Radio Boulevard
Western Historic Radio Museum
 

NATIONAL  COMPANY,  INC.

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

   PART 4

Restorations in Detail - 1940 HRO Senior SN: 463-K,
1946 HRO-5C SN: 184 0009, 1947 HRO-7T (Black Wrinkle) SN: 189 0396

  Collector's Photo Gallery of HRO Receivers

Comparison to HRO Contemporary Competition

Suggestions for Best Performance
 
by:  Henry Rogers WA7YBS

A Pre-WWII HRO-C being used in Czechoslovakia (note map)

PART 4
 

1940 HRO Senior Receiver  SN 463-K - Restoration
 
The following is an article I wrote in 2006 for use in  "Assembling an Authentic Pre-war Ham Station" that detailed the restoration of my 1940 HRO Senior SN 463-K.
Temporarily deleted - but not lost. Here it is again....

I have had HRO 463-K for over 40 years having purchased it from Fred Winkler, an old radio collector friend of mine, for $40 back in the early-1980s. I never did much with it except acquire a complete set of coils (it did have the matching C coil with it,) replace some missing tube shields and a general clean-up. I thought back then that the performance wasn't as good as other HRO receivers that I had (and had sold) but I never investigated further. So, the HRO became just part of the pre-war equipment that was on display but not used. A rekindled interest in early HRO receivers prompted me to take another look at this old relic of mine. With a little coaxing the HRO did seem to function. An alignment also seemed to help but generally the receiver didn't have the sensitivity expected and also gain across the band was not consistent.

A Broken Contact on the Mixer Section

A complete re-cap job was performed on the HRO but rather than just replace capacitors with no regard for under chassis appearance, I decided to re-stuff all of the paper wax caps to preserve the original looks of the assembly.

Performing this type of detailed rework turned up something unusual - a factory miss-wire that was not caught during test and alignment at National. The error was that C9, the bypass cap for the 2nd IF transformer secondary, was connected to the wrong side of R-8 which placed C9 connected directly to the AVC line. Additionally, C9 was only wrapped to the terminal strip - not soldered. Obviously, this was a "transparent" type of problem, that is, a problem that would not be caught in test because the error would not have affected performance enough to be noticed. Then one also has to take into account that the receiver was used for many years and this problem never noticed - certainly not a major concern but interesting how these sort of errors are found so many years later. When finished with the re-stuffing of the capacitors all resistors were checked for correct value. I usually allow about 25% tolerance but all of the resistors were much better than that. The photo to the right shows the finished result.

As for the top side of the HRO chassis, a thorough cleaning showed that there were a couple of minor rust areas. These were touched up using artist's acrylic that I matched to the exact color of the particular chassis or cabinet. Since every painted piece of an old receiver has aged somewhat differently the colors always require custom matching. Fortunately the areas involved on the HRO were small and touch up not noticeable.

The tuning condenser had to be removed for the mixer finger contact assembly repair so the entire assembly was cleaned and lubed while off the chassis. When the gearbox cover was removed for cleaning and new lubrication it was found that one of the anti-backlash springs had come off of its mounting peg. Just another example of minor problems that turn up while doing a detailed restoration.

Under the Chassis with Rebuilt Capacitors Installed

463-K Chassis after Cleaning

The S-meter scale was severely discolored which is typical for plastics of this time period. I happened to have a very nice original Marion Elec. Inst. Co. S-meter that had a nicely colored scale. Although the red was somewhat faded, the overall appearance was a vast improvement over the original meter scale. The cabinet and front panel were in nice shape so only minor touch up was necessary to have everything ready for reassembly.

After installing all of the tested tubes, shields, the front panel and knobs we were ready for alignment. The HRO has an IF frequency of 456kc +/- whatever the exact crystal frequency actually is. This is determined by sweeping the frequency around 456kc using a signal generator with the crystal filter on and listening for the peak that occurs at the exact crystal frequency. In my case, the crystal frequency was 456.3kc and the IF was aligned to that frequency using the S-meter as an output indicator.

After the IF alignment, it is necessary to begin aligning each of the coil sets. You have to start with the general coverage section first then proceed to the bandspread section. Switching from general coverage to bandspread requires moving four flat head screws located on each of the molded contact pin sections on each coil. The graphs give the micrometer readout to frequency relationship and usually all that is necessary is to trim up the adjustments on top of the coils. These are accessible from the top of the chassis through the eight holes just in front of the tuning condenser mount. I use a long shaft blade screw driver with the shaft taped to prevent shorts against the cabinet or tuning condenser spacers.

463-K in a 2006 Photo

UPDATE: 2018 - It's been over ten years since I restored 463-K. Over those years I was able to find several orphan coil sets to end up with eight coil sets for 463-K. Low Frequency coil sets G and H provide coverage from 100kc up to 430kc. Coil sets E and F cover from 480kc up to around 2mc. Coil sets D, C (the only "original-matching" coil set,) B and A are the standard coils sets providing coverage up to 30mc. All eight coil sets were aligned specifically to K-463 and this has resulted in the receiver being a very consistent performer. It always seems to operate fine even if it hasn't been powered up in a year or two. 

