National HRO inquiry

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David, K3TUE:
I am looking to mix things up a bit here and was wondering if anyone here can help before I post a “Wanted” post or make an actual purchase. Despite all I the advice I have received in the past, I am considering looking for a pre-war National HRO Sr. for myself. Based on what I have been able to find, and given the list of specifics below that are important to me, I believe it will have been made between 1937 (prefix R serial number) and 1941 (serial number suffix L):
- Large vent holes and side louvers case (prefix J--)
- Grey chassis (prefix K--)
- Square IF cans (prefix R--)
- External crystal on crystal filter (--suffix L)
- Cast gear cap (--1941)
- Preferably backlit round bakelite S meter (prefix T--), but any meter (even a hole where a meter once was) is fine for now
- Preferably 697 dog house power supply (1939--), but a working one is all I need for now
- Preferably screw-type antenna/ground terminals (prefix ~R--), but a push set (even a broken one) is fine for now
- Preferably 6.3V non-octal tubes (--1941), but since the octals were added during the war, this should not be a problem if it is pre-war

I know there are many here with experience with the HRO series of receivers. If anyone can think of a good reason why looking for a pre-war HRO, aside from greater rarity and cost, please, let me know.

k4kyv:
If you are looking for a pre-WWII HRO because you want a vintage classic, look  for the black interior, round i.f. cans, German silver dial, crystal filter with the chrome plated plug-in crystal, s-meter with 0-5 scale, 2.5v tubes, etc. Those first came out in 1935.  The later pre-War ones with the grey interior and rectangular cans don't work a bit better.

You will find it to be an excellent receiver for 160-20m (if you can get all the coils).  Expect to let it warm up for 30-45 minutes before it becomes completely stable.  Image rejection is good all the way through 20.

On the down side, it was built before we had any 15m band, so there is no bandspread coil for that band.  And, its performance is anaemic at best on 10.  Images are almost as strong as the desired signal, sensitivity is poor, it drifts, and there will likely be a 60~ FM hum on the HF oscillator, making all signals sound mushy with the BFO on.

Oh yes, one other thing.  The BFO injection is way too low. You don't get a good solid CW tone, and not enough injection to demodulate SSB.  They use a small coupling cap, probably about 15 pf, made of a little square piece of bakelite with a metal plate riveted over each side.  Don't remove it; just let it serve as a terminal strip and bridge a larger capacitor across it.  I used a small mica, about 50 pf.  Later on, I tried a 100 pf, and it actually made a crude synchronous detector; the BFO would brute-force lock on to the AM carrier if I carefully zero-beated the signal.

The wartime version is a lot like the pre-War one, except they had gone to 6.3v tubes.  Some of the deficiencies of the earlier model had been corrected, but not completely until they came out with the HRO-5. If you work mostly the lower bands, you can't beat the early model as a performing classic.  When I finally replaced mine with the 75A-4 as the main station receiver, I didn't see a radical improvement.  The stability, dial calibration and selectivity were noticeably improved, but the sensitivity up through 20m was about as good on the HRO as with the A4.

The HRO-7 came out in about 1947.  It is a lot like the HRO-5, but it is ugly as sin; the same guy must have designed the cabinet for the Gates BC1-T broadcast transmitter.

KM1H:
The HRO was introduced in 1935 with 2.5 and 6.3V large pin tubes, the latter for battery use as Millen wasnt satisfied with the 6.3V hum level for AC use. He changed his mind around 1938-39 and the 6.3V was a standard until the late WW2 models which were metal octals. There are strong rumors that the 2.5V lasted so long was only due to a huge overstock. After all the NC-100/101X had octals from their 1936 introduction.  The Navy however wanted the old glass jugs in the RAS/RAW/RBJ but they were produced in far less quantity than the others.

National produced a 15M coil for the HRO-7 and one with the correct front panel for all earlier models; they are very scarce. There were even a few built to order E coil versions that were 160M BS only; these were done in the Service Department and I had one in a HRO-5A1 when I worked there.

For performance Id go with a 1937 R series or later model as significant and minor changes were completed by then. Some say the IF's are a bit sharper in the square cans; I havent measured that to confirm.
The late 1946 HRO-5A1 is the best of the black wrinkle models as it had a noise limiter as well as the excellent crystal filter "borrowed" from Hammarlund.

Drift can be mitigated to a good degree with a TC style ceramic cap across the oscillator tuning cap as written up by Loren Windom, W8GZ, ages go. I dont have the reference handy at the moment but will dig it up later.

Carl
National Radio 1963-69
Service Tech, Service Manager

k4kyv:
I still have a NC-101XA that I retrofitted with some ceramic negative temperature comp capacitors, installed right in the oscillator section of the coil catacomb for each band.  I was able to get the drift to practically zero on a couple of bands by experimenting with various combinations of capacitance and negative temperature coefficient. I had intended to use it as my main station receiver, since with only one rf stage, it was  less prone to cross-modulation with strong signals, plus it had the direct-reading dial.  But about then I acquired the 75A4, and it worked so well that I abandoned the project.  I still need to replace the i.f. cans I had tried to modify with some unmodified ones I have on hand, and get that receiver working again, if nothing else, to use for trading material.

The greatest problem I had was finding a wide variety of neg temp coefficient capacitors of different capacitances.  It seems they were once widely available, but about the time I started the project in the early 80s, the variety of ones available was dwindling.  I recall using some small capacitance values as high as N1500 and maybe even N2700.  I can't find those any more. Last time I looked, about the only thing available was N750.

As far as the i.f. selectivity of the HRO goes (the NC-101 series uses basically the same i.f. strip), the  response is too peaky.  At 20-30 dB down, the selectivity is about 7 kc/s wide, but it rises to a peak of < 1 kc/s, so when you zero in on the AM carrier, there is enough sideband cutting to hamper the high frequency audio response.  With the 101XA I had tried modifying the coupling of the i.f. coils to flatten out the nose of the response, but didn't achieve much success.  The crystal filter is great for CW.  I could zero in on an AM carrier and with maximum selectivity, almost completely get rid of the audio with the S-meter still reading the same as  with the filter off. It was fun somtimes listening to music (they actually played music on the AM band back then) with all the audio absent except for the thump of the deep bass.  But the old single pole phasing type filter was useless for normal AM reception.  I eventually added an outboard box with some Collins mechanical filters for better selectivity.  It was easy with the old tubes with the grid cap on top.  I had to run a couple of wires under the chassis to accommodate the AVC line, but modifications were minimal and completely reversible.

After I started using the 75A-4 I transferred the external filter box to that receiver, because I wasn't satisfied with only the three selectivity positions offered by the stock receiver.

WA2SQQ:
I'm not sure what version, but my friend NO2EL has one for sale. He had it at the hamfest last week and I'm pretty sure it didn't sell. Contact Noel and mention I sent you!

Bob, WA2SQQ

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