Brief History, Circuit
by: Henry Rogers, WA7YBS-WHRM-Radio Boulevard
|Was the NC-400 National's over-priced, cheaply-built and never-appreciated masterpiece that somehow has just not been given a fair chance in comparisons with its contemporary competition? Not very many hams ever purchased or even used the NC-400 receiver back when it was a new receiver. As a result, not too many "first-hand" 1960 ham opinions about the receiver ever became popularly-known. All that seems to have become common knowledge was that the NC-400 was produced in small quantities and that has resulted in its "rare and expensive" status among today's radio collectors. But, was it a "good receiver?" Practically every modern, technical-collector review I've read has basically the same information and ultimately the same opinion about the NC-400's construction and its performance. I've based most of the technical information in this write-up on the NC-400 I own, sn: 543 0006, and also on a very nice condition NC-400 that I aligned for a fellow collector a few years ago. Those readers that happen to own a NC-400 (or have worked on one) will most likely agree with my opinions presented in this article,...but, who knows? Read on and see,...|
The NC-400 was introduced in 1959. It was obvious from the astounding
list price of $895 that the intended customers were going to be
commercial users along with possibly military and government purchasers ($895 in
1959 was equivalent to $9200 in 2023 dollars!) Many of the features of
the NC-400 were necessary for commercial-military applications but
weren't required by or even of interest to the ham users.
The five, selectable, fixed-frequency, crystal-controlled receiving channels or the diversity options were certainly for military or commercial use. National did produce a compatible Diversity Modification Kit DMK-400 to allow setting up two NC-400 receivers in dual diversity. Either frequency or space diversity were possible and it was also possible to set up triple diversity using three receivers (eliminates 99% of fading) but the antenna system for space diversity requires quite a bit of real estate which limits the use to high-level commercial or military users.
An external crystal controlled oscillator adaptor, MX-400, allowed up to 50 different crystal-controlled frequency channels to be selected (used as in the External Osc. position.) Commercial or military RTTY communications might have had a use for the MX-400 where "channel switching" would eliminate the difficulty of accurate tuning (but the RF and Mixer stages must still be manually tuned in these set-ups so one has to switch to the desired band and also tune the dial.)
There was also the MFH-400 Mechanical Filter Housing that allowed removal of the Crystal Filter in the NC-400 to allow installing up to four Collins mechanical filters in its place. Collins Radio Company had developed the mechanical filter in the early 1950s and was providing them in several of their military-commercial receivers and even in their ham receivers. Where a specifically defined passband was necessary, with vertical walls and a flat top, the mechanical filter delivered that type of selectivity.
These NC-400 accessories were specifically intended for military or commercial use. The exception would be the crystal calibrator XCU-400 that provided frequency markers at either 100kc or 1000kc. The crystal calibrator was necessary since the tuning dial scales are vague in resolution and any dual-dial receiver with main tuning and bandspread tuning requires a calibrating marker for accurate set-up,...well, as accurate as you can get with that type of tuning system. >>>
|>>> Other features
that were military-commercial and included as standard features in the
NC-400 circuitry were the crystal-controlled BFO option and the IF output (both
for RTTY.) These were features not normally found in a ham receiver and
seem to confirm that National was aiming at the commercial market. That
shouldn't be a surprise since the "commercial market" is mentioned in
the NC-400 dealer flyer shown further down the page.
NC-400 production estimates vary widely - from an incorrect low of around 200 receivers up to perhaps as high as 1000 receivers (highest NC-400 serial number I've personally seen was in the 600s.) Probably a lot less than ten percent of production went to the "well-to-do" hams and SWLs who could afford the "stratospheric" price. The remaining receivers were sold to commercial and government users. Among the more commonly mentioned government users was the FBI where the receivers were installed in some field offices for various purposes. There are reports that some NC-400 receivers were obtained in used condition from government sales mainly from the FBI. Certainly other government users and the commercial users found the NC-400 filled their requirements and the exorbitant price was paid either by tax-payer dollars or company requisition funds. Even my NC-400 profiled in this article was a "commercial receiver" originally owned and used by WHAS, a radio-tv station in Louisville, Kentucky. The NC-400 was advertised up to about 1963.
Front-end - The NC-400 is a double pre-selection receiver (two TRF amplifiers) and is also dual conversion above 7.0mc. Seven tuning ranges cover from .54mc up to 31mc. The band spread dial is calibrated for the HF ham bands with a logging scale provided for general coverage. The conversions take place at 1720kc and 455kc of which the latter is derived from a crystal-controlled oscillator. There is a "plug-in" feature for the crystal filter assembly which can be removed and replaced with an optional mechanical filter assembly that allowed the user switchable access to a maximum of three different mechanical filters for bandwidth selectivity. The receiver uses 18 tubes (add one more tube for the Crystal Calibrator if installed.) The LO tube heater is operated from a higher heater voltage winding on the power transformer that has the voltage dropped thru a 4H4-C current regulator to provide stable heater voltage regulation. A dual triode 6BZ7 is used for the LO and for the switchable Crystal Oscillator.
IF Stages - The three IF amplifier stages actually are dual IF strips that utilize the same amplifier tubes. One IF strip is for SSB and provides 14 tuned circuits to achieve a fixed 3.5kc bandwidth. Upper or lower sideband can be selected with the bandwidth switch as "SB1" (LSB) or "SB2" (USB.) The operator can also select other bandwidths for the SSB mode but the BFO may require a slight adjustment other than using the SSB arrow on the BFO knob. The Crystal Filter can't be utilized in SB1 or SB2. The fixed 3.5kc bandwidth was thought at the time to be ideal for SSB and the switch sideband selectability was to ease tuning difficulties in the SSB mode of reception. In CW, the crystal filter is switched into the circuit instead of the first set of SSB IF transformers after which the CW signal continues thru the first IF amplifier tube and thru the remaining SSB IF strip. In the CW mode, AVC is not utilized and the RF must be manually adjusted. It is, of course, possible to tune in SSB signals in the CW mode to take advantage of the Crystal Filter and manual RF gain control, if desired. AM signals are switched to the second IF strip that utilizes six tuned circuits and provides switchable bandwidths of 16kc, 8kc and 4kc. The VS (Very Sharp) provides 3.5kc bandwidth with ten tuned circuits plus the VS position selects the crystal filter to provide another fives steps of selectivity down to 150hz bandwidth. Of course, as experience is gained in operating the receiver, the user will discover that any mode of reception can be routed through any of the IF sections to take advantage of the various types of IF sections and selectivity available. Also, that CW and SSB on the Mode switch turns on the BFO while AM turns off the BFO.