Rebuilding the 1946 HRO-5C Deluxe (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 directly 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 "rebuilding" to SN: 184 0009. This is a rare receiver and this rebuild could always be revisited and upgraded to a "restoration" if a correct vintage tuning condenser assembly and a correct and unmodified dust cover were found. Finding a complete set of vintage Sprague paper capacitors (for their shells) would be easy.

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 HRO-5 AC coil (15M BS) with a HRO-7 15 meter graph installed. At the time SN: 184 0009 was originally sold, 15M wasn't a ham band yet. The AC coil was certainly a later creation that was put together from parts after 15M was opened (there weren't any 15M BS coils available for the HRO-5A1.)

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. The black wrinkle finish paint has a heavy texture.

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 "way below" average,...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. Note that the left side gear teeth are not worn since they never engage the worm gear but the right side gear teeth are half-worn-through with extreme wear.

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 Matte Black was very close to the original black on the MRR rack. Krylon's the best!

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 an 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 (and the HRO-7 had the BSW in parallel with the B+ switch.)

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. I hope, someday, that an correct vintage tuning condenser assembly and an undamaged correct type dust cover can be found. At that point, I'd redo this refurbishment as a professional-quality restoration.

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.

RESTORATION UPDATE NOTE: 2024 - I'm still looking for a good condition, complete HRO-5A1 type tuning condenser and gearbox for 184 0009. I did see one come up on eBay but it had severe corrosion and one of the bakelite bearing caps was broken. Once I've located a good tuning condenser/gearbox, I'll have to concentrate on the dust cover problem. I think the original damage can probably be repaired by aluminum-welding the hacked-out lid. Of course, the entire dust cover would have to be stripped first, then heli-arc welded and then grind the weld smooth. Then the dust cover could be repainted black wrinkle finish. Once, that work was accomplished, I'd redo all of the capacitors by installing them into original Sprague shells. That's the plan anyway, providing I can find a good condition, complete HRO-5A1 tuning condenser and gearbox to start things going.  

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

Restoring the HRO-7T "Black Wrinkle Finish" SN: 189 0396

These black wrinkle finish HRO-7T receivers were likely a special order for a commercial user. Other than the paint job, the receiver is a standard HRO-7T. When found, this black wrinkle finish HRO-7T was in very nice cosmetic condition. It had all of the original components under the chassis. Although it appeared almost "unmodified" a detailed under-chassis inspection revealed many minor modifications that had been carefully installed with no holes drilled and no original parts removed. Of course, with septuagenarian paper-dielectric capacitors still in place, it was doubtful that the receiver would work at all. But, after a careful inspection, it was powered up and it did marginally function on "all original parts." We all know that you can't have "all original" and "fully functional and meeting spec" when dealing with "all original" 75 year old electronic devices. So, this restoration is primarily electronic and, since I'm trying to keep as much originality as possible, all of the original paper capacitor shells were restuffed to preserve the under-chassis appearance. For cosmetics, I did have to repaint three coil sets to match the receiver's black wrinkle finish so there's some info on how to disassemble HRO coil sets down to the bare panel and put them back together after a repaint job.

Preliminary Inspection - Apr 8, 2024 - The condition of this HRO-7T is excellent inside and out. In some ways, it appears to have been used very little. The top of the chassis is nearly perfect. Under the chassis looks about 99% original (appearances can be deceiving.) A cursory inspection revealed that there are two very small gauge wires that have been added and the B+ switch wires to the B+ toggle switch look like they are spliced together but,...no drilled holes, one extra component and no components missing. Further analysis will require cabinet removal but, as is typical, the PW-D is stuck on the tuning shaft. Since the PW-D almost comes off, I think with a slight "nudge" it will be easily removed.

My guess would be that this receiver was "refurbished" sometime in the past. I doubt the PW-D is original, it's almost perfect. The cabinet has some very minor "touch-ups" although those appear to be fairly old. The chassis doesn't appear to have been aggressively cleaned since the ink-stamped tube IDs are all present and in good shape. The power cable has a replacement Amphenol four pin plug installed and some of the woven cloth cover is worn indicating an average level of use by former owners. Overall, I'd guess the receiver is mostly original cosmetically and only minor, reversible modifications were incorporated.

Disassemble the Cabinet - I had to dismount the PW-D first. It would almost come off but not quite. I took off the gearbox cover and inserted a "stop" so the gearbox couldn't move. I then gently rotated as I pulled the PW-D and it came off very easily. I dismounted the remaining knobs. I removed the bottom cover. The rear panel screws were removed and the rear panel and lid assembly was taken off. There are a few control nuts to remove along with the coil extractor levers (these must be dismounted.) The four rubber bumper feet also have to be removed. Then the chassis can be removed from the cabinet.

HRO-7T "Black Wrinkle Finish"  SN: 189 0396   ca: 1947   "as found condition"

Modification repair or removal description will be in italics,...

1. The IF cable that runs from the Accessory socket (pin 4) to the 2nd IF plate connection (tube socket pin 3) was disconnected on both ends and another shielded cable was added that connected pin 4 on the Accessory socket to a small mica capacitor that was connected to the 2nd IF amp grid at the tube's grid cap. Reason was probably for an external BFO but that's just a guess. Injecting the BFO at the input of the last IF amplifier stage will provide amplification of that BFO level plus the signal resulting in a much stronger heterodyne plus signal combination at the detector. This was probably installed to improve SSB reception. Removed the added shielded cable and capacitor. Then the original shielded cable was reconnected to the Accessory socket and the 2nd IF plate, as original.

