Radio Boulevard
Western Historic Radio Museum
 

NATIONAL  COMPANY,  INC.

 HRO  - COMMUNICATION RECEIVERS - "The Cream of the Crop"

   PART 4

Restorations in Detail - 1935 HRO SN: E-50,  1935 HRO SN: F-16, 1946 HRO-5C SN: 184 0009

  Collector's Photo Gallery of HRO Receivers

Comparison to HRO Contemporary Competition

Suggestions for Best Performance

 
by:  Henry Rogers WA7YBS

PART 4
 

1935 HRO Receiver SN E-50 - Restoration

E-50 was donated to the Western Historic Radio Museum by Jim Larsen AL7FS of Anchorage, Alaska in September 2011. E-50 was given to Jim by Val Anderson of Seward, Alaska. In the 1940s, E-50 was owned by Anderson's brother-in-law, Carl Carlson of Anchorage, Alaska. Carlson may have gotten E-50 through his son-in-law who worked for the CAA in Alaska at the time. By the 1950s, E-50 had gone to Cordova, Alaska and when Carlson died in the 1950s, the receiver was shipped to Val Anderson. E-50 had spent most (if not all) of its existence in Alaska. The receiver was stored for many years in Anderson's basement and, due to the cold, humid coastal environment of Seward, E-50 ended up with some rust issues. Additionally, several years of modifications had left E-50's originality somewhat compromised. A couple of the modifications suggest that the receiver was rack mounted (possibly when used by the CAA.)

This article is rather lengthy and changes style towards the end when I had started using a "journal" approach to these restoration write-ups that have the work progress as a daily entry, like a log. Since the restoration of E-50 took seven years from start to finish, I guess some evolution in my writing style must have also happened.

The photo left shows the underneath of the chassis of E-50 as received. The five orange paper capacitors are the only original Micamold brand types that haven't been replaced with other types. There are a handful of original white ceramic leaded-end resistors still in the circuit. Most resistors appear to have been replaced just after WWII judging by the types used. Wires that aren't black are non-original. Note the Mixer socket and the Detector socket are octals and all of the associated parts are replacements. The shielded wire from the Antenna terminal is not original.

Despite the "rough" look of E-50, it's a very restorable receiver. However, E-50 is a "long term" project. That is, the missing parts will take some time to find. The extensive rework necessary will also take time. Some of the missing or incorrect parts will need to be replicated which might require developing a methodology to achieve the desired results. Most likely, as certain original parts turn up, some rework will take place. This process will go on until E-50's restoration is finished (Who knew then it was going to take seven years!)

The chassis is painted with nitrocellulose lacquer and we are going to repaint using the original type of black nitro-lacquer paint. This means the complete removal of the old paint is not entirely necessary. The new paint will not react with the old paint and the places where the new paint covers old paint will not show since the edges are sanded and feathered. The easiest way to get at all of the rust is to just use sandpaper and wire brushes to remove it. The end result can be seen in the photo below.

The chassis then needed the miniature tube socket hole filled. It can be seen in the "before" photos above and in the "sanded" photo below. Bondo was used to fill the hole. After sanding down the Bondo, it has to be primed with lacquer and then sanded lightly again. This seals the Bondo for final painting. If I was doing this repair today I'd use "USC All-Metal" which is a metal-filled body repair paste that's activated with a catalyst. It's very strong though easy to work. It does require priming before painting.

HRO Cabinet and Panel Painting - The original black wrinkle finish used on all early radio equipment was a two-part process that used a base coat of Nitrocellulose Lacquer followed by a spray-on catalyst that was heat-activated, usually requiring oven-baking to produce the finish. Today, all we have available is a single-part process that can work quite well if the proper conditions are provided for the wrinkling process.

First, one has to observe how the original cabinet and panel were painted. Since the base coat was smooth nitrocellulose lacquer, this will appear as gloss black. Note that all inside surfaces of the cabinet and panel are gloss while the outer surfaces are wrinkle. The outer surfaces was where the catalyst was sprayed. When baked to activate the catalyst, the result was gloss black on the inside and wrinkle finish on the exterior.

Before painting, wipe down all of the surfaces with lacquer thinner. This is to remove all grease, oil or other contaminates. To duplicate the original finish appearance, we first have to spray the gloss black lacquer on all of the inside surfaces of the cabinet, the lid support lips and the back of the front panel. On the removable front rail, spray gloss black on the inside surface and only on the lip that supports the lid. Let the gloss black lacquer dry overnight.

To do the wrinkle finish successfully you should only do one surface at a time. If you try to paint the entire external surface of the cabinet, you're sure to have a problem heating the entire paint job enough for the wrinkle to appear evenly. Before painting, I set up at least two heat lamps that are nothing more than 100W incandescent lamps with the aluminum bell reflectors as the primary heat source. I also have a handheld heat-gun to apply heat to specific zones that are the usual problem areas - like edges or corners.

Now, I move on to the next surface. At the edges, I shoot the paint from over the surface being painted rather than from the opposite side. That way the edges will blend and not have an "over lap" of paint that won't match correctly. When I do the top lid, I set it up so that the lid is down on the supporting lips. That way, the gloss black will be protected from the wrinkle finish and the end result will look like the original application did - see photo below.

The wrinkle finish is very, very soft at first so it's best to avoid any contact with the wrinkle finish "surface." I have to move the pieces carefully as I set up to shoot each side. When the painting is finished, I'll let all the pieces cool down for awhile. It's best to let everything set undisturbed for about one hour. After an hour, the wrinkle has set up enough to handle and move the pieces indoors but the wrinkle finish will take about two days to develop it's hard, durable surface. The longer it ages, the harder it gets. Avoid mounting anything to the wrinkle finished surface for at least two days.

Replicating the Pearl Button Switch  -  I tried to paint an old ivory push button switch with Revlon Perfect Pearl fingernail polish. The Revlon "paint" never looked correct so I was looking for an original pearl button switch.

Thanks for the Parts - Thanks to Scott McDonald KA9P for the small pointer knob for the Selectivity control. Thanks to Bill Fizette W2DGB for the two National ceramic tube sockets needed. Thanks to Jim Allen NU6AM for the small red "NC" pointer mounting screw.

Paper Capacitor Update - A thorough examination of HRO D-65 has lead me to believe that all early HRO receivers came with Micamold brand paper caps. D-65 has 15 Micamolds with the remaining 6 caps being mostly Sprague (one Tobe.) E-50 has 5 Micamolds remaining but E-50 has an extensive repair history. It's unlikely that so many Micamolds would turn up in two different HROs unless they were original caps. A check of the underchassis photo from the 1935 and 1936 HRO manuals is clear enough to read the "Micamold" logo on the capacitors in that receiver which seems to confirm that they are the original brand used.

