antenna will allow maximum response from these receivers and, of course, antennas with gain will provide the very best response.

With any of these receivers you can expect a fairly long warm-up time. These receivers need at least several minutes at least to settle down and stabilize. Frequency drift won't entirely stop but it will slow down considerably within about 20 minutes or so. Although dial accuracy is excellent, the dial resolution is poor but that's what would be expected for the time. For example, on Band B, the MT dial readout resolution is an index mark every 200kc. Even on Band D (80M) the MT index marks are every 50kc. If you calibrate the Band Spread, you can get better resolution and better accuracy but even on 80M BS the resolution is 10kc per index marker so direct frequency readout still involves a lot of guessing (or using a frequency meter if accuracy is important.) National didn't provide a Crystal Calibrator for these receivers and the National Crystal Calibrator won't work in the Accessory Socket. It would be possible to install a homebrew or kit 100kc calibration oscillator but it would have to incorporate its own switch to turn off the B+ to disable the oscillator when not in use (the B+ can't be turned off at the Accessory Socket.) Also, an external Frequency Standard could be used, a popular self-contained crystal oscillator that provided 1000kc, 100kc and 10kc markers.

The Noise Limiter is from the late-forties, so it's designed for impulse-type noise, like ignition noise. It doesn't work very well on many of the modern types of RF noise (although the NL does seem to reduce SCR-impulse noise quite well, like motor speed controllers used in modern modulated heating systems.) The S-meter response is very dependent on the tuned frequency, that is, on 80M with the RF gain at 9.5 the average AM ham signal will "peg" the S-meter while a SW-BC station in the 19M band will barely reach S-9. This is typical of S-meter circuits of the time although the RF gain level can be adjusted to compensate for "over-driving" the S-meter.

Since these receivers were designed and produced before SSB came along, the Detector and AVC are designed for AM reception. CW (or SSB) can be received by adjusting the RF gain for a proper BFO injection ratio. This is typical diode detector/BFO design of the time period. The Amplified AVC circuit can be left on for CW reception if desired. SSB reception will require reducing the RF gain control down for proper demodulation of a SSB signal. Expect a lot of drift in the CW and SSB mode unless the receiver has been turned on for at least 30 minutes.

The NC-183 and the NC-183D are both heavy receivers weighing 65 pounds. When trying to move these receivers the cabinet doesn't provide anything to "hold on to" and the rolled edges and smooth paint can limit your grip. Judging by the dents and scratches found on these receivers today, loosing one's grip and dropping the receiver must have happened more often than would be expected.  

The sum it up,...the NC-173 and the NC-183 operate exactly like receivers designed and produced in the late-forties to sell for between $190 and $260. The NC-183D is a slight improvement but overall behaves with the same minor issues that would be expected from a receiver produced in the early-1950s (and selling for about $370.) Although these receivers were expensive, they weren't the most expensive receivers available. Their design and performance can't and shouldn't be compared to the Collins-designed military R-390A. Mentioning Collins, one has to consider that even though the Collins 75A Series or the Collins 51J Series were contemporaries of these National receivers and can certainly "out-perform" them in many ways, the Collins receivers were much more expensive and ALL Collins receivers at that time had extremely poor communications grade audio or worse (worse?,...the 51J-1 or J-2, for example.) Also, their AVC circuits and Noise Limiters barely functioned. But, Collins had everyone beat when it came to dial readout accuracy and oscillator stability.
 

Restoration Write-Ups

Apr 2022 - Another NC-183D - I must have a soft-spot for the NC-183D. I've owned several of them over the years but I always seemed to sell them off after a short time. Even the "D" shown below is actually one that I owned 20+ years ago and happened to have photographed. I sold the last one I had in 2019. I had acquired it the year before from Ham & Hi Fi in Sparks, NV as a "tech special" (cheap because it didn't work and couldn't be sold to the inexperienced collectors on eBay.) The exterior was in nice cosmetic condition. Inside, the chassis was in beautiful condition except the original power transformer was missing and a non-National replacement type (Stancor) power transformer had been installed. I had an early NC-183D "parts set" that provided an original power transformer. Although it was missing, the original transformer was certainly the late-style NC-183D "S-461" power transformer (the potted type, like the filter choke is.) All I had available was an early-style "D" transformer. At least, it was from a NC-183D.

Then I recapped the receiver (the paper dielectrics only using Yellow Jacket polyfilms) and that was followed with some tube replacements and a full IF/RF alignment. I used it for about a month and then I sold it to a local ham who had been looking for a NC-183D on eBay. I included the NC-183D "parts set" in the sale.

In 2022, I found out the ham I'd sold it to had put the NC-183D in storage and hadn't even been using it because of some microphonic tube problems. I offered him a trade of some ART-13 transmitter repair work he needed for the return of the NC-183D receiver. I repaired the ART-13 (burned-out field winding on the Autotune motor - required a new motor installation) and got the NC-183D back. Its serial number is 430 0135 indicating it's a fairly late production receiver probably from one of the last NC-183D production runs. Production of the NC-183D ran until 1958 while the HRO-60 was produced up to about 1960 (the latest production run HRO-60 serial number reported is on run 505 but the HRO-60 production starts at run 357, then 366, 393, 425 and the last two runs are 459 and 505.)

The trade also included that same pretty nice NC-183D "parts set" that's been "kicking around" for quite a while (I had to "take it back" as part of the trade deal - and I'm glad I did!) The "parts set" was an unfortunate victim of hamster modifications that seemed to zero-in on the dual-IF (tertiary style) part of the circuit. Rampant physical damage and circuit hacking pretty much relegated this NC-183D to "parts set" status. "Parts sets" are really a necessity for restorations and rebuilds. Except for storing them, they are really convenient to have. The serial number of the parts set NC-183D is 357 0668 (run 357 was also used for the first HRO-60 receivers.) However, as a side note,...this "parts set" has been scavenged now to restore a NC-183D, a NC-183 and even a NC-173. It's really just a "hulk" now.

Not my current NC-183D,... I had this one in 2003. I bought it with the matching NC-183DTS loudspeaker at Reno Antiques for $100, a bargain even then. Unfortunately, I sold it a few weeks later out of the Western Historic Radio Museum to a visiting ham looking for a vintage receiver.

But,...back to the NC-183D problem,...the "microphonics" turned out to actually be "motorboating" that was caused by a leaky 6BA6 tube used as the 2nd IF amplifier. I had been all through this receiver just a couple of years before and had replaced all of the paper dielectric capacitors, so I was pretty sure the problem would have to be "tube related." I replaced the bad tube and the receiver was "back to normal." Now, I'm using this NC-183D to drive a 1950s vintage 15" Jensen coaxial speaker housed in a 1940s vintage Jensen KM bass reflex cabinet. Wow! Great sound,...even some of the SSB signals seem to have extended bass in their audio. The NC-183D is one of the few post-WWII ham receivers that can provide enough audio power to drive the Jensen KM bass reflex box effectively. Though P-P audio was common on many "high-end" ham receivers pre-WWII, after the war, most ham gear came with "military-grade" audio that was promoted as "communication audio." The favored audio response for "communication audio" is 300hz to 3000hz - great for listening to a carbon mike used on the battlefield - not so great for anything else.

 My current NC-183D was located in the shop  

It's setting on top of the 1940s Jensen KM Bass Reflex Box with a 1950s Jensen 15" coaxial speaker inside. The NC-183D is one of the few communication-type receivers that can really drive this speaker box and produce a lot of bass response. 8 watts of V-T audio is available and up to 11 watts peak audio. The shop antenna used is a 130' CF Inv-Vee fed with open feed line to a Nye-Viking MB-VA coupler. Note the T-368 transmitter behind the NC-183D and KM box.

I now have an original NC-183DTS loudspeaker, that's the 10" speaker in a matching table cabinet, so I probably won't be using this Jensen KM set up in the future.

Or, so I thought,...I moved the NC-183D back out to the shop in June 2024 where it's again connected to the Jensen KM box and used as the station receiver with the W6MIT 300W transmitter.

SN 430 0135 Revisits the Workbench in 2024

Here's what I think still needs to be accomplished,...

1. Clean the top of the chassis. I can tell I didn't do that. It's not real dirty but it could use a little detailing. Cleaned with WD-40 and Glass Plus because it really was pretty dirty
2. Replace all ten of the tube shields. They're oxidized and darkened. I should have some good condition shields of this type,...I ended up wire brush cleaning the originals but the "parts set" may provide better ones
3. Clean dials, dial covers and meter glass. The meter glass looks like it has something on the inside of its glass. Cleaned with Glass Plus, the S-meter had to be disassembled to clean the smoky dirt on the inside the meter
4. Test all of the tubes. I did this in 2018, so it should be done again. Replaced three tubes,...two tested at minimum and the other one, one of the 6V6GT tubes, had a severely cracked bakelite base
5. Clean the tube sockets. I don't think I did this six years ago, but who knows? Used DeOxit applied with a small paint brush
6. BFO frequency control shaft seems very loose. It does work but the control shaft is loose feeling. I used two knob felts to lessen movement. Big problem inside BFO can,...broken ceramic base on air variable C.
7. Clean cabinet. Other than a "wipe-down" I know I didn't do this. I can also detail the paint since there are several tiny scratches. The scratches didn't clean off with Glass Plus,...already lots of old "touch-ups"
8. Dial covers Band Indicators,...I don't know if I'm going to try to recondition these or not. The white fill paint is present but it's very thin and showing quite a lot of wear. Nothing works for this problem except replacement
9. Check alignment, it should be okay but we'll see. RF Tracking alignment needed
10. I'll shoot all new photos but I'll save the one of the NC-183D on the Jensen KM box and the 2003 NC-183D.
 

Chassis Test - With all of the tubes reinstalled along with their shields I applied power to the chassis. I had the NC-183DTS connected and a ten foot test antenna. The receiver functioned with the matching NC-183DTS speaker but I was noticing a very slight "roughness" in the audio quality and, with the BFO on, CW was slightly garbled (but definitely noticeable.)

Chassis Cleaning - I thought this NC-183D was pretty clean but it really wasn't. I used WD-40 to loosen the grime and then Glass Plus to remove the WD-40 residue. The tuning and band spread capacitor area was very dirty and there was an oxidized deposit on both sides of all of the metal dividers between each section. I had to use WD-40 to remove it,...it did just wipe off so not too bad, just odd. I had to use acid brushes, long handle paint brushes, Q-tips and paper towels for cleaning implements. Afterwards, I applied a small amount of DeOxit to the rotor contact springs on both variable capacitors.