Det/AVC/Audio Output - SSB and CW signals are routed to a Heterodyne Detector/BFO that is essentially a product detector. AM signals are routed to a diode detector. Tone control is a two-position switch that was only intended for noise reduction. The noise limiter is also a basic clipper type. The audio is 1 watt output from a single 6AQ5 tube with either 600 Z or 3.2 Z available. The matching loud speaker was specified as the NTS-2 (also specified for other National receivers of the time.)
|When was National Radio Co., Inc. Created Anyway? - By referencing advertising and some National equipment manuals, it appears that National Radio Co., Inc. was created in the late-1950s. As to why the radio manufacturing was split-off from National Company, Inc.,...it might have had something to do with the government contracts that were becoming a large part of National Company's actual radio production. The communications receiver competition and National's position in the hierarchy of the existing radio companies along with National's faltering economic future as a producer of "top of the line" radio communications equipment might have also been a factor. National was still a major contractor for the Navy and was producing the FRR-2 system along with other Navy gear but their position within the ham radio marketplace had been slowly waning ever since James Millen left the company in 1939 (despite QST's unwavering support.) It's interesting that just when National Company was moving into building and supplying radio parts in the late-twenties with the Browing-Drake Regenaformer, there was already an existing "National Radio Company" located in Arizona but this company was sued out of existence in the late-1920s. Also, starting in 1928, all "legal radio manufacturing" in the eleven Western states was controlled by Gilfillan Bros., Inc. who had the exclusive licenses from RCA although the National Radio Company-Arizona suit seems to have not been related to the Gilfillan Bros., Inc. Any online history search seems to prefer "National Radio Company, Inc." and, although there is acknowledgement of National Company, there never is any information as to when or why a subsidiary "National Radio Company" was created.|
General Critique (Brutal, but True)
Cheap Parts - Some of the parts used for the NC-400 seem inconsistent with its high selling price. Most of the knobs appear to have been "leftovers" from old National television sets with these plastic knobs having flat-springs inside the hub and then pushing onto half-shaft controls. The knob nomenclature is printed on the knob brights and is so miniscule as to be almost unreadable except with magnification. Sometimes the S-meter scales will have a "splotchy" appearance when illuminated. The Antenna Trimmer air variable capacitor is mounted to a thin metal bracket that is mounted extremely close to the band spread tuning condenser. The long adjustment shaft provides enough leverage to cause the bracket to flex with each adjustment of the control making the tuned frequency "wobble" as the Antenna Trimmer is adjusted. The BFO control shaft is also very long with two half-shaft spring couplers that flex making BFO adjustments difficult. Also, the long shaft, even though it's a fiber material, can mechanically induce microphonic noise into the BFO circuit.
Antiquated Tuning System - National wasn't alone in continuing to produce a variable-C tuning system with a dial readout that couldn't resolve the tuned frequency "to the kilocycle" as Collins equipment could (the military Hammarlund SP-600JX also had poor dial resolution.) The NC-400 dial scale resolution is vague and if you have the optional crystal calibrator you can then find markers at every 1000kc or 100kc points but most of the scale index markers are in 10kc increments so anything more accurate than 10kc is just guesswork. The tuning is accomplished using a "pinch-wheel" the works on the rim of each dial that can "slip" if allowed to accumulate dirt and grease. Most military and commercial users would have the necessary heterodyne frequency meter available for accurate frequency measurements. National couldn't even install band-in-use indicators that actually worked and, even if they did work, they consisted of crude, "spring-loaded" red pointers that were supposed to align with the band-in-use scale (but never do.)
Flexible Chassis and Frequency Drift - Out of the cabinet, the chassis is moderately flexible and that can't be good for alignment which, of course, has to be performed out of the cabinet with the chassis on its side. Getting a NC-400 to "hold alignment" on the highest frequency band while getting the chassis back into the cabinet is next to impossible. The flexing of the chassis plays havoc with the adjustments and usually (if you're OCD, like me) you'll be pulling the chassis back out to adjust the highest frequency band tracking for a second or maybe even a third time. Additionally, operating the "mega-torque" band switch also seems to cause minor alignment issues on the highest frequency band. The operation of the band switch feels like each detent sends "shockwaves" through the chassis and it probably does! Other "flexible" assemblies and brackets also cause other frequency instabilities. Frequency drifting in the LO and BFO is rampant and never seems to stop (it slows down after about a half-hour.) No wonder MX-400 external crystal control oscillator was an option and the five channel crystal control of the LO and the crystal control of the BFO were built-in options,...crystal control was probably the only way to stop the drift.
Over-Priced For Sure - All of the proceeding complaints seem very inconsistent with a $900 receiver built around 1960. A disappointment to the ham buyers at the time? Possibly,...if there were any. For hams, the NC-400 was priced higher than the excellent SSB ham-bands-only receiver, the Collins 75A-4. I can't remember ever hearing any ham lamenting over his long-gone NC-400. As far as military users, there were never any military contracts for the NC-400. That leaves the users like the FBI or similar government agencies that probably were just monitoring signals and found the NC-400 adequate for that. A direct and accurate frequency readout was still relatively new then and using a heterodyne frequency meter was probably "standard procedure" for all government and commercial users. But for a few hundred dollar more, these agencies could have bought the Collins 51J-4 and put away the frequency meters (and, when the 51J-4 was tuned to a signal,...it stayed put. Zero frequency drift without any optional equipment! And, by 1962, the 51S-1 was available with the same specs but at half the size and half the weight!)