2. Modification to the BFO plate voltage utilizing pin 6 on the 6J7 socket (unused pin) to route the regulated +150vdc for the BFO plate. Since the BFO operates at around 456kc and, in the late-forties the BFO was primarily for CW, National design engineers thought that it's stability wasn't as critical as the LO stability. Besides, regulating the plate voltage on the BFO won't keep it from drifting during warm-up. The VR tube was to keep the LO stable with variations in the AC line voltage that then resulted in the B+ changing. The AC line variations could come from household appliances turning on and off or any fairly large current demand switching on the local AC power line. The VR tube kept the LO plate voltage constant and improved stability but it didn't compensate for thermal frequency drift. At the time, most operators knew the HROs required about 30 minutes to thermally stabilize and that any drift was much more noticeable when in the "band spread" set up. It seems likely that this mod was incorporated first and found to not work and then mod #1 was installed. In both cases, the probable motivation was to improve reception of SSB signals. All of the original components were still present so return to original just involved reconnecting the circuit back to original and soldering as needed.

3. Modification of the 6V6 screen. It had been wired to pin 5 of the Speaker socket as original but then also wired to the regulated +150vdc. This reduces the available power from the 6V6 audio output tube by reducing the Screen voltage by 33%. It might have been an attempt to reduce audio distortion although the receiver is for communications not listening to the AM-BC band. Originally, pin 5 was "jumped to" pin 4 B+ on the speaker plug only and that required using an original National MCR speaker or at least knowing what the design intent was. All of the original wiring was present so return to original just involved rewiring Speaker socket back to the original configuration.

4. The AVC line had two wires missing and two small gauge wires were added to replace the missing wires (original wires damaged?) The new wires were then routed around the chassis differently than original. The AVC line main junction to the switch was a cold solder joint. I think this mod was related to a failure of the AVC switch that ended up doing much more damage than just replacing the bad switch would have. Non-original wires removed and authentic wiring installed and routed as the original wiring had been. Correct color insulation wire was used for replacement wires. Correct type of switch was wired in using correct type wire and correct color insulation. Solder joint problems fixed.

5. As mentioned the B+ switch had no wires connected to it and it wasn't an original switch either. I thought the B+ wires that had gone to the switch were spliced together but they were just "cut" and unconnected inside a shrink tubing insulation. This then had the receiver relying on the shorted BSW terminals to route B+ through the receiver. Since the B+ switch isn't the original type and neither is the AVC switch, I salvaged a pair of correct type toggle switches from a "parts set" NC-183D from about the same time period as this HRO-7. I'm not sure why the standby function (B+ on-off switch) on the front panel was eliminated (see #7.) The reason for the front panel B+ switch was to allow the coil sets to be changed with the B+ turned off. Correct type of switch installed for B+ switch. Original wires used for the connection. Towards the end of WWII, the new HRO-5 receivers had changed the BSW switch from being wired in series with the B+ switch to being wired in parallel with the B+ switch. While this eliminated the need to install a jumper across the BSW terminals for receiver-only operation, it also made the B+ switch operation dependent on the remote T-R relay's auxiliary contact position. Normal T-R set-up would have the aux contacts closed during receive and that would disable the B+ switch for front panel turning off the B+ for coil set changing. This "parallel connection" of the BSW terminals with the B+ switch was used in HROs from 1945 up to 1949. By the HRO-50, the BSW was changed to three terminals that allowed the user to select how the T-R auxiliary contacts would operate the receiver B+. National's intent on this 1945-49 BSW change was probably due to set-up problems during WWII with operators that didn't bother to read the manual so they didn't know that the BSW terminals had to be "jumped" in order for the receiver to operate in a "receive-only" surveillance station.

6. Plastic tie-wraps were installed on the wire harness from the AVC junction to the RF Gain control and B+ switch. I removed the plastic tie-wraps and installed waxed-string lacing as original.

7. The B+ was incorrectly wired (from the mods, no doubt) and that's probably the reason the B+ on/off switch was disconnected and B+ leads to the switch were cut. Five different B+ wires were involved. The B+ from the power cable should have been connected to the BSW terminal 1 along with one wire to the B+ switch. Then on BSW terminal #2  B+ should have been connected to the main B+ junction wire along with the wire to pin 4 of the loudspeaker socket. Then loudspeaker socket pin 4 should have had a wire connected to the B+ junction to the VR tube load resistor. I unsoldered all five of the B+ wires involved and confirmed their origin-connection in the circuit using a "ohm meter." Then each wire was soldered to the correct destination-location in the receiver circuit. Apparently, while installing these modifications, the perpetrator mixed up the five B+ wires and rather than "straighten out" the mistake, the perp just cut some wires and jumped others to complete the B+ circuit. Of course, the B+ switch no longer functioned and an original loudspeaker couldn't be used,...but that's typically how hamster mods are executed anyway. After straightening out the B+ wiring problem I thought I had better confirm that the receiver would be operational. I measured the DCR on the B+ line and it was about 30K ohms so no hard shorts. Using a spare 697 power supply, I powered up the HRO-7 on all original parts, untested tubes,...I know,...everything that you shouldn't do,...I had coil set F installed and a ten foot wire "test antenna." After the receiver warmed up, I tuned in several AM-BC stations. The receiver basically functions now but an alignment is necessary and definitely the paper capacitors need replacement. Also, this was just the chassis and the speaker was connected with clip-leads, so just a "quickie test" to see if the HRO-7T was now successfully "de-mod'd" and back to a "functional-original."