Tube Shields - These are the push down type that are held in place with four vertical flat spring fingers. I checked my junk tube shield boxes and none of these type of tube shields are really in very good condition. Since these are all of the original tube shields, I decided to wire brush the originals using a steel bristle tooth brush. They don't look too bad after that treatment,...and they are the originals. NOTE: I checked the bag of loose parts with the parts set NC-183D specifically for the tube shields. They are in better condition so I may harvest those. Trouble is I could only find nine shields in the bag and there were ten originally,...and they aren't the same brand as the original set of tube shields,...bummer. It's better to keep the all-matching originals.

S-meter - I had to take the meter out of the case to clean the inside of the glass and to dust the meter scale. The improvement was very noticeable. I don't know how the "smoky film" got inside the meter but it was really obscuring the meter scale,...weird.

NC-183D Top of Chassis   SN: 430 0135
This chassis is very clean. Unfortunately, the power transformer, although it is a National type from an early NC-183D, isn't original to this late NC-183D chassis. See photo below for what the original S461 potted power transformer looked like. There may have been a second type of potted power transformer used in NC-183D late production that closely resembled the S669 Filter Choke. It was painted the same dark gray color as the choke and was also identified as S461.

Power-up and Testing - I listened on 20M for quite a while using the Collinear Array antenna. Heard a couple of EA-Spain stations working US hams. No distortion in the SSB signals. Tried CW and good clear heterodyne now. Listened to a couple of SW-BC stations and only the Chinese PRC stations seem to have good audio. They are strong here in the morning. I listened later in the late afternoon and copied XSQ on 16.989mc, Chinese Marine Beacon. Later, in the early evening, I copied SVO on 8.424mc, a Greek Marine Coastal station, Olympia Radio, located in Pirgos, Greece. Also, copied TAH on 8.431mc, a Turkish Coastal station in Istanbul. All of the coastal stations use CW Morse for their identification.

Reassembly - The chassis has to be installed into the cabinet to check RF tracking accuracy. Before I do that, I have to double-check that the dials are mechanically synchronized with the tuning and band spread condensers. Then, when the chassis is in the cabinet, the dial covers can be "zero'd" and then the accuracy of the RF tracking checked.

The chassis was installed into the cabinet. The dial covers had been installed first and then the chassis can go in. Once the chassis is installed, then the dial covers can be mechanically set for zero and then the RF tracking accuracy can be checked. The accuracy was checked on 20mc, 15mc, 10mc and 5mc WWV. None of the dial indications were "right on" with half of one division being the typical error. So it looks like I'll be doing a RF tracking alignment.

RF Tracking Alignment - This was just a very minor tweaking of the adjustments. The tracking was pretty close but the NC-183D can be aligned for a "right on" readout on the dial. That's what the minor adjustments accomplished for all bands.

NC-183D Under the Chassis
It was a surprise to find that I had already installed these Orange Drops six years earlier.

E - Dial accuracy okay. Not much on AM-BC to allow judging the audio quality. Super-accurate tracking on Band E requires moving a coil that wraps two turns around the outside of the LO coil. This coil is difficult to move very much due to the heavy gauge wire used. The placement of this coil controls the middle part of the band tracking. The band ends usually are tracking but the center of the band is off slightly.

80/D - Strong signals on 80M band. Using the logging scales for frequency reset will require some future set-up.

40/C - Strong signals for both ham and SW-BC, 5mc WWV and 10mc WWV are "right on frequency" SW-BC in the 31M band sounds great, TAH and SVO copied in the 8.4mc region. 

20-10/B - Strong signals from ham, SW-BC and utility stations. 20M loaded with signals, 15M busy during contest weekend, 10M busy during contest weekend. 15mc WWV and 20mc WWV are "right on frequency." Copied XSQ 16.998mc and XSG 16.954mc and Trenton Military 15.035mc USB.

6/A - Signal generator is no problem to receive but real signals are sparse and what is heard is unidentifiable. An actual 6M antenna could help but I don't think there's much activity around here. Propagation undependable.

The NC-183D is very sensitive and I really think anything that's "on the air" can easily be received provided propagation conditions allow for it. Selectivity is much "tighter" than the NC-183 and is especially noticeable when tuning on Band B. The Crystal Filter would only be needed if the QRM was very close to the operating frequency. Tone control works quite well at reducing highs. BFO injection is sufficient for SSB and CW signals but proper demodulation will require some reduction of the RF gain. The AVC can be left on when tuning CW or SSB.

NC-183D  SN: 430 0135

The NC-183D after its bench revisit was completed. I took this shot to show the illuminated Lucite dials and the illumination of the S-meter after its internal cleaning. The worn band indicators are a detraction that's impossible to fix. BUT, take a look at the last photo in the next section. It's the same receiver but with excellent condition replacement dial covers (provided from a "parts set") installed.

Are We Ever Really Finished With a Restoration?

Later NC-183D Potted Power Transformer S461 still in the second "parts set" SN: 396 0086

More Rework in the Future? - Here's what I'd do IF I had another (a second) "parts set" that had the specific parts I needed and they were in good usable condition.

1. Dial Band Indicators - The only fix for the worn band indicators problem is replacement with good condition original dial covers.
2. Power Transformer - The S461 potted transformer is the original type that was missing when I got the receiver. See photo to the left showing how some types of S461 transformers looked. There were other suppliers of NC-183D potted transformers that are slightly different shapes and different colors but they too are identified as S461.
3. BFO Assembly - I used the BFO off my NC-183D "parts set #1" for the NC-173 restoration and it was identical to the earlier style BFO. I need a late-style BFO for examination of the mechanical "tightness" of the air variable shaft and to see if the internal assembly is identical to the early version. If it's identical, then there's something wrong with the BFO tuning air variable in the original BFO of SN: 430 0135. I don't want to pull the original BFO assembly unless I have a replacement, if necessary.
4. Dials - The dials on my "parts set #1" are actually of a better build-quality than these later original dials that have several internal (from the factory) blemishes. The blemishes are little bits of white that apparently weren't cleaned off before the Lucite dial was mounted (rivets) to the backing plate.
5. Cabinet - I thought the "parts set #1" cabinet was in better condition than this original cabinet that has had lots of touch-ups, including trying to cover scratches from prying off knobs and "fat finger syndrome" around the band switch. A closer examination revealed that the "parts set #1" cabinet has one blemish that's rather severe, a super-deep scratch (well into the metal) by the band switch nomenclature. 

If another NC-183D "parts set" (#2) turns up with the needed parts, 430 0135 will go back on the bench. "Parts sets" seem to always be a necessity when doing full restorations. It's unfortunate that high shipping costs have usually made the total cost involved when buying a "parts set" prohibitive.

SN: 396 0086 became a "parts set" (I think) because of the filter choke (pn S669) overheating. There's black wax that has oozed out around the bottom seam indicating it got very hot but (amazingly) the choke still tests okay (for DCR only.) More than likely, several "leaky" bypass capacitors probably increased the load on the B+ causing a slow but constant heating in the choke when the receiver was operated for long hours (a shorted filter capacitor would probably have blown the fuse but might also have caused the choke winding to go open quickly without the long-term heating.) Sometime after that problem, the Crystal Filter assembly was completely removed. Both tuning knobs are gone and the Band Switch knob has a pointer knob (so-called "chicken head" type.) All of the vacuum tubes were still installed along with all of the tube shields. The cabinet looks like a typical "parts set" with severe scratching on the panel area. The receiver obviously was kept as a valuable "parts source." Much later, these types of radios are found by "estate pickers" and then show up on eBay. The "trick" is to find a seller on eBay that realizes the receiver that he's selling IS NOT restorable and IS A PARTS SET that's priced accordingly.

Considering Parts Set #2 was able to supply a good working S461 power transformer, an excellent pair of dial covers, an excellent MT dial and BS dial, a good working BFO assembly and a good VR load resistor 3900Ω 10W (original sustained a broken wire lead at the component body,...happened while installing the power transformer) the price of $125 for the receiver seems reasonable. Unfortunately, the shipping of a 70 pound package from Ohio to Nevada ended up being more than the cost of the junk receiver. But, if the parts I used had been purchased and shipped separately, the total cost would have probably been even more,...and, without a doubt, the time involved would have seemed endless.

Further power transformer testing was performed with 120vac applied to the primary winding. 680vac was on pins 4 and 6 of the rectifier tube socket (all tubes were removed) and 5.8vac on pins 1 and 8. The 6.3vac winding still had the dial lamps installed and the winding measured 6.7vac. I let the transformer remain powered up for 30 minutes and it remained cold to the touch. It should be good to use.

Extracting the BFO assembly requires unsoldering three wires and removing two 6-32 nuts and washers to dismount. Inspection should reveal if there are any mechanical problems.

Transformer Transplant - This transplant operation was somewhat lengthy in that the S641 had to be removed from the parts set. Then the transformer in 430 0135 had to removed. Then S641 was installed into 430 0135. Straight forward rework with no problems (except I broke R42 while swapping power transformers.)

BFO Transplant - I wanted to see why the BFO frequency control shaft was so loose on 430 0135. It became apparent after dismounting the BFO and looking inside. The ceramic base for the air variable capacitor was broken (cracked through the bearing hole) and that prevented the spring from providing the proper pressure on the rotor shaft thrust bearing. This is the second BFO of this type that I've come across with a broken ceramic mount. Apparently, if the BFO frequency knob is hit hard directly from the front and the knob has some panel clearance, that "hit" can push the BFO shaft back with enough force to break the ceramic mount inside the BFO can. Indication of this problem is a very loose feeling to the BFO frequency shaft and any lateral movement of the BFO shaft. I inspected the BFO that I had taken off of this parts set and it was in good condition. It had the same part number stamped on it as all of the NC-173, NC-183 and NC-183D receiver BFOs,...so they are all interchangeable. The installation of the parts set BFO was straight forward with no problems.

Original type S461 Potted Power Transformer and new BFO installed

NC-183D   SN: 430 0135
The good condition replacement dial covers are installed and they really make a difference. Compare to the last photo in the "Revisit the Workbench" section above. Those are the old dial covers,...it's easy to see the difference. The original type S461 potted power transformer is much larger than the older style power transformer. S461 remains cool during long operations. In the photo, the receiver is tuned to 15mc WWV with S-meter showing S9+30db, antenna is the Collinear Array.