Nickel plating generally should be done over copper plating or brass. I had to try to nickel plate pot metal. I've plated aluminum with no problems, so I thought the pot metal would do okay. Well, the first attempt turned splotchy and dark, a sure indication that too much current was being used for the electroplating. The second attempt, at reduced current, also turned dark.  >>>

All of the work described in the above sections was done while I was living in Virginia City. The 2012 move to Dayton, Nevada certainly seemed to stop this project for awhile.

Original PW-D Found - Finally, after over two years of "keeping an eye out" for a nice condition German Silver PW-D, one has turned up. I fitted this correct PW-D to E-50 and also installed the correct red "NC" diamond to the pointer and the transformation was stunning. What a difference an original German Silver PW-D makes. 

Sometime later, I was going thru some junk and ran across a Supreme Diagnometer that happened to have the exact pearl push-button that was also used in the early HROs. I swapped the ivory (painted pearl) push-button that had been in E-50 for the "real" pearl button in November 2016.

UPDATE: NOV. 2016 - Just so readers know that I am still interested in finishing E-50, I recently found a correct "short boss" RF Gain knob for E-50. I've also found a roll of the correct wire with black cloth insulation for replacement of some of the wiring. An original-type pearl push button is installed and the S-meter scale is now the correct, early-style scale.

photo above: E-50 as it looks as of Nov. 2016.

To get restarted, I stripped out all non-original components and non-original wire. This allowed me to see that about 75% of the wiring was original and the majority of resistors were the correct National type. Only five of the capacitors were Micamolds. I still haven't exactly worked out how to get the paper shell to look like a Micamold but I think orange paper labels made from copying the original Micamold labels might work. Experimentation will be necessary when we get to that part of the restoration work. As a last resort, I could use Sprague shells since National went to that brand in 1936.

Wire - Almost all the hook-up wire in an HRO receiver is black cloth-covered stranded wire that is about 18 gauge. Since I had pulled the harness out intact for the chassis repaint, most of the original wires that were left went back into the chassis as a semi-complete harness. This meant that the tube heaters and many other wires were already present and connected correctly. All that was needed was to replace non-original, replacement wiring and to install missing wires. There were two non-original tube sockets that were mods from the past. I had replaced these with original National ceramic sockets. I was able to use the original heater wiring to connect up these sockets.

Well,... here we are again, the beginning of Summer 2017, and the various "new" radio restoration and documentation projects are getting backed up. I had to set aside E-50 due to beginning a major restoration project,...the 1935 USN RAA-3 Long Wave Superheterodyne receiver. This ultra-rare, 465 pound, three-unit receiver will consume a lot of restoration time for a while,...at least until the major work is finished. Then, during the winter, I should be able to insert E-50 into the work schedule. E-50 is close to completion with only resistors and capacitors remaining. The next session of work on E-50 will complete this long-running project.    To be continued,....

I went over to KB6SCO's storage unit on October 13th and was helping him clear out some junk. John gave me a cigar box that was "filled" (literally) with nothing but leaded-end resistors. About half of them were white ceramic with blue ink "hand-written" values while the rest were painted color-coded or paper labels over the white ceramic body. Just what I needed to complete the restoration of E-50.

The next work session on E-50 should complete the restoration. I've decided to just replicate the Micamolds as close as I can by building them by using salvaged paper capacitor shells and covering the shells with replica (well,...close copies) orange paper "Micamold" labels. Sounds bad but should look good,...I think. The resistors hopefully can be found in the cigar box but if the values needed aren't there then the leaded-ends can be used to make correct value replicas. More details when the work gets started again in a couple of months - winter project.

Finished up installing the few remaining wires that were missing. Also completed the connections to the 'phone jack and to the S-meter and S-meter push button switch.

The next step is recreating the Micamold paper capacitors. I have created the artwork for Micamold paper labels using the computer. I plan on printing the labels (probably a few per page) on orange colored paper. These will then be cut out and applied to vintage paper capacitor shells. Then a new capacitor installed in the shell and the whole assembly covered with bee's wax. This isn't how the original Micamolds were made - they used the "rolled end" method and their caps weren't wax coated. But, I think the end result will look pretty authentic and at least the caps will have "Micamold" on the label. I might try dipping the assembled replica capacitor into yellow-tinted lacquer as see if that looks more like the original Micamold caps. But, if that doesn't work out, then there's still the bee's wax option.

Dec 6, 2018 - Pulled from "stock" the new capacitors needed. 14 - 0.1uf-600vdc,  7 - 0.01uf-600vdc and one .22uf-600vdc. These are polyethylene dielectric types and all of the caps are rated at 630vdc. Their small size allows insertion into vintage paper shells. I also had to account for C7 being listed as the wrong value in the parts list. It's listed as 0.1uf-200vdc but it should be 0.01uf-400vdc. This subtracts one .1uf 200vdc from the parts list and adds one .01uf 400vdc to the list totals. Although all of the caps used will be 630vdc rated, the replica labels will have the original values shown. I'm using all vintage late-thirties Zenith Radio Company capacitor shells. No particular reason other than that I have a lot of them and they are orange. When finished, the replica Micamold caps will look moderately authentic. 

Dec 9, 2018 - Made all of the Micamold labels needed. 11 - .1uf 200wvdc,   3 - .1uf 400wvdc,  7 - .01uf 400wvdc and 1 -  .25uf 400wvdc. The artwork was copied onto orange paper with three labels per sheet except for the seventh .01uf 400vdc and the one .25uf which used a single sheet of orange paper each.

Next was to melt-out all of the old capacitors from the Zenith shells. That process took about 30 minutes. The capacitor wax is melted using a heat gun. I use long needle-nose pliers to hold the cap by one lead end and heat it up until the wax runs off into a metal wastepaper basket. Then the shell can be pulled off and the junk cap insides dropped into the basket. Then the shell is wiped down with a paper towel to remove as much wax as possible. This has to be done quickly while the shell is still hot. The shells, the repro labels and the new capacitors were all sorted into plastic bags to keep everything straight and in order.

Dec 11, 2018 - Stuffed all cap shells with proper poly caps. Secured new caps inside shells using hot melt glue. Then the cap shell ends are filled with dark brown sealing wax. This process goes along pretty quickly because I'm essentially making new capacitors rather than removing an old cap from a receiver, rebuilding it and then reinstalling. Probably took about an hour and a half to stuff 22 capacitors.