8. I had wondered why the S-meter scale was so dark (it seemed much darker than usual) and when I pulled the meter lamp I found it was a #46, the screw-base equivalent of a bayonet-base #44, so that resulted in a .25A lamp right next to the meter scale (I guess I was lucky it didn't burn a hole in the scale.) I should have a good condition original scale that I can use as a replacement. I installed a #40 lamp which is the screw-base equivalent of a #47 drawing 155mA. Checked the spare meters that I have and all are WWII or earlier style Marion Electric meters. These earlier meters are subtly different than the 1946 and later meters in that the font used on the later meters for the numerals and lettering was changed to a smaller and finer type of font. Also, on the later meters the location of "S UNITS" and "DB OVER S-9" are over the scale rather than under the scale. The differences are very apparent, IF it's pointed out. So, I'll have to just leave this scale as-is, until a later style meter, with a good scale, turns up. Luckily, almost all National receivers made from 1946 up to about 1950 used this type of meter so it shouldn't be too hard to find one with a decent scale. Actually, after I cleaned up the meter, the scale doesn't look too badly darkened and when illuminated it appears like a typical HRO S-meter.
NOTE:  After about one week of fairly regular operation, about 30 minutes to an hour per day, the "new" meter lamp failed. These aren't vintage lamps but new ones made outside the USA. I've installed another one out of the same package. If it fails quickly I'll replace it with a vintage USA-made #40 lamp.  

9. Although it's not a mod, the "nati<NC>onal " emblem does have a lot of pitting and a lot of discoloration. There are two types of this emblem. One has posts and is mounted with push-on clips. The second type has bend-over tabs. I found the correct type (posts and push-on clips) on an old HFS "parts set." It was an easy transfer. The replacement emblem, while not perfect, is in considerably better condition and is very a good match to the condition of the cabinet.

Gearbox Lube - The green grease is all on the top part of the split-gear that never meshes with the worm gear. I removed most of the green grease since it's really not necessary. I then applied a similar type of grease to the bottom of the split-gear and to the worm gear. These later gearboxes have a lot of small openings that might have been for a spray-type of lubrication. There are spray cans of heavy oil that turns to grease after spraying. Available at farm-tractor supply stores.

Tested Tubes - It appears that almost all of the tubes installed were NOS and installed fairly recently. All metal tubes were General Electric types. The 6V6GT was General Electric. The 6C4 was a Tungsol and the 0A2 was an Amperex. All tubes tested "as new" so these tubes have seen very little, if any, use.

AVC Switch - This was a "ball lever" toggle switch and it was incorrectly mounted for horizontal operation rather than vertical operation. I had to replace the switch with a correct "bat lever" toggle switch that was oriented vertically. I had to replace the wires that ran from the AVC circuit up above the chassis to the switch so they were the proper type of hook-up wire and the correct color code.

HRO-7T Chassis Top of SN:189 0396
Other than the LO and VR tubes and the missing Ant-Gnd terminals, the HRO-7 chassis is very similar to the HRO-5A1. Note the red mica cap on the 2nd IF grid,...it's a mod.

Cleaning - I used Glass Plus and a short bristle paint brush to thoroughly clean the cabinet. I then wiped the semi-gloss panels with a dry flannel cloth to bring out the luster (but not to polish.) I used a right-angle awl tool to remove the push-clips on the National emblem and then installed the replacement emblem using a 3/16" nut driver to push on the clips. I wiped the cabinet down with a very light coating of "3in1" oil that was then wiped off with a clean dry cloth. This will hide small scratches or scuffs and also helps with dried-out finishes. It's much better than Armor All that smells awful and leaves the item "slippery" feeling.

Reassembly - Nothing particularly difficult in the reassembly. Mounting the coil extraction levers requires using a 3/8" socket, a U-joint, an extension and the socket wrench (1/4" drive) and one has to watch the round spacer placement, the rectangular metal reinforcement piece. When tightening the nuts, watch that the lever's extraction projections clear the slots in the front panel when lifted up (like when extracting a coil set.) The meter panel is mounted using the mounting nuts and locking washers from the AF Gain and Limiter controls. There is some limited movement of this panel that will allow better visibility of the S-meter by placing the meter mount as low in the panel as possible before tightening the control nuts. The knob flutes only required dusting and that was done with a short bristle brush. The PW-D was installed per the procedure. The receiver was turned around so the back was towards me. I then installed the back panel and lid assembly. I don't tighten these screws until I make sure the lid is setting square in the lid-well. After this, the rubber feet are installed and then the bottom panel can be installed.

Making the Best of the "All Original Parts" versus "Performance That Meets Original Specs" Conflict

"All Original Parts" Problems - Expecting paper dielectric capacitors that are over three-quarters of a century old to function without problems isn't realistic. The fact that the receiver does somewhat operate and receive stations is a surprise. Surely a testament to the quality of Sprague capacitors. But, these 75+ year old capacitors certainly have some leakage current that will skew bias voltages, limit signal coupling functions and load-down voltages where the capacitor is used for bypassing along with overheating the associated load resistor.  >>>

S-Meter problem was a wire inside the meter had been pushed, probably by the lamp bulb, to the point where it was resting against the needle rear pivot point and that seemed to "lock" the needle and kept it from moving. I had measured the DCR and that indicated that the coil was okay so it had to be something internal. It was an easy fix that required meter disassembly and then just using a small tool to push the wire over to the side where it belonged. Reassembled the meter, installed it in the receiver and tested,...worked fine.