Finalizing Things - Mar 24, 2024 - I double-checked the power transformer wiring. The bottom terminal plate is exactly like one on the old style power transformer so the installation was easy. With the NC-183D still out of the cabinet, I applied AC voltage using a Variac,...just in case. No problems and the receiver came up as expected. I installed the replacement dial covers into the cabinet and centered them before snugging-up the mounting screws. One has to be careful not to rub the dials or dial covers with a dry cloth as this will statically charge up the Lucite and that attracts dust rapidly. Discharge by using a damp paper towel on the back of the dial covers. The dials don't charge up as much since they are riveted to the copper backing plate. I just used a soft paint brush to remove the dust before installing the chassis into the cabinet. When installing the rear panel and lid, first install but don't tighten the chassis bottom felt-cup feet. Next, install all of the rear panel screws loosely. Make sure the lid is setting squarely in the lid-well. Then tighten the rear panel screws "one at a time" watching that the lid remains squarely in the lid-well. Once the rear panel screws are snugged-up then tighten the bottom felt-cup feet. This method will keep everything square and the lid will set flat and be easy to lift up. The knobs were then installed. I had marked the BFO shaft so I would know how to set the knob. When tightening the dress nuts on the switches and phone jack, only tighten these "finger-tight" - just enough so they don't move when operated. Install the bottom cover to complete.

Operation - The receiver is still on the work bench, so it's connected to the Collinear Array via a 50' length of RG-58U coax. The operations seems to be just as it was before the two transplants. Dial accuracy after reassembly is still accurate.

I dismounted both dials from Parts Set #2 and thoroughly cleaned them. They are very nearly perfect and straight.

NC-183D Dial Removal Procedure - To remove the dials, first loosen the dial hub set screws. Then loosen the two set screws on the dial drive gear. Using a small rod (<0.250" diameter) push back the dial shaft through the dial hub but not all the way through the drive gear hub. Then you can lift the dial straight up and out. It automatically disengages the pinch wheel and there's no bending or stressing any of the parts. When reinstalling a dial, insert the dial into position and engaged with the pinch wheel. Then push the dial shaft all the way forward through the drive gear and through the dial hub. Adjust the position of the drive gear, the dial (dial stops in relation to the lower end of scales on the dial and full mesh of the tuning condensers) and then tighten the set screws. Test for proper operation. NOTE: Be careful when pushing the dial shaft through the dial hub that the drive gear says engaged with the split-gear on the tuning condenser shaft and do not push the dial shaft all the way through the drive gear hub. If you do, you'll have to reset the anti-backlash on the split-gears on the tuning condenser drive gear. You only need to push the dial shaft back just enough to clear the MT or BS dial hub.

Mar 28, 2024 - I performed the usual ritual to get the chassis out of the cabinet. This was followed by taking the BFO assembly off of the chassis. Looking inside I could see that it was easy to apply DeOxit to the rotor contact on the air variable. The on-off switch could be seen by looking along one corner at the top of the can. Using the red spray tube, DeOxit was also applied to the on-off switch. I then reinstalled the BFO assembly.

While I had the DeOxit out, I also cleaned the band switch with DeOxit and a small paint brush and I also cleaned the double-conversion switch contacts (another task I neglected to do the first time 'round.)

The MT and BS dials were replaced following the procedure shown to the left. These new dials were from Parts Set #2. I mechanically aligned the two dials as indicated.

With the receiver chassis still out of the cabinet, I connected the speaker and a test antenna to verify that the BFO action was improved and it was. Very stable now. It drifts as the receiver warms up but just a steady drift,...nothing erratic. The cleaning of the BFO rotor contacts might have caused the improvement or it could be that this time I routed the BFO wires like the original BFO assembly had been routed and also moved a capacitor that was laying against the wires. But, this was "out of the cabinet."

Out of the three NC-183D cabinets I now have, the original one is actually in the best condition, so it will continue on as the "proper" cabinet for 430 0135.

Mar 29, 2024 - The chassis was installed into the cabinet as described before. A slight adjustment on the BS zero using the dial cover adjustment was required. There was no significant change in performance other than stability in the BFO seemed to be improved (at first.) Both dials are now excellent. Of course, the blemishes on the original dials were so small they wouldn't even show in a photo. But, when using the receiver, one's eyes are only about 18" away from the dials, so these tiny but numerous blemishes are very apparent. These dials from Parts Set #2 are extremely nice, nearly perfect.

Weirdness in the Workshop - Apr 3, 2024  - The BFO is erratic acting with the receiver in the cabinet. I remembered that I was having this exact same problem with the NC-183 a couple of months earlier and that instability problem went away when I moved the receiver to the ham shack. So, I moved the NC-183D to the ham shack. The receiver powered up with the normal, expected drift but nothing erratic. I left the NC-183D running for about an hour. I went to 20M CW and the signals were perfectly stable. This is weird because it suggests that the problem is in the upstairs workshop. This might be one of two things. The antenna coax comes from the ham shack to the workshop using a 50ft length of RG-58U. BUT, I've also tried the Pixel Loop (also located in the ham shack) connected with about 100' of RG-6U and the instability was still present with either antenna so that should eliminate that as a source. Anyway, antenna problems shouldn't change the receiver LO frequency or BFO. The only other thing I can think of would be random fluctuations in the AC line voltage or (more likely) at the AC outlet that I use in the workshop. Watching the AC line with a 'scope might show something but if there's a problem it might be due to current draw so I'd have to power up something. At any rate, the problem apparently isn't with the NC-183D but is in the upstairs workshop itself.

Apr 7, 2024 - "On the Air" Assessment - I operated the NC-183D with the Viking 1 transmitter using the Collinear Array antenna. This was on the Nevada Mil-Rad Net. Reception was excellent and the audio reproduction spectacular, even though I was just using the NC-183DTS 10" loudspeaker. Never a problem with copy even though a couple of times some signals approached the noise floor. On the audio, I thought I'd be able to tune to one sideband or the other to capture more audio highs but apparently the actual shape of the IF bandwidth and its narrow nose attenuates the signal rapidly as it's tuned off of center. Signals sound best when the carrier is tuned "on the nose." The Tone control can increase the highs if more are desired but I kept the Tone control on 3. The Noise Limiter was a surprise because normally they are ineffective on modern types of RF generated noise but the 183D NL worked on our modulated blower-speed furnace RFI noise. At the end of the net, one signal went close to the noise floor and the furnace was running creating a S-7 noise level. I was surprised that the NL allowed reducing the blower RFI noise to almost nothing and allowed good copy of the weak signal. Naturally, most of the AM signals are much stronger than the furnace blower RFI (average net AM signals run ~ S-9+20db) so they effectively over-ride the furnace QRN. Since the mode of operation was AM, drift was not even noticed. About 45 minutes into the net, I turned on the BFO to see how stable it would be and its heterodyne never varied in the time I had it turned on. I did use this very same NC-183D "on the air" one time before this and that was about six years ago after I had rebuilt it the first time and it was a good performer then allowing solid copy on weak signals. But, as mentioned before, I sold it but after three years was able to "trade it back" although it languished out in the shop after getting it back,...for a couple of years! Now, six years after that first use of this NC-183D, and after another rebuild, another alignment, then several partial parts swap outs involving two other NC-183D "parts sets" and then even more bench time,...I think I'm done with this NC-183D. These receivers are an excellent choice for the vintage AM ham station where the operator wants to enjoy just about the best audio reproduction along with great selectivity, a fine Noise Limiter, competitive sensitivity and an impressive bench presence.

June 14, 2024 - Out to the Shop - I really wasn't using the NC-183D upstairs so I've decided to move it back out to the shop where it can be reconnected with the Jensen KM bass reflex box with the Jensen 15" coaxial loudspeaker. Before, when I was running the NC-183D with the Jensen KM, the receiver had several problems that have now been corrected with the multiple trips to the workbench. I moved the KM box to be a little further back in the shop, about ten feet from the operating position. I used 30' of 18ga. "zip cord" that I routed up to the ceiling and across using cable hangers. Then the speaker cable drops down to the KM box's new location. I did the same thing with the antenna coax coming from the "Big Rig," that is, I routed the RG-58U out the top of the transmitter up to the ceiling and used two of the cable hangers that held the transmitter RG-8U and then dropped the RG-58U receiver coax down to where the receiver is located. I tested the "sound" by listening to a couple of local AM-BC stations (the only thing they're good for is testing) and with the KM's new location further away the bass becomes very noticeable, which is good. When the KM box was very close and directly in front of me, all I could hear were the highs from the tweeter (it's a coaxial speaker) but now the audio spectrum seems to be reproduced with a mellower, slightly bassy sound, which I find pleasing. I'll alternately operate the NC-183D and the RACAL RA6117A with the W6MIT 300 watt homebrew transmitter. The shop antenna is a 130' Inv-Vee fed with 44' of open feed line to a Nye-Viking MB-VA antenna tuner.

Restoring the 1948 NC-183  SN: 241 0262

This NC-183 had excellent cosmetics on the outside but unknown hidden terrors were lurking inside. Was it commercially-owned receiver or just a amateur station receiver belonging to an enthusiastic ham? It seemed that nearly constant operation had resulted in hours upon hours of wear and tear on almost everything that moved inside the receiver. Countless, senseless repairs had been accomplished using randomly-found parts installed with the most indifferent workmanship, definitely pointing to amateur-level maintenance. It soon became evident that the receiver had been in daily operation by a "five packs-a-day" chain-smoking ham that must have blown his cigarette smoke into the receiver hour after hour, day after day, year after year. The cigarette smoke combined with the humid environment from where the receiver was used left a nearly impenetrable coating on the chassis. After a close inspection, I'm sure the "as-found" condition showed that SN:241 0262 was an over-used ham receiver that was operated long hours by a heavy smoker and the receiver's amateur-level maintenance was performed in a technically incompetent manner. The interesting thing is that the exterior of the receiver was in excellent condition and that can be deceptive when relying just on photographs for pre-purchase initial overall condition inspection. Well, this receiver's restoration should provide some interesting commentary and observations.