Is it Really That Ugly? - As far as looks,...the front panel and cabinet design must have been conceived by one of National's part-time interior decorators since the paint scheme uses three different shades of gray in both smooth and wrinkle along with black winkle finish with chrome trim strips on the sides of the cabinet. Parts of the front panel are transparent plastic with back-screened nomenclature in white with then a coat of gray paint (backside) applied giving a slight depth to the panel and nomenclature. Then there are both gray plastic knobs and black plastic knobs along with four small aluminum knobs. The NC-400 was an attempt to follow the late-1950s trend that many radio equipment manufacturers were beginning to adhere to, that is, believing that if the radio "looked modern" maybe it could find a place in the living room as part of the decor and that might increase sales. Collins even advertised their S-line with a housewife admiring her ham-husband's small and modern-looking station located in the den. National ignored the "smaller and lighter" trend with the NC-400 being a very large and very heavy receiver. At any rate, many manufacturers followed this modern decor trend with most of them creating some of the ugliest radio gear on the planet (Drake was at the forefront of producing the ultimate in ugly radios with the 1A that looked like a lunch pail with a tuning dial on one end and the R-4 and T-4 with front panel nomenclature designs that looked more like children's toys than actual ham gear.)
National's advertising artwork never flattered the NC-400 (just take a look at the dealer's flyer shown above.) For some reason, the NC-400 receiver is just not very photogenic. It will always look much better "in person" than it does in photographs. Of course, that's just my opinion. The first NC-400 I saw "in person" I thought looked pretty cool. But, if you're an owner who finds that the NC-400 is just too repulsive to look at, you can always listen to the receiver with the "lights out!"
|Is 1kc Dial Accuracy all That
Important? - All manufacturers in the 1950s and
early 1960s had to
have some sort of response to the Collins receivers that were on
the commercial and ham market. It probably started with Technical Materiel
Corporation's published comment,
"People who require frequency meters should buy
frequency meters - not make them double as communications receivers."
This little gem of frustration
was in a TMC Bulletin regarding the GPR-90 and revealed their
feelings about the "Collins versus TMC" comparisons that were
common at the time. But, the GPR-90 was a $495 receiver, much
less than the Collins 75A-4 and a whole lot less than Collins
51J-4 general coverage receiver. However, Hammarlund's SP-600
was about the same price as the NC-400 and it had the same lack of
accuracy in the tuning resolution. But, the SP-600 doesn't have
a bandspread feature as an additional complication to the already vague
tuning resolution. Heterodyne Frequency Meters were everywhere
at the time and all military and commercial users knew how to
get the frequency accuracy required by employing their "Freq Meter."
In fairness, Collins achieved their 1kc dial accuracy by limiting the span of each tuning range to only 1000kc. The legendary accuracy of the Collins receivers at the time was in the accuracy and linearity of the PTO and to the crystals used in the Crystal Oscillator. The permeability tuning allowed using a precision threaded shaft to accurately move a ferrite core in and out of the PTO precision-wound inductance. The accuracy was unbeatable with PTO generally being linear over a 1000kc span (ten turns generally) to better than 0.50kc when they were new. But, how important is 1kc dial accuracy? If you've ever used a heterodyne frequency meter to determine where in the electromagnetic spectrum your receiver is tuned, you'll know what a pain that was. Time consuming because of setting up an extra piece of equipment that generally had a separate power supply, it had to be calibrated each time it was used, a small wire antenna was required so the "freq meter" oscillator would radiate enough signal for the receiver to pick up - a real pain. So, along comes Collins receivers that are as accurate in their dial's frequency readout as almost any heterodyne frequency meter. When using a Collins receiver, you knew where in the electromagnetic spectrum your receiver was tuned. There really wasn't any argument that the Collins tuning system was "decades ahead" of the variable-C tuning used by most manufacturers at the time. The disadvantage, of course, was that a general coverage Collins receiver will have 30 tuning ranges, each spanning 1mc. But, if you knew where you were supposed to be listening and it was important that you be "on frequency," you could set up the Collins receiver to the needed frequency in seconds.
Another popular quote was "If you want a frequency meter buy a Collins,...if you want to hear them buy a National." This quote's logic is weird since the assumption is that "if you know where in the electromagnetic spectrum your receiver is tuned by directly reading the tuning dial, you won't hear anything" or, another interpretation would be "you can't know where you're tuned if you want to hear any signals."
There were several other comments and quotes that sought to diminish the importance of Collins receivers' and transmitter's dial accuracy and the "no drift" aspect of their oscillators. Or to deride the "communications audio" found in all Collins receivers or the seemingly primitive AVC and detector circuits their equipment employed. In the end, with the exception of the AVC and detector circuits, all of the other Collins receiver attributes could be found in most communications receivers built after the late-sixties.
So, is 1kc dial accuracy important? Since every modern receiver (or transceiver) is able to display received frequency out to two or three digits past the decimal point, the obvious answer is yes. Since nowadays there are many "add-on" digital frequency readouts available for older gear that work off of the LO frequency. These devices have to be "programmed" to read correctly. Usually these digital frequency readout devices aren't particularly cheap so tuned frequency readout accuracy is obviously an important feature to some operators. Collins just happened to be first at providing an accurate dial readout and that set the trend towards receivers and transmitter-VFOs providing the frequency readout accuracy that we all expect today. Is the zero-drift important? No frequency drift will even be tolerated today (except it is expected to a certain extent on vintage radio nets.) So, all of these old quotes are just manufacturer and owner defenses against developments that were the inevitable improvements that ultimately became the "standards of performance" expected today.