I performed a "quickie" IF alignment and that got the Crystal Filter working correctly. There was also some significant improvement in the sensitivity but, overall, the output level of the receiver is still way down from what is normal for a HRO.

I'm sure the remaining problems are due to leaky paper dielectric capacitors that are of a septuagenarian status. Like most vintage electronics, it seems one can't have "all original components" AND performance that meets original specifications. A choice has to be made, either a "shelf Queen" or a well-restored, operational and usable receiver. Restuffing the original Sprague capacitor shells is the best compromise for both originality and performance. Everything under the chassis will look original (and the Sprague capacitor shells ARE the original component exterior) but new capacitors inside the original shells will allow the receiver to meet its original specifications. Quantities needed will be for 0.1uf - 16 capacitors, for 0.01uf - 9 capacitors and for 0.25uf - 1 capacitor. Also, for 10uf 25vdc electrolytic -1 capacitor and for 25uf 25vdc electrolytic - 1 capacitor. As per my procedure nowadays, I'll make a detailed drawing of the bottom of the chassis showing the location and orientation of each capacitor plus its correct value. Then all paper dielectric and electrolytic capacitors will be removed so the original capacitors can be melted out of the shells. This work is performed outside (messy and smells a lot.) Then the shells can be brought inside and stuffed with polyfilm caps of the correct value that are secured with a fill of brown colored hot melt glue. If I recoat the rebuilt capacitor shells with bee's wax, it's almost impossible to distinguish an original capacitor from a "rebuilt" capacitor. Then the restuffed capacitors can be reinstalled into the receiver. All solder joints must be wicked, if necessary (most joints can have the excess solder melted off of the joint and removed with soldering aid tools.) Then the component lead  can be wrapped correctly and soldered. Nothing looks more like amateur work than "hook splices" or "tack soldering."

But, back to the "restuffing" prep,...it's much easier and faster to melt all of the old capacitors out of their shells in one setting. I used a handheld heat gun to melt the old wax. To remove all 26 old capacitors takes about an hour to accomplish. Doing this task outside is much easier, since the job is somewhat messy and does smell a bit.

NOTE: I'm sure all of the paper capacitors were original to the receiver. All capacitors were Sprague except for the two 0.1uf 600vdc caps were Aerovox and the one paper capacitor on top of the chassis, C2 which is the 1st RF amp bypass cap on the tuning capacitor rotor to chassis. This cap was a Cornell-Dublier .01uf 600vdc Type MD. 

The two electrolytic capacitors are rolled end types that are sliced open, old cap removed, new cap inserted and then the shell is hot melt glued closed. The only precaution is to make sure the slice is opposite the component value nomenclature which puts the slice next to the chassis, out of sight. No problems.

The .25uf 600vdc cap was quite different and appeared to be oil-filled since there was red rubber sealant on the ends. This capacitor also appeared to be slightly leaking the oil. I had to "unroll" one end to remove the capacitor because there was a mounting clamp around the body that was secured by a rivet. I had to drill two holes in one end to have a way to turn the metal cap body inside to cardboard shell. I had to slice the shell also to get the metal cap body out. It was an oil filled capacitor but nowhere on the body of the capacitor does it indicate that. The installing the .22uf 630vdc polyfilm was no problem. 

Apr 23, 2024 - Continued installing the rebuilt capacitors. This takes a little more time because I have to desolder the joint to remove the old capacitor lead and provide a mounting hole for the rebuilt capacitor. This provides a much better solder joint than using "hook splices" or just wrapping the new lead around the terminal. It takes longer but it looks like (and is) the professional rework technique. Three sessions of an average of about 45 minutes or a little over two hours total time was required to complete the installation of the rebuilt capacitors. The HRO-7 chassis is very similar to all of the preceding HRO models in that access to all components is very easy. This allowed the replacement job to be completed rather quickly. Only C2 remains to be replaced and it's located on the top of the chassis and is connected from the 1st RF grid-rotor to chassis ground. This capacitor connection is a little different in that National connected the capacitor in the middle of the buss wire that runs from the coil finger terminal up to the tuning condenser rotor. The other end connects to a ground lug on the tuning condenser mounting platform.

HRO-7T SN:189 0396
Under the chassis after the capacitor rebuild. It's interesting that ALL of the capacitor shells are original to this receiver. Mostly Sprague with two Aerovox (.1uf 600vdc) and one C-D (C2 on chassis top.)