Found a complete set of 2.5vac filament tubes - 4 - type 58, 3 - type 57, 1 - type 2B7 and 1 - type 2A5. Tested all of the tubes to ensure they were all functional. Installed the tubes into E-50.

E-50 is now ready to test. 

photo left:  The underneath of the E-50 chassis now. My restoration attempts to "recreate" the original appearance of the early HRO receivers. The capacitors are very close replicas externally but have modern polyfilm caps inside. The white ceramic resistor are either originals or recreations that use vintage leaded-ends installed on correct value vintage carbon resistors that are painted white and labeled with blue ink. The wire is either original or recreated replica type wire. The only deviation in the actual wiring is the Audio gain pot and 2B7 grid bias follows the later production change (run F.)

RF Alignment - Coil set C adjusted with no problems. This coil set is an "F" prefix serial number and, while still an early coil set with white charts, it does have the larger contacts that started with the F-run. Coil set B was an early coil set with the small contacts but it did align with no specific problems. Checked D coil set, aligned, already pretty close. Worked well, listened to Pacific NW AM hams. 
 

photo right: looking into the cabinet with the lid up shows the top of the chassis details of E-50. All aluminum pieces were treated with Sodium Hydroxide (Easy Off Oven Cleaner) to clean and then impart the matte finish to the metal surface. The tuning condenser was totally removed when the chassis was restored and repainted. At that time, the condenser was thoroughly cleaned, detailed and adjusted. The gold finish on the crystal filter housing was accomplished using amber-tinted lacquer. Note the interior of the cabinet is gloss black as is the lip around the top opening while the exterior of the cabinet is wrinkle finish.

S-meter Problem - doesn't adjust and is "pegged" at FS when the button is pressed. Just to be sure that this meter (it came out of a junk RME-69) would work, I measured the DCR of the meter coil at 34 ohms. I then measured D-65's meter coil at 32 ohms. Close enough. Rechecked wiring and found that there was an error in how the push button switch was wired but correcting this didn't solve the problem. The meter movement is dependent on the plate current in the AVC controlled stages. The plate current is dependent to some extent on the value of the cathode resistors. The 1st RF cathode resistor should be 300 ohms but it was 1.2K. The 2nd RF should be 300 ohms but was 390 ohms (not bad.) The 1st IF 300 ohm was 490 ohms and the 2nd IF 300 ohm was 450 ohms (pretty far out.) Replacing the 1st RF cathode R helped. I later replaced all of the 300 cathode resistors with rebuilt white ceramics using 330 ohm CC resistors with leaded ends installed. In the later HRO manuals the values of the two S-meter bridge resistors are listed as anywhere from 200 ohms to 2500 ohms implying that they were selected during test. The values of the bridge resistors will determine the current through the S-meter. I checked H-103 and found the bridge resistors were 1.2K. I paralleled some 3.3K resistors to result in 1.2K for the bridge resistors but this still didn't correct the problem. Further checking found the problem. The screen voltage bleeder resistor R15 had been marked as a 100K (in blue ink) and when I checked its value it was around 100K so I thought it was okay. When checking the schematic closer, I saw that the value of R15 should have been 30K. I lifted one end of the resistor that was installed and it actually measured open. Luckily, in the cigar box of leaded-end resistors I found a 30K original that I then installed. This got the S-meter working correctly. The values of the bridge resistors actually function better at 1.2K as that allows the RF Gain to be fully advanced and the S-meter can still be set to zero. With the original 2K value, the RF Gain can't be set higher than 9.5 before the S-meter can't be adjusted to zero. I built two leaded-end resistors with the value of 1300 ohms and installed these for the two S-meter bridge resistors. Now the RF Gain can be fully advanced and the S-meter can be adjusted to zero.

photo above: Second production run HRO sn: E-50 completed. Note that the VHT wrinkle finish doesn't have the exact same wrinkle pattern as original "two-part" wrinkle paint but the color is very good match. I didn't put a "super polish" on the bar knob skirts although they do look a bit brighter than in this photo. I left the toggle switch "balls" worn to shown the "patina of use and age."  I prefer a "good condition original" look to the restoration.

A few things I'd do differently,...I wouldn't use "fake rivets" as I did on E-50. The original style rivets are available and the setting tools are also available and both are easy purchases on eBay. I do wrinkle finish a little differently now and I think that would have produced a "deeper" texture. I think I should have given the chassis more coats of lacquer. The originals seem to have very thick coats of paint. But, I think E-50 turned out great and it works very well.

The total time was just over seven years! That's a long time for a restoration, but I wanted to use original parts for those that were missing and locating originals just takes whatever time it's going to take. I didn't do any advertising for parts, I just waited until what I needed turned up. I could have reduced the time by advertising but things like a "good condition, nickel-plated PW-D," or a "small red <NC> dial pointer" are hard to describe to "non-enthusiasts" when advertising for parts.

Update: Dec 22, 2018 - In going through all of the spare coil sets, I found an A coil for E-50. It's not the earliest type but is one from late-1935 with a "J" prefix serial number. I replaced the black background charts with copies of the white background charts for an A coil. Tested the A coil set's operation in E-50 but, as would be expected, the coil set needed to be aligned to the receiver. Afterwards, E-50 was receiving SW BC stations around 15.3mc quite well. I'll keep watching for an early, small contact, coil set for A and also for coil C. E-50 has small contact D and B coils sets but coil sets A and C are large contact, later types.

"Detailing" 1935 HRO sn: F-16

"Detailing" is where a few minor things need to be done to a receiver but a full restoration isn't necessary. Cleaning, servicing and alignment may be required. Sometimes minor cosmetic corrections are required. Detailing doesn't require replacing capacitors or other components other than perhaps a few defective tubes. F-16 was that type of receiver. It was in good shape but certain things were incorrect. The following write-up covers what was needed to transform F-16 into a very nice condition (but not exactly original) early rack mount HRO receiver.

The electronics were typical of an old receiver that had been used up to the 1960s but never restored. The original capacitors had been replaced around 1960 using Sprague molded types with the red nomenclature, not black beauties (but similar.) The resistors were mostly original but many had small carbon resistors added in parallel to "trim" the value back to the proper resistance. There were no "cut and hack" modifications.

photo right: F-16 "as found" with wrinkle finish coil panels, sticker adhesive and dirt  

photo above: F-16 shown with its homebrew, rack mount, spare coil storage box

It was obvious that F-16 had been in storage for quite a while. Some surface corrosion was visible on a few parts but mostly the chassis was covered with lots of grease and dust. Another problem was the heavy glue residue from two stickers that was left on the front panel. With a little electronic cleaning and testing, I had F-16 working quite well. I suspected that the good quality capacitors from 1960 would probably still function. I listened to the receiver for a few weeks to see what the reliability of the somewhat vintage components would be. No problems were encountered. Even the "as-found" alignment was very good.