Refurbishing the NC-400
I've had my NC-400 on the work bench twice. When I first got the receiver, it was a "tech special" that not only didn't function - it would blow fuses if power-up was attempted. The repair was followed by an alignment and the receiver was used for SWLing occasionally. Six years later, I tried using it as a station receiver with disappointing results. So, now, a year later, in 2023, I'm again going though the NC-400 to see if it's possible that it might be a practical, great-performing station receiver,...well, maybe just a good usable receiver.
|Finding the NC-400 sn: 543 0006 - Sometimes things just come to you when you least expect it. That was the case with this NC-400. Back in August of 2017, I was casually looking around Ham and Hi-Fi in Sparks, Nevada. On the bottom shelf of their long line of shelves I spotted an unusual receiver,...it was the first NC-400 I had ever seen "in person." I remember thinking "it doesn't look that ugly in person. In fact, it's kinda cool looking!" It was pretty dirty and one of the two tuning knobs was broken with a large section of it missing. Overall, it looked complete and original. A few days later I got a late-afternoon call from Don W7SSB, the tech at H&HF, telling me that the NC-400 blew fuses when trying to power it up and, if I wanted it as a "Tech Special," I could have it "as-is" for $100. I told Don I'd be over the next day to pick it up. A copy of the manual was included along with two NOS-boxed tuning knobs.||2017 Problems - Bad Multi-Section Filter - The problem of "fuse blowing" was caused by a shorted filter capacitor (from what I've read that seems to be a common problem on NC-400s.) Luckily, no power transformer damage occurred. I "tack soldered" some electrolytic capacitors in as a "quickie test" to see if that stopped the fuse blowing and it did. I ordered an exact replacement multi-section from Antique Electronic Supply in Arizona. The 4H4-C was missing and a 6V6 substituted. This was "no problem." Also, the 6BZ7 tested "dead" - really, no emission at all, even though the heater was illuminated. I tried a recommended substitute tube for the 6BZ7 but found that tube wouldn't even function as an oscillator. I eventually performed a thorough search through the tube storage up in the loft out in the shop and found a few "NOS, in the box" 6BZ7 tubes. I guess the upshot would be to use only a 6BZ7 tube - no substitutes. The substitute would have only been for testing purposes anyway, but I couldn't even get the the substitute to oscillate, so no receiver operation. I replaced a few weak tubes and performed a full alignment (a couple of times on the highest frequency band.) The NC-400 seemed to work okay. I used it a few times but never put it into a station at that time.|
Another Refurb for 2023
|Power-up After an
Idle Seven Years - After I got the NC-400
refurbished and aligned, I never used it as a station receiver. I apparently continued to have minor interest in the NC-400
because somewhere in the past few years I managed to find an
original and mint condition manual. About a year ago (in 2022) I
set the NC-400 up with the W6MIT homebrew, "the 1625 Rig" transmitter
and used it one time on the Vintage Military Radio Net on 75M. I wasn't
impressed with the performance or with the very poor audio reproduction and I
immediately disassembled the station and reinstalled the British RACAL
RA-117A that I had been using with the W6MIT transmitter (now THAT'S a
Then, a couple of months ago, I found an original Dealer's Flyer for the NC-400. With a renewed interest in the NC-400 starting around April 2023, I kept thinking that I should be checking the NC-400 out again. After all, it might pick-up signals really well after a little more rework. That finally happened on May 22, 2023. The initial signal reception tests were on 20M SSB and the results were dismal. Frequency instability was rampant. SSB was next to impossible to demodulate. AM worked fairly well but CW seemed to over-drive the receiver unless the RF gain was cut back and SSB was extremely difficult to tune in and be able to understand the speech. I tried Variable IF bandwidth and it was slightly better. SB1 and SB2 didn't seem to work well and were very unstable. A few times the receiver stopped pulling in signals entirely. Changing bands seemed to get it working again ("shock therapy" provided by the band switch detent spring!)
So, it sounds like the NC-400 needs another "going over."
|Cleaning Switches - I
didn't do this the first time back in 2017, I don't know why,...maybe I
was out of DeOxit or something. Anyway, I'm paying for that now with the
intermittent contacts and instabilities. I rarely "spray" DeOxit since
that gets the DeOxit all over the chassis and other components where you
don't want it. Also, "spraying" tends to put way too much DeOxit on the
switch. You are forced to spray pots but I always turn the spray head
for minimum and just spray a "shot." At any rate, for switches, I spray
the DeOxit into a little cup and then use a small paint brush to apply
where needed. The main tuning condenser and the bandspread condenser
rotor contacts appeared dry. There was some residue of the original
grease but that was also dry. The switch contacts for the mode select in
particular needed something. The switching to SB1 or SB2 didn't even
seem to work. The contacts weren't oxidized but just really dry. Same
with the band switch. All controls and switches were exercised during
the DeOxit treatment. The improvement was noticeable and stability was
improved somewhat. The SB1 and SB2 now worked, so that helped.
The band switch has this "mega-torque" detent system that jars the receiver whenever different bands are selected. I applied grease into the detent to see if that would help but it didn't do much. I think the spring needs to be replaced with a lighter-weight spring. It shouldn't be too difficult since it's located right at the bottom of the detent assembly.
|4H4-C Design Problem or
- Typical of 4H4-C current regulators, in this receiver it had been replaced with a 6V6
as a substitute in 2017. I had a good quantity of 4H-4 current
regulators that I installed then. Now, I've discovered that the
4H-4 has slightly different specs than the 4H4-C. The 4H-4
seemed to work in
the circuit. Here are the differences:
4H-4 - 4.3 to 10.0 voltage range rated at 500mA
4H4-C - 4.5 to 7.5 voltage range rated at 440mA to 470mA
I've used the 4H-4 type in my HRO-60R for years with no real issues. The 4H4-C is rated to regulate to a closer tolerance but the 4H-4 will also regulate to 6.3vac if set up to do so.