Main Dial Drive - The drive gear shaft was frozen in the bearing. I found that dirt and grease had hardened and heat broke it loose. I still had to disassemble the entire drive because of rough, grating feel to the tuning. I pushed the shaft out of the bearing and found the set screws had galled the shaft. I filed the galled area to allow easy reassembly. I had to clean and lube the washer-spacers and the shaft. When reassembled everything worked smoothly. The dial rim was not engaged into the pinch-wheel so by loosening the pinch-wheel mounting I could get the dial rim into the pinch-wheel. Although both of these dial drive mechanisms for both dials need to be thoroughly cleaned, it is working smoothly now with some slipping due to the dirt-contamination. Later, I took a closer look at the Band Spread dial mechanism. I could see that the drive gear had quite a lot of wear that was probably due to excessive anti-backlash pressure and a lot of tuning. I had to disassemble the Band Spread drive to clean and relube. After that the tuning was still rough-feeling caused by the excessive wear to the drive gear teeth. I harvested an excellent condition drive gear from the NC-183D "parts set" (the gears are the same part number and exactly alike) and installed it into the Band Spread tuning. I adjusted the anti-backlash split-gear for just enough pre-set for proper operation and then "snugged up" everything. Now the Band Spread and the Main Tuning drives are both very light feeling and the pinch-wheels shouldn't have any problem driving the Tuning and Band Spread condensers.

Removed and Cleaned Dials - I used Glass Plus to remove the nicotine staining then polished with a soft flannel cloth. Wrapped with paper towels for protection until I'm ready to reinstall. Finished dials are shown to the right. The remaining discoloration is inside the plastic and can't be removed.

S-Meter - Dismounted the S-meter for later cleaning. Stored in the parts box.

NC-183 Dials after damp-cleaning. This is about as good as these acetate-phenolic dials can be cleaned. The discoloration is deep in the plastic and can't be removed. Though the red looks totally faded, when back-lit, the red becomes very visible. The cleaning also helped to increase the contrast of the black nomenclature against the amber background.

Photo above shows the chassis before any cleaning. Grease and dirt combined with humidity and tobacco smoke,...yuk!

Noise Limiter - Frozen Shaft - Heat was used in combination with WD-40 to work the control shaft loose. It took several applications but finally it was working normally. I noticed that someone had turned the NL control while the shaft was frozen and actually rotated the potentiometer. I had to dismount the RF Gain pot and the NL pot to get everything oriented correctly. Ultimately this pot tested as defective and had to be replaced with an original NL pot that was harvested from the NC-183D parts set. Luckily, the NL circuit hadn't changed in the evolution of the receiver model so the pots were identical.

Replaced the Filter Choke - I removed the open choke and installed the spare unit that had come out of a HRO-60, it was the same part number. Since the original was open, it's possible that its open condition actually "saved" the power transformer from subsequent damage from those "let's plug it in and see if it works" types that expect miracles from three-quarters of a century old components.

Cleaning - The entire chassis was filthy. Most of the greasy dirt came off fairly easily using WD-40 followed by Glass Plus. I found that just Glass Plus worked somewhat better on the smoke contamination. The chassis is high-quality cadmium plated so it cleans up nicely. I used a brass bristle brush with lots of Glass Plus. Since the brass is softer than the nickel it only removes dirt. The aluminum pieces can only be cleaned using an acid brush or similar soft-bristle brush otherwise the aluminum will be scratched. The stamped part numbers on the individual components used a very durable ink that is resistant to Glass Plus which is nice. Some severe tobacco staining required denatured alcohol, probably to break-up the tar component of the cigarette smoke deposits. That was then again followed by Glass Plus cleaning.

Photo above shows the cleaned chassis but without dials mounted or tubes installed. That's the HRO-60 filter choke installed, it's the exact same part number as the original choke.

Audio Gain Pot - This was a "splined split-shaft" replacement that didn't allow mounting the knob correctly. Easy to replace,...or so I thought. Examination of the NC-183D "parts set" revealed the change in that receiver when compared to this earlier NC-183. The AF Gain pot incorporated the AC power switch on the NC-183D, so I had to go to the "used pot box" for a proper type of potentiometer. I found a couple of 500K Audio taper pots that looked very much like the original style (these were Clarostat types.) I selected the one that tested the best and installed it. I'm not sure what the original pot's taper was since it's not mentioned in the parts list. The Audio taper will allow a slow increase in audio gain to about 60% advanced, then a rapid increase in gain for the remaining 40%. It makes the actual "listening-level of volume" easier to adjust. Photo right shows the new Audio gain pot installed.

The .05uf 600vdc capacitor is split-open. Note the ceramic disk is connected from suppressor-grid to ground - not an original component.

Replacing Capacitors - There are thirty paper dielectric capacitors that will need to be replaced. There aren't any weird values, just the typical .1uf, .01uf, .05uf and one .25uf. The .05uf capacitors, of which there are four, are the rectangular brown molded plastic body types. The photo to the left shows one of the four .05uf 600vdc bakelite capacitors that split apart (3 or the 4 are split-open.) The other capacitors are the typical Aerovox brand tubular wax-covered types. The mica capacitors are usually only replaced if found defective. There are five electrolytic capacitors. The filter capacitor is a dual multi-section both 10uf at 475vdc. There are two cathode bypass electrolytic capacitor and one addition power supply (bias line) electrolytic. There's a .1uf capacitor across the filter choke to somewhat tune the filtering (parallel LC at resonance is a high Z, probably tuned for 120hz.) I'm not going to bother with "restuffing" the original capacitor waxed-cardboard shells.  I'm going to use orange drops for all of the replacement capacitors. I've never had any trouble with the polyfilm "yellow jackets" but their bright yellow color and small size make any recap job look like the cheapest parts available were used. Orange drops aren't too much better in the "looks department" but the Type 715P uses polypropylene as the dielectric material and they are "more accepted" by rebuilding technicians for a recap job because of their reputation of quality. Photo right shows under the chassis before any capacitor replacement. The three "as found" replacements are easily spotted in the photo (the blue cap next to the replacement Orange Drop is the wrong value. The Orange Drop is also the wrong value,...hmmmm,...hamster junk box "close-enough" parts.)

Under the chassis before any rework

Touch-up Paint - I bought some Testor's model enamel paint (small glass jars) to mix up an exact "touch-up" paint for the very light gray with a touch of silver. I had to do the same thing for RACAL's "Admiralty Gray" which is actually a very light green color. The approximate mix for the NC-183 & NC-173 light gray with a touch of silver is Machine Gray (5 parts) + Silver (4 parts) + White (3 parts) + Yellow (1 part) + Brown (a dab.) Use a mix of gloss and flat paints to end up with semi-gloss (3 gloss to 2 flat is about right.) The mix-hue when wet has to be just slightly lighter than the cabinet paint because most paint colors dry slightly darker. The mix will get you close but each cabinet has aged differently so you will have to vary the mix slightly to match. Also, match using sunny daylight from a window or two for ample natural light that won't skew the colors. Don't have any lamps on,...lamps tend to skew the color match. I didn't apply the touch-up paint until after the receiver was completely reassembled. The last photo in this restoration write-up section shows the NC-183 after the rebuild was completed and the touch-up paint applied.

I also reinstalled the plastic dial cover. The dial index fiducials were cleaned and needed just a little straightening before installation. The dial guides (these keep the dials from rubbing against the dial index fiducials) were cleaned, adjusted and reinstalled but these have to be removed before the chassis is reinstalled into the cabinet. The pinch-wheel drives, the dial index fiducials and the dial guides all have to be adjusted after the chassis is installed into the cabinet.

Jan 22, 2024 - More Cabinet Cleaning - In some types of light, I could see there was still that yellowish smoker's residue on the cabinet. I wouldn't recommend the following treatment unless it's tried on someplace out of sight first. Denatured alcohol seemed to be the best for removing the most difficult of the smoker's residue. I tried the inside rear panel first and the smoker's residue came off and, most importantly, the paint was not affected at all. I continued on, cleaning all sides of the cabinet using denatured alcohol and clean paper towels so I could see what was coming off and it was always the yellowish brown color of tobacco smoke. I never saw any indications that the paint was being affected but I'd say that 99% of the smoker's residue is now removed from the cabinet. Jan 23, 2024 - Degreasing - I used DeOxit to thoroughly clean the band switch. To make sure that all of the grease was removed from the contacts, I used a small paint brush to apply the DeOxit just where it was needed. There were places that had grease applied that needed more detailed cleaning, like two or three brush applications, to remove the white-colored grease that had been used (unbelievable!) The Tuning and Band Spread condensers were also "greased" at all of the rotor contacts and required additional cleaning. I had to use DeOxit and a brush to clean the rotor contacts that had been greased with that old white grease. I sparingly relubed the Tuning and BS condenser ball bearings with red wheel bearing grease worked into the ball bearings ONLY. I wondered about this white grease smeared everywhere and remembered that back in the days of TV repair shops, older TVs had a revolving turret, channel-changing Tuner. There was a product then called "Tuner Grease" that was supposed to be applied to the TV-tuner contacts. It was supposed to decrease the wear, ease the "channel-changing effort" and provide better contact. Anyway, that might have been where the idea for smearing the grease everywhere in the receiver came from.

The bi-metallic temperature compensation device for the Local Oscillator is behind the LO tuning condenser stator. This device was only used on the NC-173 and the NC-183 receivers. The NC-183D used a converter 6BE6 tube and a different method to control f vs temp drift

More caps replaced - I found that the plate load bypass on the Mixer had a replacement plastic molded cap that was 0.1uf but the value shown in the manual indicated that it should be a .05uf. Additionally, one end of the plastic 0.1uf hadn't even been soldered. This plastic cap was replaced with a .05uf 600vdc Orange Drop. The two IF amplifier screen load resistors were obvious replacements being 2W resistors. But, even these 2W resistors had swollen from being overheated by the original leaky bypass capacitors that were still in the circuit (if the original 1/2W resistors burn up,...replace them with 2W resistors that can tolerate the capacitor leakage current,... but don't change that cap! No excuse for this incompetence but I've found it many times in various rebuilds.) I replaced the resistors with the correct dissipation CCs and the capacitors with new Orange Drops. There were three .0068uf ceramic disk capacitors on the suppressor grid to ground on the two RF amps and the Mixer that were removed since the suppressor grid is already grounded and these disks aren't shown on the schematic.

Jan 24, 2024 - Completed installation of all 30 of the Orange Drop capacitors. Every joint was unsoldered, cut leads removed, the terminal solder wicked out and then the capacitor lead installed and soldered. There were no "hook splices" used. I found that one original bypass capacitor on the B+ line had just one end cut. I didn't check but it's probably shorted and that was a quick way to get the receiver semi-working again (the ham's aversion to actually replacing THE defective component.) I found that many of the resistors were replacements that were installed with the typical "wrap and solder" technique or the even lazier approach of tack-soldering. This type of poor workmanship is often found on receivers that were maintained by the ham-owner.