Detailing was going to require the following:

1. Replace wrinkle finish coil set panels with crackle finish coil set panels on A, B, C and D. White frequency charts will also have to be transferred to the crackle finish panels.

2. Replicate a typical meter clamp for the original meter to mount correctly. Fill meter mounting holes in the front panel. Shim the meter to fit the panel hole correctly.

3. Clean front panel of label adhesive residue and then recondition the front panel paint.

4. Flatten the finger-lift projection to create a rack mount type dust cover.

The exchange is easy to accomplish although it's a little time consuming. First, all of the individual coil shielded assemblies are removed from their respective panels. Four coil assemblies for each panel. This only requires loosening the mounting screws. Then the coil frequency charts are removed and the coil panels cleaned and touched-up as necessary. Then the proper charts are installed onto the correct panels. Then the correct shielded coil assemblies are mounted to their new respective panels.

It takes a couple of hours to do the exchange with cleaning taking up most of the time spent.

Filling the Meter Mounting Holes - The easiest method is to use 5 minute epoxy as the filler. I used masking tape on the front panel to cover the holes. I then fill the holes with epoxy from the back of the panel. The tape assures that on the front of the panel the hole fills will have an absolutely flat surface. After the epoxy has set up, I remove the tape. On the back of the panel, I use a razor blade to "shave" the top of the epoxy fill so that it is flat. The front and rear surfaces of the fills are then painted using black nitrocellulose lacquer to match the original paint.

The same yellow discoloration was on the PW-D. This dial is German Silver plated, so you have to be careful. I disassembled the PW-D so I was only working with the Index dial. Again, lots of Glass Plus and a very light touch with the brass brush removed the yellow contaminate and left the Index Dial the correct flat silver finish. Be sure to test the reaction of the brass brush and the German Silver. In my experience, the brass seems to be a little softer so it doesn't scratch. But, brass brushes may vary in the type of brass used for the bristles. Some brass has a lot of steel in it and that might make it hard enough to scratch German Silver. Test on the back side of a knob skirt first.

Photo right shows F-16 after the detailing.

Rebuilding the 1946 HRO-5C (HRO-5RA1 with SPC, 697PS & MRR Rack)  SN: 184 0009

Rebuilding is the term for a restoration that doesn't adhere to strict originality in component replacements or in mechanical repairs. Modern poly-film caps can be used as replacements for paper-wax capacitors. Other components may also be replaced with modern manufactured substitutes or similar vintage parts. The object is to have the receiver function correctly but still have an original external appearance. With the following receiver, the HRO-5AR1, a worn-out PW gearbox determined the overall restoration approach. Other destructive modifications also influenced our decision to apply the "rebuilding" type of restoration to SN: 184 0009.

I had to take the receiver and the SPC out of the rack to be able to move it easily. SN: 184 0009 was installed in the correct MRR rack although the side trim pieces weren't present. The SPC unit had a Type 697 power supply mounted on top of the coil box. This 697 was built on a regular chassis using a potted power transformer, potted choke, can electrolytic and a 5Y3 rectifier tube (serial number was 200 0009 indicating the original 697 power supply.) Also, mounted on top of the coil box was an aftermarket 100kc crystal calibrator (Hammarlund) that was wired into the 697 using a fairly crude-looking harness. The calibrator power switch was mounted on the left side of the SPC panel (a stock hole for a pilot lamp was used.) There were four coil sets in the SPC and one in the receiver. Coil sets D, C and B were matching "0009" serial numbers but coil A wasn't matching and the last coil was a rack mount HRO-7 AC coil (15 meters, black wrinkle panel.) At the time SN: 184 0009 was originally sold, 15M wasn't a ham band yet. The HRO-7 coil was probably purchased when 15M was opened.

The receiver, being a rack mount, has a dust cover. Originally it didn't have a lid so access to tubes required dust cover removal. A former owner modified the dust cover to have a lid. Although relatively neatly done, the design wasn't thought out very well since there isn't a stop for the lid. To keep the lid from resting on the tuning condenser, a screw with electrician's tape wrapped on it was wedged into one of the upper dust cover mounting screw holes (these two upper dust cover screws were cut off.) The receiver itself appeared to be in good, not extensively modified condition.

The receiver was called the "HRO-5C" or "HRO-5C Deluxe Receiver Combination." In earlier versions of a deluxe, rack-mount HRO receiver with rack speaker and coil storage, this type of combination was available as early as 1941. This version is from late-1946 and consists of the HRO-5RA1 receiver, the SPC coil storage box and loudspeaker panel with 697 chassis type power supply mounted on the coil box behind the loudspeaker. These units were mounted in the MRR table rack that included side mount trim rails and three stainless steel trim strips. 

Shown to the left is the listing for the HRO-5C from National's Catalog No. 700 that was published in the 1947 edition of the ARRL's Radio Amateur's Handbook. No prices are shown in the listing but the standard HRO-5TA1 was selling for around $300. This price included the speaker and power supply along with the four standard coil sets. The HRO-5C was probably priced around $375.
 

MRR - Very good condition. Missing the two side trim pieces. Has one chrome strip but missing the other two.

SPC - Seems to be in very good condition. Aftermarket (Hammarlund) 100kc crystal calibrator added. 697 has replacement electrolytic.

HRO-5RA1 - CW Oscillator variable capacitor was missing the arm that actuates the BFO switch. Most original capacitors have been replaced. One Sprague .1uf 600vdc molded cap was just laying in the chassis. Appeared to have broken loose somehow (tack soldering.) PW-D wasn't assembled correctly. I removed it and corrected the assembly and reinstalled. The gearbox was very noisy and rough feeling in areas of tuning. I removed top cover to inspect the gearbox and found that the teeth were practically stripped off of the right side gear. I've never seen a PW gearbox with this much wear-damage. It will have to be replaced. S-meter scale had darkened quite a bit. I have a very good condition, correct style meter to replace it. PW-D had a "fat finger" wear-ring surrounding the knob. I have an excellent condition replacement PW-D. Receiver front panel was very good but will need to be "touched up" in a few places. In the past, this HRO-5RA1 was apparently used "hours on end." I've never seen so much wear on knobs, the gearbox and several other parts. The receiver was obviously repaired many times over the years. The workmanship was below average, almost worse than "hamster" rework. The soldering joints for installing replacement caps were all "tack soldered" joints which resulted in many compromised connections. Well,...on to the repairs.