I connected the 697 power supply and the loudspeaker,...it's actually an older HRO speaker that does have the proper output transformer on the speaker so it is compatible with the HRO-7. I had built-up a five pin plug with wire leads from pins 3 and 4 (6V6 plate and B+) and then pin 4 jumped to pin 5 (routes B+ to 6V6 screen.) The HRO speaker was connected with clip leads to the plate and B+ wires from the five pin plug inserted into the speaker socket. I first used a 50 foot long test antenna and coil set F was installed (lower AM-BC band.) I had the 697 powered by a Variac that allowed the first application of AC voltage to be accomplished in a moderate "ramp up" just in case something happened. But, power up was without any issues and the HRO-7 came to life with lots and lots of volume. I had to reduce the AF Gain to 2 and the RF Gain was switched on (max) allowing the S-meter to function and AVC switch was on. I tuned in KKOH 780kc and the S-meter was pegged which is sort of normal for a HRO on the AM-BC band tuning in a quasi-local 50KW station. The audio sounded excellent with no noticeable distortion. I tuned in a few other AM-BC stations and with the non-local stations I would get S-meter readings that indicated everything was normal with the meter.

I connected the Collinear Array antenna to the HRO-7 and switched over to coil set A so I could tune in 20M hams and 19M SW-BC. The BFO coil needed a slight tweak for a better range of adjustment. It did produce a strong heterodyne. SSB stations were demodulated with the RF Gain at 8 and the AF Gain at 7. I tuned in a few SW-BC stations and a few hams on 17M. 

I tried the Crystal Filter and it functioned as expected. The TONE switch is a little "touchy" in that it does work but the switch handle needs to be wiggled sometimes to get it to go into LOW. I had given it a shot of DeOxit earlier but maybe another shot might help (it did.)   >>>

Next, is to do a careful IF alignment. Then original coil set D can be aligned. Coil sets F, C, B and A have gray panels along with the spare D set with the gray panel. I'm going to disassemble those coil sets and repaint the panels in black wrinkle finish. The coil assembly mounting bracket is riveted to the back side of the coil panel which complicates the repaint. I've drilled out the six rivets on other coil sets then do the repaint and just remounted the bracket with screws and nuts. The screws and nuts are hidden when the coil set is fully assembled. I'll align these coil sets first to confirm they are functioning correctly, then I'll do the cosmetics.

A Matching Loudspeaker? - I have a very poor condition MCR-type speaker cabinet that had been included in a trade a while back. Sometime in the past, someone painted it machine gray color using spray-can type paint (a very ugly, very soft, lumpy paint job.) This speaker had a 6" loudspeaker with a badly ripped cone that was repaired with several layers of Scotch Tape (unbelievable.) Essentially, this speaker is "junk." I'm thinking about stripping and repainting the cabinet black wrinkle finish. I'll want to install an eight-inch loudspeaker so I'll have to make an eight-inch speaker adaptor plate to take the place of the six-inch adaptor plate and then install a proper eight-inch PM loudspeaker. I have several 7000Z to 8Z output transformers that can be mounted on the loudspeaker. Unfortunately, I don't have the exact heavy-texture, woven pattern brown grille cloth but I do have a very close texture-match to the cloth that's black (with a slightly brown tint in color) that might be believable as "originally installed" because of the loudspeaker cabinet being painted black wrinkle. Of course one troubling thought is,...I'm not really sure that these Black Wrinkle Finish HRO-7T receivers even came with a matching MCR loudspeaker in the first place.  More on this MCR restoration at the end of this write-up,...

Before the IF/RF alignment, coil set B when on BS for 20M, the Audio Gain was on 7 and the RF Gain on 8. Now, the moderately strong signals on 20M BS are received with Audio Gain on 7 and RF Gain on 5. Big improvement. All of the coil sets were fairly close in alignment so they functioned okay. Interestingly, all of the coil sets were noticeable "off" on the RF2 alignment only. I suspect that the coil sets were aligned to the receiver when it still had the loose set screws on the RF1 and RF2 side of the tuning condenser shaft along with the original septuagenarian capacitors and compromising modifications. However, a thorough "de-mod job," new caps and a complete alignment of the coil sets to the rebuilt receiver have provided a big improvement in performance.

Repainting HRO-7 Coil Set Panels

First, I have to remove the four coil assemblies. This is easy and just involves loosening four screws for each assembly on the mounting bracket. Each coil assembly is already identified so it's not really necessary to keep the assemblies separated but I'm going to have three separate boxes that each contain all of the parts for a specific coil set. It will make reassembly easier. Next, the coil graph has to be removed by unscrewing the four retaining screws. Then the lever-bumpers are removed. They mount with two screws, lock washers and nuts for each bumper.* The last piece to remove is the coil assembly mounting bracket.

Unfortunately, this piece is riveted to the front panel. The six rivets have to be drilled out to remove the bracket. I just drill off the "rolled end" of the rivet and then drive out the rivet with a punch. Now the coil set panel has all of the parts removed and it's ready for prep. Panels stripped of brackets and bumpers Apr 27, 2024

While the coil set is disassembled is a good time to go through each of the coil assemblies. Each one should be cleaned of dust and spider webs. The components should be checked for broken wires or bent plates on the C trimmers. Also, the inside of the shield can should be cleaned. Since the coil set probably should be aligned again after this painting job, apply a small amount of DeOxit on the trimmer adjustment so that it rotates easily and is making good contact. Coil set F cleaned, reconditioned and aligned Apr 28, 2024. Coils sets A and B are already very clean. Coil set C is very dirty. Cleaned C on Apr 30, 2024.   >>>

*This describes the early style plastic-type of lever bumper. Later bumpers are metal and swedge-mounted which complicates their remounting after removal.