I tested the electrolytic capacitors by lifting one end and then using a capacitance meter. One of the 25uf capacitors tested at 15uf. The 10uf tested at 17uf and the other 25uf tested at 40uf. The filter electrolytic can't be checked unless it is disconnected from the choke. The choke ties the two positive terminals together and the dual unit has a common negative. In essence, the two sections are in parallel. Tested in this manner, I read 15uf which seems to indicate that the two sections are low in capacitance. However, since I've already replaced all of the paper dielectric caps, I'll go ahead and replace the five electrolytic capacitors too (since the receiver chassis doesn't look original anymore, anyway.)

Under the chassis after the recapping job. For some reason, the Orange Drops look pretty good,...well,...better than Yellow Jackets

I installed new 10uf 450vdc electrolytic caps for the filters. I didn't remove the original capacitor but I did disconnect the two positive wires on each terminal. These "pairs" were then soldered together with a lacquered sleeve installed over the joint. I then connected the two 10uf electrolytics into the circuit and placed them in a way they were somewhat hidden and not obvious. Interestingly, the NC-183D changed the filter capacitor values to 40uf for each electrolytic capacitor in the multi-section unit. The three bias filter capacitors were installed in the same manner as the originals. I had to use a slightly higher value on the negative bias voltage line, the original was 25uf and I installed a 47uf 50vdc instead. The two cathode bypass electrolytics were the original values of 10uf for the 1st AF amp cathode and 25uf for the P-P 6V6 cathodes.

Unbelievably when I was looking in the parts bins for fuses and plugs, I saw a little three pin plug laying in one bin. I turned it over and it had two small diameter pins and one large diameter pin. I tested it in the NC-183 and it was a perfect fit for the audio output connector socket (that was a lucky find,...I didn't know where I was going to find that type of plug.)

The next issue is the height of each dial above its associated pinch-wheel. This is controlled by the height of the front frame of the tuning condenser assembly above the chassis top surface. The studs and nuts that tighten the front of the condenser assembly in place were loose when I first inspected the receiver. In tightening them, of course, I didn't have the dials installed so I didn't know if the height was correct or not. Later, with the pinch-wheel assembly mounted, then the engagement of the dial rim into the pinch-wheel is governed by the height of the front of the tuning condenser which is adjusted with the two top nuts on the two front mounting studs which sets the height and the nut under the chassis that locks the mounting in place. Unfortunately, when assembled, the pinch-wheel flywheels block access to the bottom nuts so the flywheels have to be removed before the bottom nut can be tightened. But, the compression of the pinch-wheel depends on the flywheel being mounted, so you can't test the drive unless its fully assembled. This makes the entire adjustment a little more difficult than it should be. I had to do one dial and pinch-wheel assembly at a time and set the tuning condenser height one side at a time. It's a very slight adjustment difference between the dials slipping and the pinch-wheel gripping effectively. Trial and error will eventually find the correct setting. It took me two times on the Main Tuning and four times on the Band Spread, so finding the correct settings will require a little bit of patience.

NOTE: It's also possible to move the mounting of the pinch-wheel bearing a little bit. This method can be used if the condenser height is about right and just a slight adjustment is all that's needed. Sometimes the tightening of the mounting nut will change the angle of the pinch-wheel assembly slightly so you do have to watch exactly what happens as you tighten up the nut.

The tuning condenser assembly front mounting stud and "height setting" nut on the Band Spread side of the TC assembly.

Jan 28, 2024 - Cabinet Installation and Knob Cleaning - Installing the chassis into the cabinet isn't too difficult. The mountings are the four bottom felt-cupped feet and the chassis front is pulled flush with the inside cabinet-panel wall using the dress nuts for the Phone jack, the Send-Rec switch, the AVC-MVC switch and the Radio-Phono switch. Once these are snugged-up then the back panel can be installed. This has two screws that secure the rear of the chassis to the back panel and several screws around the perimeter to secure the back panel to the cabinet. These rear panel screws are all Philip's head sheet metal screws. It's important that ALL screws and lock washers (if originally used) are installed. The cabinet rigidity depends on all of the screws being present. With the chassis mounted in the cabinet, then the dial guides are installed. These mount with two hex head 6-32 screws and external-tooth locking washers each. The guides will have two fingers in front of the dial and one finger behind the dial. The guide prevents the dial from rubbing on the index fiducial.

Dial Guide on Band Spread dial. The two center hex head screws adjust the position of the index fiducial

Jan 29, 2024 - Installing the lid isn't too complicated. There are right angle stops on the hinges that only allow the lid to go up to the vertical position. The lid has to be almost closed to insert the 6/32 screws through the rear panel. Then the lid is put up vertically and the stops will hold the screws in place. Then the nuts and lock washers can be installed from the inside of the cabinet.

This completes the rebuild of the NC-183 as far as the mechanics and the electronics (except for installing the bottom cover.) Now, I'll use the NC-183 daily for an hour or so for a few days to do a sort of "shake-down" and if no problems crop-up we'll then proceed on to the IF and RF tracking alignment. I listened today with the Collinear Array as the antenna. Even 20mc WWV was strong. Trenton Military on 15.035mc USB from Ontario, Canada was easily heard. I switched over to a Collins 270G-3 10" loudspeaker that really improved the bass response. I listened to a couple of SW-BC stations and the strong ones sound quite impressive.

Jan 30, 2024 - I adjusted the position of the dial index fiducials to exactly match the dial stops on the gear drive and the position of the tuning and band spread condensers set at full mesh. This is the preliminary mechanical set-up that proceeds any actual electronic alignment. I then checked a few frequency marker stations and I was really surprised at how close the electronic tracking alignment already was. WWV 15mc was very close and WWV 10mc was 100kc off. On the AM-BC band all of the daytime locals were very close to their assigned frequencies. The Band Spread was set to the "SET" mark at 180 of 200 on the logging scale. The receiver seems to be performing much better than I would have expected and that indicates that the alignment is probably pretty close. Even the Crystal Filter seemed to be working pretty well. When I do the alignment in a day or so, I'll find out just how close it really is. To eliminate the need of flipping through the pages of the manual for the adjustment locations, I made copies of the top chassis layout and the bottom chassis layout. These two pages aren't with the alignment procedure in the manual so having loose copies simplifies locating the correct adjustments while performing the alignment.

NC-183 Installed back in the Cabinet

NC-183  SN: 241 0262
The finished receiver has turned out to be an excellent performer. I've been using it "on the air" and it has proven to be a dependable vintage ham station asset. Production run #241 was probably the third of NC-183 receivers and that would put the build date of this receiver around early-1949.

Feb 1, 2024 - Alignments - IF - The NC-183 IF seemed to be tuned to 451kc. I "rang" the Crystal Filter crystal and it was resonant at 457.8kc, so the IF wasn't really very close to where it should be. Using 457.8kc as the alignment frequency, I connected the RF signal generator to the Mixer stator through a .1uf capacitor. The IF and Crystal Filter were aligned as specified in the National NC-183 manual. Then the Amplified AVC and the BFO were adjusted for 457.8kc. Before the IF alignment I couldn't get the S-meter to respond much more than about S-4 on 15mc WWV, now it showed S-9+, so the IF was certainly improved.  NOTE: One thing I noticed had to do with my having both the National manual and the Sam's Photofact for the NC-183. The National IF alignment has several steps specifically for the Crystal Filter alignment that pays off with its proper operation. The Sam's doesn't have a Crystal Filter alignment procedure and just instructs the technician to "peak" the adjustments in the Crystal Filter at the IF frequency. In fact, the Sam's doesn't even have the procedure to "ring" the crystal to determine the correct IF. If the Crystal Filter and IF are aligned as instructed in the Sam's Photofact the receiver will work okay but the Crystal Filter won't operate correctly. Use the National NC-183 manual procedure.

Alignments - RF Tracking - The RF Tracking wasn't too far off. Just little adjustments to get the dial indications to be very accurate. Since I did the mechanical set up first and apparently nobody in the past had gotten into the inductance loops for adjusting the low end, just adjusting the trimmers accurately got the entire tracking to be excellent.

Finishing Up - The bottom cover was really greasy on the side facing into the chassis. I had to use WD-40 and a brass bristle brush to remove the grease. This was followed by Glass Plus to clean the WD-40 residue. The bottom cover is mounted with three sheet metal screws at the rear, one sheet metal screw in the center and a 6-32 binder head machine screw and flat washer in front. The photo to the left shows the NC-183 after the rebuild. Note that the dials, when illuminated, look bright and the nomenclature is crisp. The red isn't as faded-looking when the dials are illuminated. The photo was taken after I did the "touch-up" paint job. The "touch-up" paint was mixed as described in Jan 18th log "Touch-up Paint" above.

NC-183 Performance After the Rebuild

Ham Operation - I've paired the NC-183 with the Johnson-Viking 1 transmitter. They are both approximately from the same time period, 1949 to 1950. The antenna is the Collinear Array. I had to set-up a DowKey relay to work with the Viking 1 relay drive that is parallel with the Plate switch and then to use the DowKey auxiliary contacts to operate the remote standby on the NC-183. The Viking 1 transmitter doesn't have push-to-talk so turning ON the PLATE switch places the transmitter into operation and, via the DowKey, places the NC-183 in standby. The DowKey T-R contacts switch the antenna to the Viking 1 when the PLATE is turned on. This particular DowKey has the spring-loaded disconnect in the receiver-side coaxial fitting barrel as added protection.  >>>

The Viking 1 with VFO and the NC-183 station

1. L21 IF Transformer Secondary (Output side) & NL Control/Switch - The peak adjustment on this was very shallow and a barely noticeable change in output level regardless of changing the position of the slug. Since I had the AVC on and was using the S-meter for peaking, the AVC action might have been controlling the final IF output since it goes to the detector and AVC line. This output also goes through the NL pot (that was replaced but was still causing problems by attenuating the signal unless the ON-OFF switch was jiggled around.) Since I did see a small peak, I didn't investigate at the time. On the NL control,...by exercising the NL control, the NL function started to work but the NL switch function is still problematic. Both the NL pot and switch should have been cleaned with DeOxit and I didn't do that. Spraying DeOxit into the NL pot access hole corrected the problems in both the pot and in the switch. NL and switch operation are normal now. To test the L21, I connected a VTVM to the diode load and input a 457.8kc signal to the Mixer grid. I adjusted L21 and it behaved exactly as it should, although it was slightly off peak. I peaked the IF to 457.8kc (the Crystal Filter's crystal f.)  