I decided to use the RAS-5 tuning condenser because the  RAS-5 receiver is damaged by excessive modifications and unnecessary cutting and hacking. It's beyond restoration. Removal of the tuning condenser assembly wouldn't affect the RAS status at all. The HRO-60 receivers were actually complete, working sets although they had serious cosmetic problems. I didn't want to relegate the HRO-60s to "parts set" status. With thorough cleaning and use of the cad-plated gearbox cap, when installed, the gray paint of the RAS-5 tuning condenser shouldn't be too obvious.

PW Gearbox photo shows the extreme wear and damage to the gear teeth. The problem was caused by an excessive anti-backlash setting. Worm gear was extracted before the photo was taken.

More Repairs - Repaired the BFO adjustment condenser by replacing the on-off switch and installing a replacement switch arm. The B+ on-off switch was a modern toggle switch which I replaced with a good original HRO B+ switch. Rebuilt the S-meter to use a brighter scale. Replaced the thumb nuts on the ANT-GND terminals with good originals. Replaced the chassis to GND wire connection with original spade lug and correct type wire. Cleaned and touched-up the front panel. Installed all missing lock washers, screws, nuts and other missing hardware. Installed replacement PW-D. Cleaned and installed all original knobs. This completed the cosmetic and mechanical rebuild. Next is the electronics rebuild. 
 

June 10, 2019 - Preliminary Test - I connected a "dog house" 697 power supply and a MCS table speaker to the HRO-5RA1 just to see how it would function with the RAS-5 tuning condenser. The one "loose" capacitor was the cathode bypass on the first RF amplifier. This was reinstalled before power-up. With the AC turned on the receiver eventually showed some signs of "life." Nothing could be heard while tuning around the 40M band with a 20ft wire for the antenna. I injected a 456kc signal into the mixer grid and a strong carrier resulted. I injected a 5mc signal into the antenna and tuning the receiver to 230 on the PW-D I did hear the signal generator. However, it sounded like the generator was acting as the LO. In another test, I injected 5.455mc into the LO grid lead with the LO tube pulled. I tuned in WWV 5.0mc using the signal generator. This showed that the receiver was functional except for the LO.

June 11, 2019 - LO Repair and Yet More Repairs - I checked the LO components and found that the screen load resistor was about 4.5 meg ohms. It should be 47K ohms. Further checking revealed that the LO coil contact fingers had one finger severely bent to the point where it wasn't contacting the coil button. I replaced the bad resistor with a NOS 47K carbon 1/2 watt. I scavenged another LO finger contact assembly from the RAS-5 "parts set." Additionally, testing the Audio Gain pot (not an original part either) showed it was intermittent. I pulled the original Audio pot out of the RAS-5, rebuilt it (you can rebuild the originals) and then installed it. Upon power-up, the HRO-5RA1 now functioned more-or-less correctly. A quickie IF alignment and a quickie C coil General Coverage alignment had the receiver working pretty well. However, there are still a few minor issues that are probably caused by the menagerie of mis-matched capacitors that had been "tack soldered" into the circuit over the years. The receiver would certainly benefit from a complete capacitor replacement job. Since the tuning condenser is obviously not original, it wouldn't make sense to restuff cap shells for the under-chassis appearance. Just a standard recap on this one (well,...almost standard.)

These two photos right show the back and the front of the SPC unit before repair. Note the chassis type construction of the 697 power supply. The filter capacitor is a replacement type. The chassis toggle switch selects either 115vac or 230vac operation. The parallel 15 ohm 2 watt CC resistors and the toggle switch were probably for some sort of speaker muting that was never incorporated. The tag on the front of the coil storage door is the standard 697 power supply tag.

Coil Set Cleaning - All of the coil sets needed to be examined closely due to the lack of technical ability found in the repair work performed on the receiver in the past. Each coil shield was removed, the coil examined, cleaned and a very small drop of DeOxit applied to the trimmer capacitor adjustments. The exterior panel and chart plates were cleaned. These coils were now ready for alignment.

IF Alignment - The "quickie" IF alignment I had done earlier had shown that former alignment was pretty far off. Checked the crystal filter active frequency and found it to be 455.2kc (spec is 456kc +/- 2kc.) The IF alignment is standard procedure except for the crystal filter. National provided a really easy alignment procedure for the later crystal filter used in the HRO-5A1s, NC-200 series and the RAO receivers.

RF Tracking - It's very common when aligning a HRO coil set to find that the low-end tracking has never been adjusted over the years. It seems that if any alignment was done, just the high-end trimmer was adjusted and that was all. As expected the low-end tracking was off by quite a bit, 60kc off on coil set D and 30kc off on coil set C. Coil set D has a padder capacitor so adjusting the low-end was easy. An inductance "loop" adjusts the low-end tracking on coil set C (also B and A.) Almost always, the inductance loop is right up next to the wall of the coil form so there's apparently no adjustment left. However, return the loop to the center of the coil form and then readjust the high-end trimmer. Now, adjust the low-end with the inductance loop. This "reset" procedure usually works for getting a C, B or A coil set to track correctly. On Band Spread, all coil sets have padder capacitors so low-end tracking is easy to adjust.

RF tracking adjustment is tedious on all HRO receivers because each coil set has to be adjusted twice, once for General Coverage and then again for Band Spread. 

BFO Tuning Condenser Problem - After the rebuild and alignment, the HRO-5RA1 was working very well,...at least on AM. On CW or SSB, I had a difficult time finding upper or lower sideband using the BFO tuning condenser. I finally removed the variable condenser out of the circuit for a closer examination. Someone had broken one of the rotational stops and then tried to compensate by moving the remaining single stop. The correct setting is for one stop to limit further rotation at "OFF" and the other stop will limit rotation somewhere above "10." I set the remaining single stop to function with the condenser just past full mesh. I then adjusted the BFO switch arm to allow switching "ON" with the condenser at full mesh, half-mesh would be at "5" and full un-mesh at around "10." I reinstalled the BFO variable condenser and installed the knob. In operation this set-up then allowed the BFO tuning to select either upper or lower sideband since the BFO was set to 456kc at half-mesh. SSB signals are now easily to tune. Normally, if the BFO variable hasn't been tampered with you can set 456kc zero beat at "6" on the dial and everything will work fine. I had to go through this procedure because of a "hamster tamper" of the BFO variable.