The HRO-7 coil panels painted in black wrinkle.
The bottom coil (D) is original paint

The backside of the panel has to be painted first. Use matte-black paint. Krylon is best for off-the-shelf paint. Let the backside dry for a day. Then paint the front side with VHT Jet Black Wrinkle Finish. I use a minimum of three heavy coats applied in different directions spaced about 1 to 2 minutes apart. Four coats is even better. Then the wrinkle paint needs heat. I use two 250W brood lamps overhead about 18" above the panel and also a handheld heat gun. The trick to a good wrinkle finish is "lots of paint and lots of heat." But, if the heat gun is held on one spot too long the paint will "gloss" there and not match. Always keep the heat gun moving.

It takes about 3 minutes to 5 minutes for the wrinkle to start. Keep the heat gun moving to help the wrinkle move along until the entire panel surface is wrinkled. Remove the heat. Don't move the panel because the paint is very soft. Let it cool down for about 30 minutes. Then it can be carefully handled and can be placed somewhere safe to cure. Curing takes a few days. Overnight is okay but the paint will still be a little soft. A few days lets it harden better. Waiting longer, like a week, will let the wrinkle get even harder. About a month is required for full hardness but just a few days will be enough to allow for reassembly. Panels painted Apr 29, 2024. I painted all three panels simultaneously since they aren't a large surface. Wrinkle was perfect. I'll allow three days for curing.

Coil Set Reassembly - May 2, 2024 - I decided to use pop-rivets in the mounting of the coil assembly brackets to the panel. I used 1/8"x1/4" steel rivets. The head diameter is .250" and the recessed hole in the panel diameter is .215" so I had to slightly reduce the head diameter but otherwise the fit was good. The head height didn't allow the head to protrude above the surface of the panel from the recessed hole. The backside of the rivet "when set" was less than 3/16" in height so it didn't interfere with the coil assembly mounting. I had to use a protective cover over the wrinkle paint when setting the rivets, a paper towel was sufficient protection. Once the bracket was mounted, then the two lever bumpers were mounted followed by the metal "frequency to PW-D" scale. To complete the reassembly the four cleaned and checked-out coil assemblies were mounted to the bracket on each coil set (C, B and A.) Then the coil sets were tested in the receiver and an alignment was performed since each coil set had been entirely disassembled. Coil set C was cleaned and serviced and that involves cleaning and rotating the position of the alignment air trimmers so it needed the most recalibration. Coil sets A and B had been inspected and were thoroughly clean inside so when reassembled these two coil sets only needed a slight readjustment. The finished coil sets look great. The wrinkle finish is noticeably a finer texture. Even with four heavy coats the texture will still be noticeably different from the old original wrinkle paint but the color match is very good. I didn't remove the ink-stamped serial number on coil sets C, B and A (even though the SNs obviously don't match the receiver.) I wrote in pencil by the stamped SN that C, B and A have been aligned to SN: 189 0396.

Close-up showing that VHT wrinkle is a finer texture than the original paint

Converting a "Junk" MCR Loudspeaker into a Black Wrinkle MCR to Match the HRO-7T

This turned out to be a much more involved conversion than I first thought it would be. But, the end result was a stunningly different looking MCR loudspeaker.

Wooden Adapter Plate - The six-inch speaker adapter plate is made out of steel so reworking it for an eight-inch speaker would be difficult. I decided that I would use 1/2" plywood to make the adapter plate since it would be much easier to work with as a material. The size that would fit into the box is 9.5" square. The speaker hole cut-out is 7" and then there are four speaker mounting holes and four other holes for mounting the adapter plate in the box. The metal adapter plate was used as a template for marking and drilling the .250" diameter mounting holes for the wooden adapter plate. The speaker frame was used as the template for marking and drilling the holes for the 8-32 x 1.25" FH screws that are counter sunk on both sides (conical for the FH and a relief hole for the nut to retain the speaker mounting screws to the adapter plate. Once the wood-working is finished, the wooden adapter plate was painted matte black and allowed to dry overnight. Painted the adapter plate on 5-13-24

Selecting a Loudspeaker - I thought it might be a good idea to test the loudspeaker I intended to use prior to full assembly. I used a Stancor Universal Audio Output Transformer rated at 4 watts and set up to provide 7000Z primary and 4Z for the voice coil of the speaker. I tested the audio response first using a function generator driving the primary. The speaker didn't have very much response below 100hz. I next used clip leads to connect the speaker to the HRO-7 with the speaker mounted on the adapter plate and setting in a box to simulate a housing. Although the loudspeaker set-up sounded fine with no distortion, it was definitely lacking in bass response. It would be fine for military communications but I wanted more lower end audio. I decided to do a swap. The loudspeaker I was using for testing sounded great. It was installed in the older style HRO cabinet. I have several of these speakers mostly as companions to HROs I rarely use. Since this test loudspeaker did have a great sound, I swapped it for the "communications" speaker.