2. Crystal Filter switch noisy - I forgot to pull the side cover and clean the Selectivity switch with DeOxit when the receiver was all apart. The switch has pretty much "self-cleaned" since being operated often, but I really should have used DeOxit on the switch and inspected the inside of the Crystal Filter assembly. The inside cover is easily removable while the receiver is in the cabinet. There are six hex head screws to remove and then the cover can be taken off. There aren't any studs on the cover. There's quite a bit of clearance but the 6J5 LO tube and the 6SA7 Mixer tube can be removed for better access. This was a very easy task. It took about 15 minutes total to remove the cover, brush out loose dust, spray DeOxit on the switch and the air variable, clean the chassis area and reinstall the cover. Works noiselessly now. Also, the .250" diameter hole near the front right corner of the Crystal Filter top cover is an access hole to allow spraying contact cleaner on the switch. I didn't know that until I removed the side cover and saw that there was no other reason for that hole.

3. Band E low sensitivity - I checked this earlier and couldn't really find anything. I'll have another look. The only component in the AM-BC that is unique to that band is the 200pf antenna coupling capacitor and it checks okay. There's no visible damage to the Band E antenna coil either. Well,...that's what I thought but this did turn out to be a burned primary winding on the Band E Antenna coil. The burned area was not readily visible and a dental mirror was necessary to actually see the burn. How it burned with the C isolation is a mystery. The best "fix" would be to salvage the Band E Antenna coil L5 from the parts set NC-183D but to remove the Band E Antenna L5 coil would require removal of the back of the RF coil box, removing the band switch shaft and unsoldering a few wire connections including the TC buss wire to the tuning condenser. The mounting screws for the coils are located on the bottom side of the assembly plate that is also the rear side of the RF coil box. I'd have to do that entire removal on both receivers to salvage the coil and then install it into the NC-183. Since it's just the AM-BC band that really isn't listened to (and would only be used for demo purposes) here's the easiest fix,... I wound a small coil on a rubber grommet that I used as the coil form. The grommet slips down inside the Antenna coil. I only had to disconnect the coupling capacitor which was then connected to one end of the new coil and the other coil end was connected to the A2 coil terminal. With the receiver turned on, I positioned the new coil for the best signal response looking at the S-meter. This fix isn't noticeable (unless you know it's there and looked down the coil barrel) and the end result is that Band E has just about normal sensitivity now. With burned primary, KOH 780kc was S2 with the test antenna. With new primary coil, KOH 780kc is S-6 with the test antenna and runs S-8 on a larger outdoor antenna. If I was really OCD, I'd remove just the burned primary coil on the original coil form with it still mounted. Then I'd just wind a new primary coil on the original form. Since there are only maybe 30 double-layered turns of ~ 32ga. enameled magnet wire for the primary coil, it really wouldn't be too difficult. I may replace the grommet-coil later if I feel obsessed with an uncontrollable compulsion to do so. UPDATE: Mar 23, 2024 - On a "183D parts set" I discovered that it is possible to unscrew the individual coils in the Antenna section. The leads can be cut and then the coil unscrews to remove. There's a tapped 6-32 hole in the brass coil retainer and a screw under the coil mounting plate. The screw is long enough that it doesn't "drop through" the hole in the mounting plate, so a new coil can then be screwed in place and connected up. I've already removed a good Band E Ant Coil L5 from a late-build ''parts set" NC-183D. However, I soon discovered that there was a redesign on L5 with the NC-183D that did away with the capacitive coupling from the Antenna Input to the primary. This was replaced by a direct connection and a huge primary winding that must be five or six layers. It might have been due to the change in RF amplifier tubes from the 6SG7 used in the NC-183 to the 6BA6 used in the NC-183D. So, the NC-183D L5 would work since the larger primary winding isn't that critical of a change,...more to come when I remove the original L5 for close examination. UPDATE: Aug 6, 2024 - NC-183 is on the bench and ready to install the L5 from the "parts set" NC-183D. The "D" L5 terminals were cleaned of cut leads and solder. The wires were removed from the old L5 and it could now be un-screwed from its mounting screw. The terminals are slightly different on the "D" L5 but the receiver wires would still easily reach. Nothing holds the mounting screw in position so I made a small wooden wedge that I installed under the coil chassis to hold the L5 mounting screw tightly in position. Then the "D" L5 was screwed in place. Since this isn't the way the coil was originally installed, naturally, when tightly mounted, the terminals didn't line-up correctly. I had to leave the "D" L5 slightly loose to have the terminals line-up where they should be. Next, the wires were soldered to the "D" L5. The old L5 had the primary capacitively-coupled to the antenna input. But, the NC-183D is directly connected and the "D" L5 has a much larger primary winding, so I removed the capacitor and connected the primary directly to the antenna input as if it was installed in a NC-183D receiver. I used a solid wire to replace the capacitor to provide some "stiffness" to the L5 mounting. Tested the "D" L5 by tuning in KOH 780kc. This station was running S2 with the original burned primary and about S-8 with the make-shift coil inside the L5 coil form. Now, KOH 780kc just about "pegs" the S-Meter using a 50ft test antenna. S-9+40db. Antenna Trimmer works normally. I still have to try AM-BC on an outdoor wire antenna.

4. Instability on Band B - I thought this was the BFO but actually it's the LO that's wobbling around BUT at first it only seemed to happen on Band B. Band C seemed "rock steady" but, as I did more listening, the instability was apparent on all bands. Probably something in the LO section or with some of the connections. First was to install another 6J5 LO tube but I got the same instability. I resoldered some connections that involved the +150vdc regulated voltage. I also replaced the 0D3 regulator tube. The best results were when I sprayed the band switch with DeOxit (instead of brushing on the DeOxit) and then rotated the band switch several times in both directions. The other part of the LC for the LO is the tuning condenser. The NC-183 has a mechanical bi-metallic compensating capacitor on the LO circuit that has to be set-up correctly and be tightened to prevent its movement after calibration. This compensating C wasn't tighten and, in fact, was very loose and "flopping around." Also, typical staff-tech work, the lock washer had been left out at some time in the past. The entire comp-C was oily and dirty. I cleaned it and recalibrated it (.020" clearance between the brass end and the stator contact.) Reassembly included the lock washer. While I had the cover off, I went ahead and again cleaned the Main Tuning and Band Spread condenser rotor contacts with DeOxit. I noticed that the ceramic tube sockets (front end tubes) pin contacts were spread quite a bit so I used a couple of tools to push the pin contacts back tight. I resoldered a few more joints that looked bad. On the LO tube socket, the 47K resistor was a replacement that was just "wrapped and soldered" so I removed, checked its value again, cleaned the tube terminals and reinstalled the resistor correctly. So far, the instability has not changed and is still present. It's going to take isolation and substitution to determine what's causing this instability. First, the BFO needs to be eliminated so,...TEST/ISOLATE BFO - I unplugged the BFO tube and connected the RF signal generator through a 200pf capacitor to act as the BFO. Set sig gen to 457.8kc and tuned the receiver to 10mc WWV. The instability was still there,...so it isn't the BFO. TEST/ISOLATE LO - Using the same RF signal generator I substituted it for the LO by pulling the LO tube and then coupling the RF generator into the Mixer. The RF generator is set 455kc above the tuned frequency so for 10mc WWV, the RF sig gen was set to 10.455mc. The BFO tube was reinstalled and the BFO switched on. 10mc WWV came in as expected and the heterodyne of the BFO was apparent. I listened for quite some time. I never heard the instability at all. There was the slow linear drift that would be expected but no erratic changes in the BFO heterodyne which indicates that the instability is almost certainly in the LO circuit. I did a lot of other things that didn't fix the problem but finally I moved the receiver out of the workshop and into the ham shack. The stability problem seemed to be gone. As the "station receiver" being "powered up" and operated for about an hour and a half it was noted that,...when the receiver has only been on for a few minutes, the LO/BFO still drifted quite a lot. But, after about 15 minutes of operation, the LO/BFO remain perfectly steady for the rest of the net.  UPDATE: Apr 3, 2024 - Solution? I was having exactly this same problem of instability with a NC-183D I was working on. I tried a few different things that didn't work and then I kept thinking it was too much of a coincidence that the NC-183D was behaving exactly like the NC-183 had behaved and the NC-183 problem disappeared when the receiver was moved to the ham shack. So, I moved the NC-183D to the ham shack and the instability problem disappeared,...same as with the NC-183. Hmmmm,...this means the problem is in the workshop and is probably an unstable AC line maybe due to a defective AC power strip, a defective AC outlet or maybe the AC line breaker. More checking of the workshop required. Not using the power strip. Plug directly to AC outlet and problem goes away. Defective power strip.

Restoring NC-173   SN: 173 2821

If this NC-173's serial number follows National's usual encoding, production run 173 would have started before the last of the black-box HRO-5A1 and the HRO-6 receivers were built (run 184.) It would be before the last of the NC-2-40D receivers (run 183) and would be before any of the HRO-7 receivers (run 189 was for the first HRO-7 receivers.) National didn't introduce the HRO-7 until August 1947 and the NC-2-40CS receivers were still being built as RCR airport receivers in 1948. Looking at advertising from the time period, it's apparent that the HRO-5A1 and the NC-2-40D were both being produced and sold concurrent with the NC-173. So, maybe it's possible that NC-173 production started with run 173, as contrived as that run number sounds. It's also very possible that ALL NC-173 receivers use the "173" production run number and the four digit portion of the serial number is the sequential indication of when the receiver was built. Serial numbers as high as 173 4204 have been reported. At any rate, this is a very nice example of the NC-173, at least on the exterior. But, certainly some surprises will be found on the inside as this restoration proceeds.

About 18 years ago, I almost owned another NC-173 that was in mint condition and had belonged to W7NOM (Ray Westcott from Ontario, Oregon.) A collector friend of mine had acquired some of W7NOM's ham gear at an auction. He was going to donate all of the gear to my radio museum in Virginia City. Included was W7NOM's NC-173 and his beautifully-built homebrew transmitter. Out of consideration for the donation, I convinced my collector friend to keep the NC-173 and I'd take the rest of the gear. I still have the W7NOM homebrew transmitter,...this transmitter also has a beautifully-built, cable-connected external VFO.

I did see another beautiful condition (close to perfect) NC-173 at a ham swap meet in Fallon, Nevada a few years ago. The excellent cosmetic condition was impressive and, additionally, it was in completely original condition under the chassis (I actually removed the bottom cover to look!) It was bargain-priced at $100 but it was purchased by a fellow Nevada ham friend of mine.