The initial power-up using the SPC speaker produced no sound. Yet another "tack solder joint" had broken and that had left the speaker disconnected. Cleaned the joint of old solder, made the joint mechanically sound and then soldered.

photo right: This shows the top of the chassis after the installation of the complete tuning condenser and PW gearbox from the RAS-5 "parts set." It's difficult to tell the tuning condenser isn't original but note that the frame, end plates and dividers are painted gray. Also note the channel pieces on the inside of the end plates - these aren't on the later condensers. With the cad-plated gearbox cap installed, the gray paint isn't very obvious. The good news is that this RAS-5 condenser functions great and the PW gearbox operates smoothly. 

A note on painting, the first paint used was Rustoleum Semi-Gloss Black. Using most Rustoleum paint-types will result in an inferior coating and I should have known better than to use it. The can clogged up after about half of the paint was used. The "Semi-Gloss" was really a high gloss paint and had way too much shine. I sanded it down and painted over with Krylon Flat Black only to have the Rustoleum base coat (that was supposed to also be a primer) wrinkle and crack ruining that paint job. I had to strip that paint and start over. I found that methylene-chloride stripper is not carried by very many of hardware stores nowadays. I tried non-methylene "Kwik Strip" made by Klean Strip and it worked quite well. I also bought a can of Krylon Matte Black. The repaint turned out great and the Matte Black was very close to the original black on the MRR rack. 

A drawing of the side trim pieces is shown to the right - just in case you need to make a set for your MRR version rack. If you are going to fabricate these pieces, remember that the two sides are different, that is, one is for the left side and the "mirror image" is for the right side. Drawing shows the two different sides.

Stainless Steel Trim Strips - I only had one original trim strip. There should be three. One below the receiver, one above the receiver and one above the SPC unit. The original stainless trim piece appears to be a very thin, flat metal piece bent multiple times to form a "T" shape. I decided to use stainless trim strips harvested from derelict Bud cabinets. The red paint had to be stripped off first. Then about 0.5" of the strip was flattened using a metal dolly and hammer. The strips could then be inserted into the table metal brake and bent at a right angle. This left about 0.625" of embossed trim with a flat extension out the rear for mounting. These replicated trim strips were placed at the top of the SPC and the bottom of the receiver. The original was notched for the top thumbscrews and a cut-out made to allow for the bottom of the latch. These modifications were necessary to have the trim fit as flat as possible. Some adjustments were necessary to the MRR rack top piece to allow enough clearance for the trim.

Pilot Lamp in SPC? - The National artwork for the HRO-5C shows what looks like a pilot lamp in the right side of the SPC but what would it indicate? There's already a pilot lamp in the receiver along with the illuminated S-meter. There had been an extra toggle switch in this position of the SPC to turn the B+ on to the 100kc crystal calibrator when needed. I didn't really need the calibrator so it was removed. I installed a vintage red pilot lamp assembly in place of the switch but I didn't connect it to anything. I've been thinking that a more useful application than a redundant pilot lamp would be to use the extra toggle switch to have the BSW terminals wired in series external to the receiver. The HRO-5 circuit changed the BSW terminals to be in parallel with the receiver B+ switch. In this wiring, a remote relay would operate the B+ with the receiver's switch in the B+ off position. However, if you want to change coils and the remote relay is NC for receive then you can't switch off the receiver B+. If the BSW terminals were wired with the extra toggle switch in series, then that switch could turn off the B+ for coil changes regardless of the remote relay position. National probably did the BSW parallel change because of WWII uses of the receiver and the stand-alone requirements to short the BSW terminals may have caused confusion for those setting up the receiver. Certainly, National had a change planned to return the switchable B+ because HRO-50 and HRO-60 receivers allow the user to select how the remote standby affects the receiver standby switching.

Performance - HRO-5C SN:184 0009 had a pretty hard life. It was plagued with a multitude of minor problems and a couple of serious ones all caused by repair work that was careless or incompetent. But, it's working fine now and exhibits all of the positive points that the HRO-5 upgrades provided. I have one other HRO-5A1 receiver, SN: 184 1054, built towards the end of the production run. Both of these receivers perform similarly with excellent sensitivity, an easy to operate crystal filter and a less frequency-sensitive S-meter. The manual specs the sensitivity at better than 1uv which is believable. Conditions aren't particularly good this time of the year (summer 2019) but still DX signals on 20 meters are easy to find. East coast stations on 40M are pretty easy to find by mid-afternoon. I think the audio on this particular rack mount set up is about the best I've heard. Typically, rack mounted HROs using a SPC type of panel speaker will tend to sound a little thin since there isn't an enclosure for the speaker. But, this one sounds almost like listening on a table speaker. Overall, a very nice and usable vintage ham receiver.

UPDATE: Aug 30, 2019 - Like a lot of rebuilds, 184 0009 required a revisit to correct a subtle problem that continued to worsen over time. I had noticed a slight hum with the AF Gain turned to 0. Also, the was slight distortion on 20M signals (sort of slight low frequency modulation.) Finally, in late-August, the hum was becoming very apparent and signals were badly distorted. I had left the chassis mounted 697 power supply filter in place since it was a replacement and was okay - at first. It didn't take too long for it to break down - two months. I replaced the filter with new discrete axial mount electrolytics. This, of course, cured the hum problem. Most of the time, after a rebuild, you will find yourself back doing some work that should have been taken care of the first time around.   

photo above: The completed HRO-5C SN: 184 0009.
 

Collector's Photo Gallery of National HRO Receivers

E-mail us a photo of your HRO receiver along with some comments about your opinion of your HRO's performance. If you are restoring your HRO, feel free to relate any rework experiences you would like to share. Photos can be of any of the HRO family of receivers. E-mail your HRO photo to:  Western Historic Radio Museum - HRO Photo

Paint Note: The photos of G-235 and of F-09 are particularly good at showing the heavy texture of the early, two-part wrinkle finish that National used on their receivers. Note that there is a secondary pattern that is larger than the primary "wrinkle" pattern. It's unfortunate that the modern, one-part wrinkle finish doesn't duplicate this dual wrinkle texture pattern. You can reference the last photo of E-50 in the Restoration Section above to see what to expect from modern VHT Black Wrinkle Finish, one-part wrinkle finish.

HRO Senior SN: 170-H - This HRO is from Great Britain and dates from about 1940. It is owned by John G3SLX. 170-H is in great original condition and features an unusually heavy texture wrinkle finish paint. Note the plug-in crystal on top of the Crystal Filter.