Hollow State Newsletter Write-up - Among the papers that were included with the NC-400 purchase was a copy of a Hollow State Newsletter article in which Dallas Lankford related a change he had found in a NC-400 he had worked on. That NC-400 had a 6-4 ballast tube (used in RBB and RBC WWII Navy receivers) that was substituted for the 4H4-C with a slight circuit modification of a 100 ohm resistor acting as an additional load on the 6BZ7 heater (this load is actually recommended in the Amperite handbook on their ballasts.) When compared to the schematic in the manual, this 100 ohms resistor looked like an addition. Lankford related that he had installed a newly purchased 4H4-C to replace the 6-4 only to have 4H4-C instantly go open with a flash. The dramatic failure of a fairly expensive tube got Lankford to investigate the failure. Although, the ultimate find was that the "new" 4H4-C was defective (it had apparently leaked the H2 out over the years.) Lankford thought that the 6-4 mod had added the 100 ohm load since it isn't shown on the schematic but the installation looked "factory." Normally, there's a 2.2 ohm 1W wire-wound in series between the 4H4-C and the 6BZ7 heater. It seems that this series current limiter is not sufficient and apparently National added the 100 ohm parallel load to drop the 6BZ7 heater voltage. Lankford speculated that his 6BZ7 tested bad because of the long-term high heater voltage. That was interesting because my NC-400 also had a bad 6BZ7 tube when I first got it. According to Lankford, only the early versions were lacking the 100 ohm parallel load that brought the 6BZ7 heater voltage closer to 6.3vac. My NC-400 is really early being the sixth one built. Lankford suggests that probably a slightly lower value resistor should be used for the load, maybe 50 ohms. The best method is to monitor the 6BZ7 heater voltage with the 4H4-C installed to check that the value selected provides close to 6.3vac.
My Testing - So, the first thing I had to do was to actually test the 6BZ7 heater voltage. My 4H-4 ballast tube actually measured 5.5vac,...hmmm,...pretty low. The AC line input is at 116vac. Looks like I'll have to pull the chassis to check this out further.
With the chassis out, it was obvious that the 100 ohm load was present. I tested the receiver at 120vac line to see what the 6BZ7 heater voltage would be and it was still at 5.5vac. I measured the 2.2 ohm resistor and got 3.5 ohms. I added a parallel resistor to drop the series resistance to 2.2 ohms, still no change with the 6BZ7 heater running at 5.5vac. I installed a 6V6 tube in place of the 4H-4 and the 6BZ7 heater voltage went up to 7.7vac. I took out the parallel resistor to return the series current limiter to 3.5 ohms. The voltage dropped to 7.0vac. I went out to the shop and got another 4H-4 ballast tube, took out the 6V6 and installed the new 4H-4. The heater voltage was at 7.0vac. So, my problem had been a defective 4H-4. I lowered the AC line to 116vac and there was no change in the heater voltage (after all, the 4H-4 is a regulator.) Lankford's recommendation to lower the load value probably would help.
The actual problem seems to be that the NC-400 schematic was never corrected to show the 100 ohm load resistor. But, if my sixth receiver "off the line" has the 100 ohm load installed (and it does look original,) the change probably took place pre-production and the correction of the schematic never happened. Another problem is that there are 4H4-C ballast tubes and 4H-4 ballast tubes. If the 4H-4 ballast is used, a slight adjustment of the load resistance would probably "dial" the 6BZ7 heater voltage to exactly what is required. I ended up installing a 270 ohm 1W resistor in parallel with the 100 ohm resulting in a 73 ohm resistor. This resulted in 6.49vac for the 6BZ7 heater voltage which is close enough (~3% high.)
I did notice a difference in stability from when the 6BZ7 heater was running at 5.5vac when compared to 6.5vac. One wouldn't think that low heater voltage of a little over 10% down would have that much of an effect but now, at about 3% high, the receiver seems to be more stable and doesn't have erratic frequency excursions like it initially did.
Instability and Frequency Drift - However, even though getting the 6BZ7 operating correctly did improve erratic frequency excursions, there are other mechanical design or construction issues that also have an effect on the frequency stability. Things that "flex" and physically move range from the Antenna Trimmer control up to the entire receiver chassis. Adjustment of the Antenna Trimmer especially will cause momentary frequency wobbling. It's caused by the ultra-thin and long metal bracket that's mounted right next to the rotor of the rear section of the bandspread tuning condenser. The bracket is very close, probably 0.030" spacing from the rotor plate of the bandspead condenser. As the bracket flexes, the C value changes and even though it's in the 1st RF Amplifier tuning, it changes the frequency slightly causing the "wobble" as you try to adjust the Antenna Trimmer. The designer must have come up with the bracket idea when the bandspead condenser was in full-mesh and never checked just how close the condenser rotor would come when the bandspread was in its normal tuning range. Of course, the designer might have thought that since the rotor is grounded and the bracket is grounded it shouldn't be a problem. But, in the normal tuning range for the bandspread part of the rotor plate next to the bracket and the other part of the rotor plate partially meshed. This is when the bracket movement causes the most frequency "wobble." Just an unbelievably Rube Goldberg mechanical design in a $900 commercial receiver.
The entire chassis is built of relatively thin sheet metal. It can and does flex under its own weight. Just pushing down on the top of the chassis will cause a frequency excursion (when the chassis is out of the cabinet.) The tuning and bandspread condensers are screwed to a cast aluminum mount that should increase the stability. I think the cause is that all of the coils and trimmers that are mounted to the underside of the "flexible" chassis. Any torque of the chassis or pressing to distort the top surface will cause a frequency change since the coil positions move somewhat. Of course, it's much less noticeable in the AM mode but CW or SSB will show quite a bit of instability just in slight movements of controls or just pushing on the chassis of the receiver. When the chassis is mounted in the cabinet, the mechanical stability is greatly improved. The chassis bottom is supported by the cabinet framework, which helps. BUT ALL of the screws for mounting the chassis into the cabinet MUST be installed to reduce "flexing" to a minimum.