With only one RF amp and a single 6V6 output tube, sometimes the NC-173 is ignored by ham gear collectors but near-perfect cosmetics can compensate for potential performance deficiencies, especially since finding any of these receivers in great cosmetic condition is such a rare occurrence.

NC-173  SN: 173 2821  ca: 1948    "as purchased condition"

NC-173 Top of Chassis before cleaning

Under Chassis Inspection - Despite looking like the power transformer "burned up" and leaked out some of the black potting wax, it tested okay. I ran the transformer with the rectifier tube pulled but all other tubes and lamps installed and measured the voltages at about 5vac, 6.3vac and 320-0-320vac and that seems "as expected." I let the transformer run for about ten minutes. The transformer remained cold, so it's probably okay to use (although the HV-AC had no load during this test.) I'll have to see how the black wax cleans up. It's a mess under the chassis. All of the original paper dielectric capacitors and original electrolytic capacitors are still installed and the resistors don't look like there are any replacements although there are a few additions as described next.

1. Panadaptor - The RCA phono jack on the rear of the chassis is for connecting the receiver's IF to a panadaptor input. Coaxial cable has been run from the RCA phono jack to the 1st IF amplifier tube. A tie strip was added and an RC installed to couple the IF to the panadaptor via the RCA phono jack. Non-invasive because of where the RCA phono jack was installed is where the hole is to allow extraction of the band switch shaft (if necessary) so that hole was original. Coaxial cable easily visible in photo. This was an easy to remove mod. The tie point had been mounted using an existing stud and nut, so easy removal. No problems removing the RCA jack.

2. 5AR4 installed for 5Y3 - Pin-outs are compatible. Results in somewhat higher B+ since the 5AR4/GZ34 has a very low voltage drop where as the 5Y3 rectifier is a somewhat higher voltage drop. Non-invasive. Increasing B+ levels changes how some voltage dividers operate and can cause things to run hotter than design level. Add to that the increased AC line voltage we have today and this ends up not being a good idea,...I'll install a 5Y3GT tube.

3. B+ and B- Pin Jacks - The black pin jack is connected to directly to B+ and the red pin jack is connected to B- located on pin 4 on the Battery Socket. Two tightly twisted wires are used for each connection (can be seen in photo.) Unknown reason. Except for the two pin jack mounting holes, non-invasive. The twisted wires were wrapped around and through the terminals involved which required cutting the wire at the wrap in order to remove without damaging the terminal. I had matte-silver metal hole plugs for the pin jack holes.

4. Tone Control Repurposed - The 500K Tone Control is disconnected from the RC Tone circuit and has two wires (as a 500K rheostat) that are routed though a shield across the chassis to connect one wire to an unused pin on the detector tube socket used for +150vdc BFO plate voltage. The other wire connects to a 220K RF amplifier tube screen load resistor. This has the RF screen load R variable from 720K down to 220K apparently to vary the screen voltage level on the RF amplifier tube. Supplemental RF gain control? Original wire and C for Tone control are still present so this is basically non-invasive. Cut the wires and removed the shield which had wraps of soldered TC that used the RF box screws for mounting. The original Tone wire ends were reconditioned. The Tone control mounting had to be rotated CCW about 30º to have the original wires routed correctly. Soldered original wires to complete the Tone control. I removed the wires from the Detector side, then the 220K was soldered back to pin 6 of the Detector tube and the BFO coupling C was left unconnected. It's the wrong value at 50pf which is five times the original 10pf. Increasing the BFO coupling cap value was a common mod to increase BFO injection for easier SSB demodulation. Probably why the RF amp screen voltage was changed to adjustable to allow setting the ratio of RF signal level to BFO injection for better SSB demodulation.

NC-173 Under the chassis with Mods
Before any restoration, shows the mods described still installed. The coaxial cable for the panadaptor mod is easy to spot,...so is the shielded wire cable for the Tone control repurposing. Note the twisted wire going to the filter choke,...this is for the B+, B- pin jacks mod. Note that the power transformer and components in the lower left corner have a lot of black wax residue from overheating. The two molded bakelite capacitors are .005uf 600vdc. All of the .01uf 600vdc capacitors are Gudeman's Company brand (never heard of 'em,...WWII surplus probably.) The remaining capacitors are all Sprague brand (.1uf 400vdc and .05uf 600vdc.)

Removing the Chassis from Cabinet - Feb 18, 2024 - I've noticed that this procedure isn't in any of manuals for the NC-173, 183 or 183D receivers. The dials on the 173/183 can be damaged if the chassis is removed from the cabinet carelessly. Start by removing the bottom cover. Next, remove the back of the cabinet. This will also remove the lid which is attached to the back panel. Now, looking from the back of the receiver the two dial guides are easily visible. Remove the two dial guides (these aren't used on the 183D.) Next, turn the receiver on its side and remove the four felt-cup feet. Return the receiver to the bottom-down position and the rear of the chassis facing you. Notice how the chassis side flanges have to slide through the cabinet side and bottom flanges. Carefully position the chassis so the side flanges will clear the cabinet flanges and pass through the openings. This will allow the chassis to slide out of the cabinet easily.

Allen Set Screws - When pulling the Main Dial and the Band Spread Dial, I found that the set screws had been replaced in the Band Spread dial with Allen-type set screws. All of the gears also had Allen-type set screws. Only the Main Tuning dial had the original type of set screws. I suppose these Allen set screws were installed by a former owner for some reason, but why didn't he do the Main Tuning dial?   >>>

Audio Disconnected from Accessory Socket - When dismounting the pinch wheel drive for the Band Spread dial, I noticed a disconnected wire. I traced it to the Accessory socket pin 6 which is the audio input for either the NBFM adapter or the Select-O-Ject. Unknown why it was disconnected but I reconnected it to the PHONO jack as original.

Cleaning the Black Wax Deposits - This required denatured alcohol and acid brushes. Really thick wax had to be scraped off first using various tools. The terminals had to be cleaned with a very small steel brush and denatured alcohol. The wires were flushed and wiped multiple times to remove the wax. Alcohol-soaked Q-tips were also used. A very messy job. This wasn't that much black wax, probably about a teaspoon total. But, when hot it spreads everywhere. This transformer appears to have gotten hot but not to the point were it vaporized much of the wax, it looks like most of it just flowed out of the seams a bit. NOTE: Lacquer thinner would have probably worked faster but I was doing this cleaning inside the house. Denatured alcohol doesn't smell as much as lacquer thinner and denatured alcohol doesn't have the MEKs that are in lacquer thinner. NOTE 2: I'm going to retest this power transformer again to confirm that the HV winding output is correct and then let it run for at least an hour to see if it gets hot at all. 115vac input on the primary resulted in 320vac on the secondary to chassis (voltage as shown in the manual,) transformer cold to touch after an hour - no loading, so no internal problems (2-21-2024.)

Dial Cleaning - I cleaned the NC-173 dials using the same method as described in the NC-183 write-up. They turned out magnificent with very little darkening of the plastic. The red is still vibrant and the contrast after cleaning makes the dials look almost new. For comparison, look at how darkened the dials shown in the NC-183 write-up are.

The first step is to make a detailed drawing that shows the location, the connections and the orientation of each capacitor. I actually made two drawings to divide the chassis in half so the capacitor connections can be shown in detail. I also photographed at 12mp a detailed photograph also for reference (it can be enlarged to "mega size" on the computer.)  >>>

With all of the shells ready, I just use masking tape to build-up the diameter of the new polyfilm so it will stay in place when put into the shell. The orientation I use is if the nomenclature is right-side up, then the outside foil is the lead to the left. It doesn't matter with polyfilms, but that's how I install them anyway. Once the polyfilm is in the shell, then the ends are filled with hot melt glue. When the hot melt glue is cold then I color it using a brown permanent marker so it looks more like the old wax. The capacitor is then ready to install. I've found though doing this many times, it's much easier to make up all of the capacitors first. Then, when reinstalling the rebuilt caps, I can concentrate on doing neat professional job since I don't have to stop in the middle of the job to rebuild more caps. Since I cut the original caps out first, when doing the reinstallation, I'll have to  remove the old leads, recondition the solder joint, wrap the new rebuilt cap leads correctly and then solder. This ends up with the entire chassis looking almost original but having all new polyfilm capacitors and new electrolytics.

Resistors - All checked okay. One non-original 1 watt resistor installed where original was 1/2 watt. IF screen load R probably burned because of leaky screen bypass cap so they replaced the burned resistor with a greater dissipation resistor and left the leaky cap in the circuit - I've run into this quite often and there's just no explanation for it. I replaced the 1 watt with a 1/2 watt identical to the original and, of course, the leaky capacitor-culprit is going to be replaced during the "re-cap."

IF Transformers - All checked out okay, including Crystal Filter and Amplified AVC LC.

Potentiometers - AF Gain pot needed to have two wires resoldered, otherwise, no problem. The NL pot tested okay. The Tone control tested okay. The RF gain showed open. I unsoldered this potentiometer with the AC switch on the back so I could disassemble it. The AC switch has to be removed before the pot back cover can be taken off. The entire wire-wound part was okay. I shot a little bit of DeOxit where the arm brass contact rotates on a brass contact to connect to the terminal. That seemed to get the pot working again. Reassembled the RF gain pot with the AC switch and reinstalled after reconditioning the wire ends before reconnecting and soldering.

Capacitor Removal - Feb 21, 2024 - The location drawings were made and all component testing that can be performed without power was completed. I clipped out the original capacitors making sure to leave a stub for positive location identification for the proper installation of the rebuilt caps. Caps were sorted to eleven .1uf, ten .01uf, five .05uf and two .005uf, 28 capacitors total. I also pulled from stock the needed polyfilm capacitors. Next, the old capacitors need to be melted out of their shells,...an outside job,...tomorrow.

Feb 22, 2024 - The .005uf capacitors are bakelite molded types so I found two Aerovox small diameter shells to use for the replacements. I melted all of the old capacitors out of their shells. To do 28 capacitor shells took a little over one hour. The .1uf 400vdc and the .05uf 600vdc caps were all Sprague types which are difficult to melt out because they used sealing wax as an end-cap and this takes longer to melt and doesn't stay liquid very long at all. Extra cleaning is required to remove the residual sealing wax from the shell. All of the .01uf 600vdc capacitors were Gudeman's-types. These were easy because the end-caps were just bee's wax. The unusual look for the Gudeman's are the shells are a mildly bright yellow color (they'll stand out.) The next step is to "stuff" the shells with the correct polyfilm caps. Masking tape is wrapped around the polyfilm so that it fits snuggly in the capacitor shell. I verify the the correct value polyflim is being installed in the matching capacitor shell and that the orientation is correct. The .005uf capacitors were installed in Aerovox shells that were small but were for .01uf 600vdc. I had to cut the shell to have it be the correct size for .005uf 600vdc and then the ".01uf" was changed to .005uf. All of the replica capacitors are now ready to fill with hot melt glue,...tomorrow.