This RCCS modified HRO Senior belongs to Fred Archibald VE1FA, who has seen six other examples of this version of the HRO. Several versions were seen at swap meets in Canada and a couple have shown up on eBay. Photo provided by Fred Archibald VE1FA.

* Tom Brent reports that in 1975 Fair Radio Sales offered (on the front page of their catalog) these receivers "in the wooden crate as refurbished by the Canadian government" for the price of $125 with five coil sets included but no power supply. This information explains why so many of these RCCS HRO Senior receivers are found supplied with later HRO-5 type coil sets.

HRO  Performance Comparisons

The HRO Design - The HRO was the only 1935 receiver that came with two stages of tuned RF amplification provided on all frequencies. This virtually eliminated images and later became the standard method used in single conversion superheterodynes for image reduction. The use of a plug-in coil module that shielded each coil and itself was installed into a shielded compartment assured that stray losses were not encountered. Connecting the coil set to the tuning condenser via flex finger contacts and short, direct buss wire assured that tuning losses were at the lowest possible. Efficient design of each tube stage, especially the First RF Amplifier, reduced thermal noise and increased the signal to noise ratio. These design factors allow the user to experience the best in sensitivity with low internal noise. Copying very weak signals is standard with the HRO - the signals would not even be heard on some of the other receivers of the time. The other stand-out feature was the incredible bandspread. Though only available on the ham bands 80, 40, 20 and 10 meters, it gave the user unbelievable tuning with the micrometer dial providing the equivalent of nine and a half  feet of linear dial for each ham band. Tuning across the ham bands seems to go on and on - it was literally the best bandspread system available. 

Performance Today - What about the HRO performance today with modern QRN and QRM levels? The HRO is a very sensitive receiver capable of receiving DX signals that would be lost in the noise on other receivers. Of the many types of vintage receivers I've used over a long period of time, I have always found the HRO to be the most consistent great performer when it came to receiving weak DX signals. The HRO is the only vintage receiver that I've used that would receive Russian hams on 20M CW using just a wire antenna (Inv-Vee.) This was in the mid-1970s when most UA/UKs were not running much power and were always just above the noise, even in a then-modern receiver. Additionally, when it comes to QRM, the early HRO crystal filter is quite amazing. It is one of the few crystal filters that will eliminate QRM, even on AM phone operation. By tuning the AM signal "on the nose" with the crystal filter on you will notice just how narrow the bandwidth has become and a slight detuning will decrease the signal significantly. The same is true for CW but these days there is little CW activity, let alone CW QRM.

Here's a profile of four communication receivers that were the HRO competition,...

The Hammarlund Super-Pro was introduced in March 1936 (for the civilian market) as a high performance, professional receiver. It is a great receiver that was built to a high quality standard and performed quite well. The Super-Pro had variable coupled IFs, high fidelity audio, accurate direct frequency readout. It also used double preselection on all bands and used a separate power supply. The only complaints heard regarding the Super-Pro is limited frequency coverage (this is for the X and LX versions though, the SX covered 160M to 10M), high front-end tube noise masking weak signals (mostly due to improper antennas) and non-calibrated bandspread dial (it was a commercial receiver, too.) In actual use, the Super-Pro is an excellent receiver with great audio but if you are looking for weak DX signals, you will have to use a matched antenna. Hammarlund expected the user to operate the receiver into a matched antenna system and did not provide an antenna trimmer adjustment. On the audio side of things, the Super-Pro has 14 watts of high fidelity audio available and it can sound incredible when the speaker is matched to the audio output Z. The Super-Pro has the power to drive the speaker to a thunderous volume. Certainly a rebuilt Super-Pro will perform as well as a rebuilt HRO in almost every type of reception. Separate power supply and speaker are the required accessories. Photo shows the 1936 SP-10 version of the Hammarlund Super-Pro.

Using the HRO "On the Air" as a Vintage Communication Receiver

The easiest device to use is the HRO's own passband selectivity. For very near-frequency QRM,...if you're in the AM mode and the QRM is from a SSB signal, try tuning "off frequency" a couple of kilocycles from your frequency of operation. You can usually greatly reduce interference and still recover enough audio for decent copy of the AM signal. Usually two or three kc, either above or below the operating frequency, will drop the QRM enough for good copy. The SSB operators will always have filters in their rigs to limit their bandwidth to about 2.1kc. A typical AM signal has about 6kc of bandwidth. Also, the typical AM envelope has the audio information in both sidebands. >>>

Depending on the type of QRM, for instance, adjacent signals on both sides of the operating frequency, it might be more advantageous to reduce the HRO bandwidth. Using the Crystal Filter you can narrow the bandwidth down to less than half a kilocycle. It's amazing how narrow the receiver bandwidth can be and still provide decent copy of an AM signal. However, it's very important that you tune the desired AM signal "on the nose" for good copy. Of course the audio will sound "muffled" but it will still be intelligible. Of course the audio fidelity suffers a lot in this mode. But, the goal is successful copy and a completed AM QSO or net operations,...in other words - communications.

Suggestions for Best Performance

More Alignment Notes - Proper alignment also is important for best performance with any receiver. Though it is possible to find an HRO receiver that is mostly original parts and is functional, it certainly will not be working at its design limits. A full rebuild is usually required for top performance. This is also true for orphan coil sets which might seem to function okay in a rebuilt HRO but full performance capabilities require that the coil set be fully cleaned internally, trimmers should lubricated and the coil set fully aligned to the HRO receiver that it is going to be used in.  

The original coil set alignment instructions had the user remove the upper front lid support that's held in place with two screws at each end and by the dial pointer mounting screw in the center. This was to allow easy access to the coil trimmers. I've found that if you use a very long and thin blade screw driver with the metal shaft wrapped with tape (to prevent shorting to the condenser frame or the cabinet) all of the coil trimmers are accessible without removal of the upper lid support.  

The basic alignment is described in "Restoring and Aligning HRO Coils Sets" elsewhere in this article. The specific manual for the particular type of HRO will generally provide more details and should also be used when aligning the receiver. As far as the alignment of the IF and Crystal Filter, this is standard faire and as long as the IF is aligned to the exact crystal frequency, no problems will be encountered.  >>>

>>>  The IF is 456kc and the crystal will be very close to this frequency. However, sweeping the signal generator across the IF can determine the exact crystal frequency and this is the frequency that the IF should be aligned to. In fact, your Crystal Filter will function better if you perform the IF alignment with the Crystal Filter on and set for fairly narrow selectivity. This assures that the Crystal Filter is operational and can be a useful tool for combating QRM.