Along with mechanical stability issues, the BFO seems to drift rapidly for about 10 minutes upon power-up. After that, the drift progressively slows with more time and, after about half an hour, tuning is only occasionally required, maybe once every 10 to 15 minutes for a SSB signal. Will the NC-400 ever stop drifting? Probably not in the CW or SSB mode. That's why crystal-controlled oscillators were available options. Without crystal-control of the oscillators, I don't think the NC-400 could have met spec for any type of commercial facility operating RTTY and might even be a problem for commercial SSB communications. While the drift is still present in CW, it's not so much of a problem for the CW operator and is only a slow change in the heterodyne tone heard. Of course, in the early 1960s world of ham radio, drift was common and both receivers and transmitters drifted in frequency unless they were crystal-controlled. Using a VFO-controlled transmitter would result in some drift in the transmitted signal. So, at the time, the CW drift probably wasn't any problem and the SSB drift was also something that was expected, at least for hams. All one had to do was to slightly retune the receiver, which, at the time was normal operating procedure. Nowadays, drift seems to be intolerable in any mode for any type of radio operator.
- If you're planning on electronically rebuilding a NC-400, luckily most
of the capacitors are ceramic disks that won't require replacement.
There are a handful of plastic molded paper dielectric capacitors. These
are the later Sprague capacitors with a black body and yellow
nomenclature. These tend to be more reliable than "black beauties" but
still they are paper dielectric and probably should be replaced. The
resistors should be checked for value. Some rebuilders recommend
unplugging the Crystal Filter and cleaning the pins and sockets
but this does require dismounting the front panel.
The multi-section electrolytic uses four capacitors. Three are power supply filters and one is the cathode bypass for the 6AQ5. There are new-manufacture multi-section capacitors available that have the correct values (made by CE using original Mallory machinery - available from www.tubesandmore.com) and are an exact fit mechanically.
Where most socket-type set screws are encountered, they will be Bristol or spline-type set screws. The small aluminum knobs typically have maxi-torqued Bristol set screws and if an Allen wrench is used to loosen the set screw, either the wrench will be ruined or the set screw socket will be destroyed. Using a proper Bristol wrench is the only way to safely loosen the maxi-torqued set screws.
The manual has one error in not showing where L-11 is located. It's part of the IF alignment and is located on the bottom of the Crystal Filter assembly. The schematic has a few errors. The 4H4-C 100 ohm load isn't shown on any NC-400 schematic but was installed in all receivers. There is an indication of a wire marked "E" from S-9 that isn't shown anywhere. I believe that "E" was one of the terminals on S-9 that could be connected as part of the "Tune" position on the CAL switch which was provided for the end-user, probably for the MX-400 LO option. Although there's no indication of that on the schematic what the "Tune" position was for it appears to work the S-meter circuitry and probably was to provide some way to "peak" or "tune" an external device using the S-meter as the indicator. My original schematic has an paper "overlay" glued over the Crystal Filter section indicating some sort of major change or error correction. I'm sure there are several other errors, these are just ones I've found.
Mods and Write-ups - There are a few mods mentioned in Ray Osterwald's Electric Radio write-up from February 2011 and March 2011. The mods are minor changes in the value of a few resistors and a couple of capacitors, so nothing major. Ray's main concern was with the audio quality which wasn't to the specifications but, as Ray also mentions, his NC-400 was the only one he had ever worked on and it might not be representative of all NC-400s. The article is worth reviewing because there was a lot of interesting testing done with oscilloscope photos to show results.
All of the other write-ups I've seen (and read) are basic descriptions and history reports. They are,...
1. QST October 2011 - Vintage Radio section - John Dilks
Chassis Removal - Instructions not in
the Manual -
The manual has absolutely no instructions for how to remove the
chassis out of the cabinet. I guess it's not too hard to figure
out but a couple of sentences would have sufficed,...so here
they are. First, remove the four acorn nuts from the front panel
and remove the bezel. On the rear apron are six hex head screws
with flat washers that need to be removed. On the
bottom there are four hex head screws with external-tooth lock washers to remove. The rubber feet
also need to be removed. With
the receiver right-side up on the bench, slide the chassis/front
panel assembly forward out of the cabinet. When reassembling,
slide the chassis into the cabinet and be
sure to install the bezel as the next step. Clearance is tight for the
bezel fit and once it's in place then the remaining screws can be
installed. Do not tighten any of the screws until all of them
are threaded into their holes. This assures mechanical alignment
is okay within the cabinet and no binding is occurring.
>>> Why National didn't provide for all alignment adjustments to be accessed from the top (like their competition was doing) is a mystery, especially since National wanted commercial and military users to purchase the NC-400. Also, why there isn't a removable bottom plate on the cabinet to allow performing the alignment with the receiver in the cabinet is another mechanical design mystery.
The receiver does have to be mounted in the cabinet to achieve the best stability. How NC-400 receivers were ever used rack-mounted is a mystery (probably with side rails.) The cabinet frame and screw-mounting provides the support the flexible chassis requires. The NC-400 can pull in weak signals quite well, especially after a careful alignment. It probably will take a bit of listening and working with the receiver to eventually get the most out of it. The NC-400 can be a sensitive receiver. It can be selective enough when necessary and also can be set-up for a broad IF passband to enjoy some of the quality SW BC stations (are there any left?) Ownership and actually operating the NC-400 for a while will go a long way toward one's overall appreciation of the NC-400 and certainly its electronic performance is at the forefront of that appreciation. The audio quality is a pleasant surprise considering it's just a single-ended 6AQ5 that's producing the very natural sounding reproduction. However, the Tone switch is next to useless since it just reduces the high audio frequencies slightly by switching in a bypass capacitor (for $895, one would think National would have installed a true adjustable Tone control.) For the best audio reproduction, use a good quality, moderately large diameter speaker, 8" to 12" is sufficient, installed in a decent housing and the NC-400 audio will sound great. Finding the matching loudspeaker, the NTS-2, is difficult. I run my NC-400 using the balanced 600Z output to a good quality eight-inch speaker in a large wooden cabinet with a 600Z to 8Z matching transformer. The sound quality is pleasant. There already is a 22 ohm load resistor on the 3.2Z output so nothing has to be connected there when using the 600Z line. Also, the phone jack is on the 3.2Z output but since there's the 22 ohm R load any impedance phones can be used.