Assembly - Before the adapter plate is installed into the box, the grille cloth and the <NC> emblem had to be installed.  The <NC> emblem was mounted using its threaded stud with a fender washer, locking washer and nut to secure. The original installation had the grille cloth attached at the center to the stud of the <NC> emblem and then the cloth was glued to a cardboard piece surrounding the speaker opening. That was because the old adapter plate was steel. With a wooden adapter plate, the grille cloth could be glued to the wood. The grille cloth had to be glued just on the very edge and no more than 3/8" wide (otherwise the glue will show through the speaker opening.) I used a grille cloth that was a close match to the texture of the original cloth but the color was mostly black with just traces of brown (the original cloth was all brown.) The adapter plate with the loudspeaker mounted was installed and then the felt cup feet were installed on the bottom of the cabinet. A cable clamp had to be installed on one of the cup feet studs. The loudspeaker cable had to have a five-pin plug installed to fit the speaker socket of the HRO-7. Completed 5-15-24. The completed Black Wrinkle Finish MCR with 8" loudspeaker is shown to the right.

HRO-7T SN:189 0396 with Black Wrinkle Finish MCR 8" Loudspeaker
The VHT BWF paint is obviously a much finer texture but the color is a good match. The grille cloth has a pattern similar to the original and there is a slightly brown tint to the color.

The Black Wrinkle HRO-7T with the WWII ART-13A Aircraft Transmitter

HRO-7T SN:189 0396 - Rebuild Performance Details

I have to admit that the HRO-7 is an impressive performing receiver. Even though the upgrades appear to be subtle and not all that important, obviously National engineers did improve the receiver's operation and performance. While the HRO-7 still has to use a separate 697 power supply and, to switch from General Coverage to Amateur Band Spread still requires moving four little screws on the coil set each time you want to make that change, there are many new and nice improvements with the HRO-7. The coil set graphs were improved to be like a slide rule dial and this does make them easier to read than the old "X vs Y" type graphs used on the earlier HRO charts. The installation of the coil set into the coil bay is somewhat easier with the levers to aid in seating the coil correctly. Coil set removal is also much easier with the levers. The Noise Limiter is a much more effective circuit and it can be switched "off" unlike the older HRO-5A1 Automatic Noise Limiter that was always "in circuit" and cut-off by adjusting the bias level. The Crystal Filter works very well but I think it's the same type that was used in the HRO-5A1 (that also worked fine.) The TONE switch is a nice addition and can provide a "very mellow" audio effect when LOW is selected. Incorporating the S-meter switch into the RF Gain control might lead someone to think that the AVC is also controlled by the RF Gain switch but, of course, it isn't and there's obviously the AVC switch on the panel. I think the S-meter switch assured that the RF Gain was at maximum for a proper S-meter calibration,...if the AVC is turned ON. I haven't done a "side by side" comparison but the HRO-7 seems to have much more audio output available,...especially if compared to the pre-WWII HRO. The audio output has a good quality that provides a pleasant listening experience. Full output in the AM mode with RF gain at maximum is achieved at about "2" on the AF Gain. LO and BFO drift is not too bad. There's the normal thermal drift as the HRO-7 warms up but after 20 to 30 minutes I can listen to a SSB QSO (on 20M band spread) and not have to adjust the receiver for probably about 5 minutes or more (big improvement over the pre-war HRO on 20M band spread.)

Listening on coil set B set for 20M band spread, I had the HRO-7 connected to the Collinear Array antenna. Many stations from North and South America were heard. CW on 20M was very clean sounding and very stable,...very nice. I had coil set A set for General Coverage so I could listen to other types of stations from 14mc up to 30mc. Signals were copied all the way up to 29.2mc upper 10M band (although these guys sounded a lot like CBers.) Coil set C is set for 40M band spread and I did all of the listening during the day when most SSB signals are from various net operations. For SSB reception, I set up the BFO for zero-beat at about 6 on the scale. That way either upper and lower sideband can be selected. Coil set D is set for 80M band spread and this is a nighttime (or early morning) listening band. Although operating the HRO-7T "on the air" on 75M wouldn't be much of a challenge, I'm sure it will be a stellar station receiver in a vintage ham station. I'm going to give it a try soon using the HRO-7T paired-up with the WWII ART-13A transmitter, as shown in the photograph above-left.

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 in Part 3 to see what to expect from modern VHT Black Wrinkle Finish, one-part wrinkle finish.

Update: 2022 - Greg, K6SRO became SK recently. G-235 is now owned by Steve W6SSP

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 and the influx of Japanese ham transceivers were changing the operating habits of hams. 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 or transceiver!" As SSB became the dominate voice mode, the receiver/transceiver 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 during the Great Depression 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 just have to get used to the drift. Adding voltage regulation to the LO or the BFO won't help thermal drift. The idea of a VR tube was to kept the plate voltage constant when the AC line voltage was changing due to large appliances switching on and off or other irregularities on the AC line. A NPO style capacitor might help the LO thermal drift but there's still the BFO drift for SSB copy. 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 my 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 online as part of a PDF from the Antique Wireless Association. Search "The AWA Review Vol. 4, 1989" - Fisher's article is on pgs 32-44.

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 online from the AWA as part of a PDF. Search "The AWA Review Vol. 17, 2004" - Barry's article starts on page 145.

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.

Apr 2024 - Expanded the HRO-7 section and added a section on the Black Wrinkle Finish HRO-7T SN: 189 0396 receiver.

May 2024 - Restoration of HRO-7T Black Wrinkle Finish added to Part 4,  updated all four parts and performed minor corrections,  updated the information on Charles Fisher's AWA article and Barry William's AWA article.

HRO Part 1                HRO Part 2                 HRO Part 3                 Return to Home Index