Although the manual will recommend that the BFO be set to zero-beat with the control at "9" this is not really necessary and actually reduces the effectiveness of the BFO. In the 30s and 40s the BFO was for CW and upper and lower sidebands don't even exist in that mode. Today we have SSB and by setting the BFO to zero-beat at 6.5 on the scale will allow you to select either upper or lower sideband when receiving SSB. As with all early receivers, the RF Gain must be reduced and the AF Gain set to near maximum with the AVC off and the BFO on to receive SSB signals. You have to set the receiver RF Gain so that the signal to BFO injection ratio is correct for non-distorted SSB reception.

Coping with Drift - Drift is just a "fact of life" for early receivers. There is a lot of discussion today for ways to eliminate drift in old receivers but it really depends on your individual method (or preference) of operation. Almost all amateurs in the 1930s and 1940s were on CW and they were always tuning the receiver during a QSO. That was just the way it was - you followed the received signals by frequent adjustment of the tuning dial. It became "second nature" for hams to "tune around" for the best reception during a QSO. By the 1960s and later, SSB voice comms were becoming more and more popular so a different belief was beginning to be instilled into hams - "you shouldn't have to touch the tuning dial on the receiver. The received signals (the transmitter) shouldn't drift and neither should your receiver!" As SSB became the dominate voice mode, the receiver required virtually no drift from either the LO or the BFO. This has resulted in today's hams expecting "drift-free" operation in a receiver that was designed when CW was the primary mode of amateur comms and changing the frequency of the audio tone heard in a drifting signal didn't affect copy at all. Although "drift-free" design was possible, the components and circuits necessary made this expensive to achieve. Considering the economic factors of the Depression it became next to impossible to build a "sellable" receiver that didn't have some drift. The HRO will drift in frequency for about 30 minutes and then settle down somewhat. In the bandspread mode the drift is much more noticeable. This is because of the resolution of the PW-D in the bandspread mode. A small drift of a couple of kilocycles appears as a drift of five to ten "index marks" on the micrometer dial (depending on the coil set used.) So, if you want to operate your HRO "as original" you'll have to get used to the drift. Otherwise, you can change the LO coupling capacitor to a NPO type and add in zener regulation on the oscillators or build a regulated power supply for the receiver. But, original is more authentic and this is about nostalgia, isn't it?

The HRO's later kin - the HRO-50 and HRO-60 are fairly popular today with vintage AM enthusiasts, still giving competitive performance even on 10 meters. These later receivers are larger and do feature a built-in power supply, push-pull audio and a direct readout dial which seems to confirm our opinion as to why there's a lack of interest in using the pre-WWII HRO as the station receiver.

Today, the HRO is revered as one of the great designs of pre-WWII communication receiver production. Though collector emphasis and resulting high prices are on the rarer early versions, any of the HRO receivers will give the user superior performance, especially following a careful, thorough rebuild and alignment. With a production history spanning 30 years, certainly the HRO design concept was a long-lived one. It was a great receiver then and is still a fabulous performer some seventy-plus years later. It is National's "masterpiece."

photo above: HRO sn: H103 from September 1935 (fifth production run)

References:

1. "The Wonderful HRO Receiver" by William Orr W6SAI,  published in CQ magazine, May 1975 - Orr's article describes the origins of the HRO receiver

2. "The HRO Report: Dating the Early HRO " by Charles Fisher,  published in AWA, Oldtimer's Bulletin, Vol. 4 - 1989 - Fisher deciphers National's serial number system for the HRO, production history and engineering changes from 1935 up to 1941. Fisher's article contains detailed information on early HRO production history and his observations from the results of a survey that references over 70 examples of pre-WWII HRO receivers. Fisher's article is available on the web at:  FISHER: Dating the National HRO  

3. "The Evolution of the National HRO and its Contribution to Winning WWII"  by Barry Williams KD5VC - Excellent detailed history of the entire HRO line, info on British use in WWII. Barry's article is available on the web at:  Evolution of the HRO - Barry Williams

4. QST Magazines 1934 through 1939, various letters by James Millen, ads, etc. - Info on product development and upgrades, ads show evolution of the design

5. National Co., Inc. - HRO Manuals - detailed circuit description, design intent and performance expectations

6. Rider's Perpetual Troubleshooter's Manual - Vol VIII - most of the information needed for the early HRO receiver is in VOL. VIII

7. Article in Shortwave Craft  March 1935 by James Millen - Basically, this is a detailed advertisement for the HRO, circuit description and schematic

8. National Co., Inc. Bulletin #250  1936 Catalog - Information on model numbers, availability and prices

9. National Co., Inc. "The HRO" Advertising Brochure - Information on circuit design and performance

10. "Discovering Vintage Radio" - Chapter 14, "National's Legendary HRO" - This is a British book with some interesting history and information on the HRO

11. "Communications Receivers - The Vacuum Tube Era" by Raymond Moore - Excellent reference book on HROs and all other models of tube-type communications receivers

12. Thanks to all of the National Co., Inc. and HRO enthusiasts that have sent in photos, serial numbers and other information. Your input has helped to expand this article and add detailed information on the production and history of these great receivers.
 

Entirely Re-edited, re-styled and expanded with new material to match our other communications receiver articles, corrections made, new photos - Henry Rogers © September 2011

More information and photos added: November 2011, January 2012, new photographs February 2012

March 2012 - Corrections to HRO-5A1, HRO-6 and HRO-7 introduction dates   June 2012 - Minor corrections, Royal Canadian Corps of Signals HRO Senior - Fair Radio Sales info added,

July 2012 - Additions to details on HRO-5A1 "late versions" and relationship to HRO-6

Jan 2013 - HRO Junior information expanded with new photos

May 2013 - Correction to AWA reference - now Amalgamated Wireless Australasia

August 2013 - More details on two-letter prefix SNs from WWII production

January 2014 - More details on alignments of Coil Sets and IF alignment. Correction to assembly of the worm gear thrust assembly.

May 2014 - Add info and photos of HRO N-130

Nov 2016 - Update E-50 with new photo

April 2017 - More E-50 info, partial completion of the restoration project

Nov 2018 - Added info and photo of HRO F-16

Dec 2018 - Added info on completion of HRO E-50 restoration, added new photo of H-103, updated info on F-16

July 2019 - Added info and photos on rebuilding HRO-5C SN: 184 0009

Nov 2020 - Edited new info regarding HRO-5TA1 with serial number 189 0081 indicating tha some HRO-5TA1 production was in run 189 formerly thought to be entirely HRO-7 receivers.

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