That being said, the NC-400 has many, MANY quirks. First, the good quirks,...the multiple IF selections will take getting used to. You can receive CW and SSB signals (even AM signals) several different ways with combinations of the Mode switch and the IF Selectivity switch. Once it's determined that SB1 is LSB and SB2 is USB, then selecting the proper sideband isn't difficult but a slight adjustment of the BFO will be necessary on either LSB or USB for best demodulation of the SSB signal (an easy way to remember USB versus LSB is that SB2 is the "higher number" and is the "upper sideband.") You can use SB1 or SB2 in the AM mode for an interesting type of AM reception. For exalted carrier AM, you have to switch to CW and adjust the BFO for zero-beat (no AVC in this position.) It's important to remember that when you're in B, M or S bandwidths, you're using a different IF section in the receiver with only six tuned circuits. But, you can run the AM signal through the 14 tuned IF circuits by using SB1 or SB2 for a fixed 3.5kc bandwidth. Or, you can run the AM signal through VS and have 10 tuned IF circuits and also use the Crystal Filter. CW signals or SSB signals can also be run through the various IF circuitry. There are many methods to try, which does seem to backup National's advertising claim that the NC-400 was "the most versatile receiver available."
Now, the bad quirks,...the insolvable problems are all mechanical in nature. Like the flexible chassis and component mountings that cause frequency excursions whenever certain adjustments are made. Or the vague frequency readout that gets worse as the tuned frequency gets higher and higher. When actually using the NC-400 "on the air" most vintage ham gear operators will have a digital frequency counter set up to read the transmitter frequency. The NC-400 can be "zero beat" against the "tune" or "spot" function from the transmitter and the receiver will be "on frequency." But, those crappy "band-in-use" indicators that stick and never seem to point to the band you're actually on, or the "mega-torque" band switch that causes shockwaves throughout the chassis as it's operated and the cheap knobs that give the controls a flimsy feeling are other irritations that you'll eventually have to accept. The sheer size of the receiver and its 75+ pounds of bulk weight requires a lot of bench real estate but that could be viewed as a "commanding bench presence" and that might compensate for the lack of table space. In many vintage amateur radio station installations the receivers aren't moved around very often, either physically or even tuned frequency. Sometimes many of the adjustments are only made once and then left alone. So, perhaps most of the mechanical problems really won't be that much of an issue (unless you have an OCD nature and want everything to be "perfect!")
I'm using my NC-400 connected to my 75M collinear array antenna. This is a 240' center-fed dipole with 108' open feed line to a 1KW Johnson Matchbox tuner that's operated on 75M (two half-waves in-phase.) This is a substantial antenna that does exhibit a little gain (~1.9db) and does allow hearing very weak signals. But, the NC-400 noise floor is pretty high with this antenna so for the best sounding signals I'll reduce the RF gain from maximum until the S-meter reading just begins to drop. At this point the signal is still creating AVC voltage but the background noise is greatly reduced. I think that SSB signals actually sound much better with the RF gain reduced slightly in this manner. CW signals will require reducing the RF gain somewhat since the AVC is turned off by selecting CW but the CW signal is still routed through the Product Detector. Also, you can listen to CW in the SSB position and then have AVC, if desired. Strong AM signals, with using a diode detector and a different IF section, can do rather well with full RF gain but, with weaker AM signals, better results (especially with QRN present) will be with the RF gain slightly reduced. As mentioned above, you can route AM, CW or SSB signals though any of the various IF bandwidth or Mode options to experiment with what works best for the receiving conditions.
For signals outside the 75M ham band, the Matchbox allows "tuning" the antenna for a good match anywhere from 3.5mc up to 25mc. I listen to 20M a lot since it's a daytime DX band and I do most of my listening during the day. Listening to the Chinese CW Coastal Beacons is pretty easy on 16.9mc in the later afternoon. Any of the SW BC stations sound pretty good but the best broadcast audio quality signals seem to be coming out of China these days. The Chinese are producing powerful signals with lots of modulation and a wide bandwidth (well, maybe not Firedrake, but most other Chinese stations.) Radio Havana is strong too but their modulation is a little rough sounding and their announcers are annoying to listen to.
So, despite the mechanical quirks, give your NC-400 a little time. Experiment with the many different methods to demodulate signals. I think when you get used to those "good" quirks and when (or if) you get used to its unique appearance, the NC-400 can become a fine station receiver,...one that isn't seen all that often and at least you didn't have to pay the 1959 $895 cost in 2023 dollars (9200 of 'em.)
I have my NC-400 set up with the W6MIT homebrew transmitter, shown in the photo above. Though W6MIT's transmitter might be a little short on "good looks" just the opposite is true of its incredible audio quality on AM. A really stellar homebrew transmitter. At only 70 watts of carrier power, I find that using the 75M collinear array antenna is a big help. Most of the time, the "1625 Rig" with the collinear produces a good signal that is Q5 with most check-ins to the Vintage Mil-Rad Net. And, the gain of the antenna also helps with the received signals (gain is about 1.9db.) I have a write-up on the transmitter as an addendum further down this page.