Feb 23, 2024 - Hot Melt Glue - Before proceeding with the hot melt glue, I tested all of the capacitors with a Digital C Tester to verify that I had the correct value cap in the correct value shell. All 28 capacitors tested as correct. The hot melt glue is injected to fill one end only of each cap. By the time I get the 28th cap end-filled, then the first ones are cool and I can start over and fill the other side. I have a stand that can hold up to 30 capacitors allowing them to set vertically as the hot melt glue cools. The glue will contract as it cools so most of the time I fill the end so the glue is a convex "dome" that, by the time it cools, will be more or less flat. If the glue fill is left flat, then when it cools, the fill will be concave or "dished in" and won't look right. Once all of the caps are filled on both ends they're left to fully cool. Filling all 28 capacitors took about 45 minutes. When cool, then the glue is colored brown using a permanent marker.

Cathode and Bias Electrolytic Capacitors - I rebuilt the three 25uf electrolytic capacitors. I used a razor blade to split open the "rolled end" shell. I wrapped the new electrolytic in blue masking tape to get it secure in the shell. I used two wraps of scrap wire twisted to hold the shell tightly together at the slice. I then filled the shell with hot melt glue. Let it cool and then do the other side. The hot melt glue holds the slice together and the finished rebuilt electrolytic looks original, unless you look at the ends. But, when they're installed you can't see the ends very well anyway. Also, I made sure the slice was facing down towards the chassis.

Feb 26, 2024 - Rebuilding the Multi-section Filter Capacitor - These are always a headache to do but if you don't want new modern-looking filter capacitors under the chassis then installing the new capacitors inside the original "can" is what needs to be accomplished. The can has to be bifurcated to remove the old capacitors. I use a hack saw BUT, before sawing, I carefully scribe two parallel lines spaced .125" apart around the can. Then one vertical line is scribed through the two parallel lines. The two parallel lines are guides that are used while sawing the can and allow you to keep the sawing between the lines. After the can is apart and the old capacitors removed (it's not necessary to remove all of the black wax, just what's in the base part and about 1" up from the cut in the top piece. Aerovox usually didn't fill these metal cans very full of the black wax anyway so removal of the old capacitor is easy and very little of the black wax needs to be removed.) A file can be used to dress down the saw marks while keeping the parallel scribe lines visible. This allows a perfect fitting of the two pieces. The vertical scribe line also helps in exactly orienting how the two pieces should fit together. Normally, the original capacitors weren't soldered to the terminals inside (they're usually crimped because the capacitor foil was aluminum.) It's necessary to drill three small holes to allow the new electrolytic capacitor leads to exit out next to the terminals and then wrap around the terminals for soldering. Once this is done, then the can is epoxy-glued back together. I use heavy paper to make a small cylinder that fits inside the can at the joint. The paper is coated with epoxy on the side next to the inside of the can pieces and then the two can pieces are "fit" together. The excess epoxy can be wiped off the can joint with a paper towel. I then install one wrap of masking tape around the can "over the top and around the bottom" to keep the joint together tightly. After the epoxy cures, I remove the tape and then the seam can be painted with silver paint to help hide the joint. The new rebuilt filter capacitor can then be reinstalled. Time required to rebuild the multi-section is about 1 hour.

With the multi-section placed in it's clamp but not tightened yet, the can was rotated until the terminals were inline with my drawing, then the clamp was tightened. This assured that the original hook-up wires would be long enough and oriented correctly. I also had to connect the other end of the .1uf ripple-filter capacitor. This completed the capacitor rebuild and reinstallation part of the project.

NC-173 Under the chassis after cap rebuild

NC-173 Top of Chassis after cleaning - it turned out really nice. Notice how the BFO from the NC-183D looks exactly like the original "broken" NC-173 BFO (same part no.)

Next, the two pinch-wheel drives were cleaned and, when reassembled, new damping grease was used for the main shaft only in the main bearing. The pinch-wheel drive has to be assembled in the receiver chassis to make sure that the interface of the dial into the pinch-wheel is correct. Too little engagement and the dial will slip. Too much engagement and the dial tuning feels very heavy and rough. This setting is important so the receiver tuning is light-feeling but doesn't slip.

All of the tubes were tested. I needed to replace the 5AR4 with a 5Y3, I used a NOS 5Y3GT. All three of the 6SG7 tubes tested far below minimum acceptable. Two of the 6SG7 tubes barely moved the meter on the TV-7. The 6AC7 tube tested with high leakage (tests as a short.) The two 6SJ7 tubes tested very good. The 6V6GT tested very good. The 6SA7 tube tested good but I already had a NOS 6AS7 tube to install. The 6J5 tube tested very good. I found several 6SG7 tubes in the shop storage but none were NOS, so I had to select the three that tested best to install. I installed a NOS 6AC7 tube and a NOS 6SA7 tube. The 0D3 tested good. I ended up replacing seven of the thirteen tubes. 

I cleaned the rotor contacts on the tuning and band spread capacitor with DeOxit and a small brush. I gave the band switch the same treatment. I installed the S-meter. I installed the tuning capacitor cover. I used test leads to connect an 8Z ohm speaker and a ten foot test antenna. I connected a DC voltmeter to the B+ to monitor the actual B+ with the complete load on the power supply.

Cleaned the tube sockets. The intermittent problem was still present. I input a 455kc modulated signal to the Mixer grid and the signal came through strong, so the problem was in the front end. I replaced the front end tubes one by one. When I replaced the Mixer tube, everything began to work correctly. The Mixer was a NOS 6SA7 so, even though it tested good in the TV-7, when installed in the receiver it acted up (a tube tester ISN'T the final word on tube functionability - correct operation in the circuit is what's required.) I replaced the erratic 6SA7 with the original tested-good 6SA7.

Installing the Pinch-wheel Drive Assemblies - Cleaned and installed the pinch-wheel drives. I used fibrous sodium based grease that has good damping characteristics. The Band Spread installed quite easily with no slipping and smooth operation. The Main Tuning required several adjustments to eliminate slipping but I'm going to have to do some more adjusting because although there's no slipping, the tuning feels a little rough, not bad but noticeable. Slight lift on the tuning condenser frame should do the trick. The slight lift is accomplished by loosening the condenser nut on top of the chassis which will also loosen the nut under the chassis. Then the under the chassis nut can be turned out about one full turn. Then the top chassis nut can be tightened. This tightening will "pull" the condenser "up" slightly and reduce the engagement of the dial in the pinch-wheel. The end result is the adjustment can be made without disassembling the pinch-wheel drive.

NC-173  SN: 173 2621 and NC-173TS Loudspeaker
After restoration, original and cleaned dials but not illuminated

NC-173  SN: 173 2821
Rebuild Finished. I haven't done any "touch up" painting on the cabinet on this receiver. It cleaned up very well and, while it has a few nicks and little scratches, it's a pretty good example of acceptable cosmetic condition. But, in the future I'm sure I'll do the cabinet paint "touch up" and that should further improve the overall cosmetics of the receiver. The smaller size and reduced weight of the NC-173 results in a very convenient to use station receiver, especially on 80M AM, where its performance is superb.

Shake Down Results - March 1, 2024 - Tuned around 20M about 1400hrs PST. Fairly active, mostly North American stations. Tried to find Trenton Military on 15.035mc USB but couldn't hear them if they were transmitting. Tried the Chinese Mainland Maritime beacons and heard XSG 16.954mc with a strong signal steady signal (about RST 579.) I decided to tune even higher and give 15M a try, heard one SSB net mostly W5s. Went up to 10M and was surprised to hear quite a bit of activity, one JA station from Japan working stateside stations. Tried 6M and heard three unmodulated signals that weren't identifiable (beacons?) Stations on 20M CW were steady with good solid tone, no instability. Didn't hear any CW on 10M or 15M. XSG also had a solid steady CW note. RF tracking is excellent, I don't think any alignment of the LO is necessary or even desirable since it's almost perfect. A total surprise since I had the dials off. Performance is very good and I'll use it as a station receiver soon.

UPDATE: The NC-173 was set-up with the W6MIT "1625 Rig" - a 75 watt output homebrew transmitter. Antenna was the Collinear Array. Sea Trials began on March 17, 2024. An excellent receiver, no problems experienced. The one issue is the S-meter action on 80M when using a very large array antenna. Most ham AM signals "peg" the S-meter. There are two ways to compensate, the first would be to reduce the RF gain so the signal strength is at a level that's within the S-meter scaling but this method is changing the front end sensitivity. Second, is to reduce the signal level using the TRIMMER, which is what I prefer to do since that is reducing the signal level at the Antenna input. The NC-173 has accurate calibration and its audio reproduction is superb. An excellent receiver on 80M AM.

1. Manuals for NC-183D, NC-183 and NC-173 - National manuals are always a little sparse on information especially when it comes to design intention and detailed circuit descriptions. Just the basics are provided along with a parts list, schematic and alignment instructions.

2. 1950, 1956 ARRL Handbook - National Co.,Inc. - period advertising

3. Communications Receivers 4th Edition, Ray Moore - basic descriptions

4. QST magazine, issues March 1947, April 1947 and December 1947 - introduction of NC-173 and NC-183. I scanned original ads from either QST magazines or from ARRL HBs for this article for the best quality of reproduction.

Online:

1. BAMA Edebris - references to other data on all three receivers. The NC-173 schematic jpeg on BAMA (the one that isn't the cleaned-up original) contains many errors. Some values shown are incorrect, the Accessory socket wiring is left out completely and the Battery Power Socket is drawn incorrectly (the pin outs shown for remote standby would actually short the rectifier output to chassis ground,...oops.)

2. National Co., Inc. - period advertisements for NC-173, NC-183 and NC-183D are easily found on the Internet.

3. National Radio Reflector - Google e-mail group for National Co, Inc. enthusiasts. Search "mailman.qth.net" to see the list of groups and to find out details on becoming a member of the National Radio Reflector group. Thanks to those members that have sent their serial numbers to me, I've added them to the NC-173, NC-183 and NC-183D serial number log.