|SN: 543 0006 was an actual Commercial Receiver - Although a few collectors might consider the NC-400 a "ham receiver" due to some of National's advertising, very few NC-400s were sold to hams. Most couldn't afford it and those that could have afforded it would have bought Collins gear instead. Most NC-400s found today were in commercial service. The particular NC-400 (SN: 543 0006) that's the subject of this write-up was a commercial receiver that was owned by radio-tv station WHAS in Louisville, KY. WHAS wanted to make sure their ownership of the receiver was obvious so "WHAS" was vibro-etched in four different locations. The top of the crystal filter, top of the chassis and two places on the rear apron of the chassis. Many of the large radio and tv stations would have their own repair facilities and technician staff. It's likely that the NC-400 was in that environment at WHAS since it was obviously used very little and luckily it was never worked on by their staff technicians or ever had any ham-mods incorporated. As a "monitor receiver" or as a "test bench receiver," it probably was a satisfactory performer for WHAS.||Things that remain to be worked
on,...the Crystal Calibrator is a unidentified
kit that was installed in the original socket for the XCU-400
National 100kc/1000kc Crystal Calibrator. The kit can only
provide a 100kc output marker. For some reason, even though the
connections are provided at the receiver socket, this kit has a
toggle switch mounted to the side to turn off the plate voltage
(I think.) The toggle switch is a major problem since to use the
calibrator requires lifting the receiver cabinet lid and that
prevents having any accessories setting on top of the receiver.
I'll have to investigate why the front panel switch wasn't used
and if that can be employed with this kit calibrator.
I did discover that the Antenna Trimmer knob was tightly pressed up against the front panel plastic face. This really increased the torque necessary to move the trimmer and increased the amount of "frequency wobble" when adjusting the trimmer. I had lubed the shafts and the trimmer C parts to no avail. But, when I remounted the Antenna Trim knob with some clearance between it and the panel, the difficulty in rotating the adjustment was gone. Now, adjusting the Antenna Trim doesn't cause any frequency wobble unless the knob is pushed sideways or something that wouldn't be normal for adjusting. An accidental but easy fix.
The following are some details on the W6MIT "1625 Rig"
W6MIT - AM100 a.k.a. "The 1625 Rig" - Homebrew Transmitter
The AM100 transmitter was designed and built by John Svoboda W6MIT in 1997. The circuit operates around a T-368 Exciter that is used to drive a pair of 1625 tubes in parallel for the PA. The modulator is a pair of 1625 tubes in push-pull. The power output is 90 watts on CW and 70 watts (carrier power) on AM. The T-368 Exciter tunes continuously from 1.5mc up to 20mc however the output network allows antenna matching on 160M, 80M, 40M and 20M. It is possible to tune 30M while in the 40M band position. The speech amplifier is a "classic audio design" and utilizes a driver transformer from a Heathkit DX-100 and a military transmitter modulation transformer. Both the modulator and the PA are located in a shielded compartment on the left side of the transmitter. The T-368 Exciter is located on the right side and the speech amp is located under the T-368 exciter. The T-368 Exciter was purchased from Fair Radio Sales but the Exciters were sold without the 6000 type output tube. The 6000 is fairly expensive and difficult to find and also runs on a 24 volt heater-filament so John modified the Exciter to use a 5763 tube for the output. The power supplies are located directly behind the T-368 Exciter on the right-rear side of the transmitter.
photo left: The 1625 Rig out in the shop showing it on its
"roll-around" stand. This is a really convenient feature because it
eliminates the need to find bench space.
photo right: Inside the 1625 Rig showing the modular construction with the T-368 Exciter with its PTO and multiplier stages. Behind the exciter is the power supply. Under the T-368 Exciter is the Speech Amplifier section. The modulator and PA are located inside the metal box. Harness connections use Jones plugs for the most part.
Complete T-R control is provided with PTT relays that includes a receiver remote standby output and antenna input for the receiver. The microphone input uses a PL-68 type of jack that's located on the lower right corner of the front panel. CW operation keys the Exciter and the key jack is located on the rear of the chassis. Next to the front panel "spot" button is a screwdriver adjustment for the power output in the "Tune" mode. It can be adjusted from a few watts to full power (it's set for 10 watts.) The panel is a standard 19" x 10.5" size and the transmitter overall is 21" wide by 11" high by 20" deep. Weight is approximately 80 pounds. The AM100 is shown in the photo above left setting on its "roll-around" stand. This eases the problem of finding a bench location for the transmitter since it has its own mobile-stand. Although the case is molded plastic, the inside of the case is sprayed with a conductive metal coating. When the top is mounted with its eight screws complete shielding is provided.
In 2007, John revised the AM100 design with special attention to achieving good quality audio in the speech amplifier. A clipper circuit was removed and the speech amplifier rebuilt to provide natural sounding voice reproduction while maintaining a proper bass roll-off that would allow for good copy in poor conditions. A second hi-level jack was added to allow a method to input recorded material to drive the audio system. A second fan was added to keep the power transformer cool. The meter was replaced with an "easy-to-read" white scale (original was a black scale unit.)
Unlike most homebrew rigs, the documentation on this transmitter is impressive. John's manual is thorough in explaining design, intended performance, set up and operation. Plus full schematics, photographs and tube specs are provided. Without doubt the W6MIT "1625 Rig" is professional-quality in design, construction and documentation. Its performance is top-notch and it always garners great audio reports. In the past, I'd QSO'd John many times when he was using this transmitter from his Rescue, California QTH. It was a great sounding rig then and I hope it's still sounding just as good from it's new QTH in Dayton, Nevada (April 10, 2018.)
1. NC-400 Manual
2. Electric Radio - February 2011 issue and March 2011 issue - article by Ray Osterwald that thoroughly covers the NC-400 and the problems Ray found. Some easy mods that might help the audio.
3. QST October 2011 - Vintage Radio section by John Dilks covers a NC-400 found in the original box - review and general information
4. Hollow State Newsletter - 2000 reprint of article by Dallas Lankford on the 4H4-C ballast tube problems
5. Electric Radio - unknown issue - article by P. Jay Spivack N7JDT covering the NC-400 - review with some critique
6. Online - there are a few references and online articles, search "National NC-400 receiver" to find them. Some postings will be found on AM Radio group or Antique Radio group pages.
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