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 Post subject: HRO-500 challenges
PostPosted: Mar Sat 20, 2010 10:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mt. Airy, Maryland
I bought this at Dayton several years ago, and dragged it with me to Athens as I never had time to work on it back home. Symptoms were that the synth wouldn't lock anywhere, poking a signal into the IF gave zero output. A little bit of white noise from the RX, even with the mute transistor removed.

So, start with the 230 KHz IF. A couple of bad resistors and way out of alignment. Worked fine in the 5 KHz and 8 KHz positions but not .5 and 2.5. I sweep aligned the passband filter with my Agilent 8935 service monitor and the 2.5 Khz position aligned nicely, the passband tuning tracked well, but the .5 Khz position is down about 10 dB. That can be corrected by adjusting the passbband filter, but in doing so the 2.5 Khz position is asymmetrical. I decided I probably won't use the .5 KHz position anway; the problem is most likely in one of the cup core coils and would need disassembly of the passband filter to correct.

Aligned the variable IF, which went smoothly. Now, why isn't a signal getting thru from the base of that mixer into the 230 KHz IF? The spectrum analyzer in the 8935 told the story - the VFO was dead. Found every resistor had dramatically changed value, and although it was oscillating (way off frequency) the buffer wasn't doing its thing. The new resistors brought the buffer back on line.

Tried aligning the VFO. A visual inspection found that someone had just slapped on the PW dial in some random position. A lot of juggling back and forth (National sez the variable cap needs to be 13 degrees from its endpoint when the dial is at 500. Yeah right, that's easy to measure given the cap is buried under the synth chassis!) got it to the point where the over travel was equivalent on each end of the VFO.

A lot of fiddling later, got the VFO back to where it should be. The spectrum analyzer in the 8935 made this much easier. Then, I noticed the VFO was drifting pretty badly. The oscillator transistor was getting warm, which shouldn't be happening. A new transistor cured that. Must have been leaking and going into runaway.

Now time to attack the synthesizer. There are a bazillion adjustments to get it to track correctly. First was the 4.75 MHz IF. Jeez, it's oscillating. An intermittent solder connection on one of the bypass caps was causing this. Then, I noticed the scope was indicating a nice audio tone at the output of the phase detector. ***?!? Is the IF oscillating again and beating against something? Finally noticed I had the 8935's sig gen modulator on :oops:

That done, I aligned the synth according to the book. Couldn't get some of the bands to lock at the high end no matter what. Some genius had "cleaned" the synth area of the bandswitch with Lubriplate :evil: It was gummed up everywhere, had hardened, and scraped the silver off the contacts. Some Q-tips and deoxit got it working to the point where it locks everywhere, but doesn't track too well in the middle; some bands are a bit off, showing two bands in the window. Again, using the spectrum analyzer in the 8935 to find where the synth was locking made this task much easier.

The radio now has good sensitivity everywhere, but the copious crossover distortion was noticible. Again, new resistors in the audio output got that more or less under control.

THe only thing left to do is fix the S-meter, whose needle bas broken off and jammed up the works. That will require dropping the front panel, which I'm not looking forward to.

The 8935 was invaluable. Yep, coulda done it all with just a scope, sig gen, and VOM, but that passband filter woulda been a pain to do with out the tracking generator. I'm sure Carl woulda appreciated that tool when he was doing this for a living at National!


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 Post subject:
PostPosted: Mar Sat 20, 2010 12:59 pm 
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Joined: Dec Sat 08, 2007 9:51 pm
Posts: 1991
Location: Nashville TN
Sounds like that radio went thru a war.Though National made good radios .Wonder who made the resistors with that many bad.Specans sure do help wish i had a HP one they were nice but oh so heavy pewww.Angelo 8) :D

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 Post subject:
PostPosted: Mar Sat 20, 2010 2:52 pm 
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Nice story- all the usual excitement in bringing a nice receiver back from the dead. Glad to hear that you didn't run into any unobtainium parts, 'tho the meter will be a challenge. I think an HRO500 would be next on my list of most desirable radios, but I'm not sure my wife would agree with the usual pricing.
Ed W2EMN


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 Post subject:
PostPosted: Mar Sat 20, 2010 2:56 pm 
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Joined: Oct Sat 20, 2007 3:36 am
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Location: Southern NH, 03076
Knowing you John someone paid you to take it away!


Quote:
The 8935 was invaluable. Yep, coulda done it all with just a scope, sig gen, and VOM, but that passband filter woulda been a pain to do with out the tracking generator. I'm sure Carl woulda appreciated that tool when he was doing this for a living at National!


Thats the same POS filter used in the NC-270 or close enough anyway. They gave me fits in the 270's but were apparently still too new to give any HRO-500 problems. I think Dave Ishmael has become the current resident expert on the 270 and those filters.

If you had a choice would you get a 8920 or 8935? Ive been promising myself a present ever since you started bragging about them on AM.

BTW, I get to start on a HRO-500 in a few days.

Carl


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 Post subject:
PostPosted: Mar Sat 20, 2010 9:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Monterey California USA
DRMO surplused about 100 HRO-500's out here in California in the mid 1990's, new in crates. Surprisingly, many of them had the same component issues you mention, right out of the crate, although admittedly they had been sitting for thirty years in a warehouse.

Our fearless leader at station KPH - KSM just rescued one of the HRO-500's which RCA-Radiomarine used to use at the operating positions up to the early 1970's. It was used so much that the tuning knob is all worn down around the edge. However...it's DOA at the moment. :(

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 Post subject:
PostPosted: Mar Sat 20, 2010 10:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mt. Airy, Maryland
Geoff, I remember reading about those somewhere... ER perhaps. A cool score to be sure. The one I have is somewhat beat - someone painted the cabinet baby blue. It was $150 or so as I recall.

Carl, I'd go with the 8935/E6380A/E6381A. The 6380 and 6381 are similar; one is a TDMA analyzer and the other is a CDMA analyzer. Not sure why there are 3 model #s for the same (almost identical) analyzer.

The 8935s go for about a kilobuck - that's about 1/3 the price of an 8920. The differences:

- 8935s have 30 kc and 230 KC IF filters. They are standard 10.7 MHz xtal filters and the 30 KC filter could easily be replaced. I haven't done that yet on mine.

- 8935 has a plasma screen instead of the CRT that's gonna go dim.

- 8935s all come with the spec analyzer and tracking generator.

- 8935 does NOT receive or generate DPL. It will generate CTCSS and touchtones, but won't decode them.

- 8935 has an RF tools software suite. Add a directional bridge and you have a nice antenna analyzer.

- 8935 has a 100W dummy load. Some of the other HP CDMA analyzers only have a 5 watt load.

- 8935 won't count the transmitter frequency for some reason. It will give you freq error if you tune the 8935's receiver though.

I've never used an 8920, but I'm very happy with the 8935.


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 Post subject:
PostPosted: Mar Sun 21, 2010 12:33 am 
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Joined: Oct Sat 20, 2007 3:36 am
Posts: 14393
Location: Southern NH, 03076
You must have been feeling flush to part with that much green John. Wow, $150 :lol: :roll: :wink: .

Some time in the mid 90's I spent $250 for the last production run HRO-500, LF-10, cable, original manuals, speaker, NIB rack mount kits. Guess I shouldnt complain about you....we often think alike :oops:

OK on the 8935. I read that comparison at http://www.amtronix.com/e6380a.htm
but wanted a hands on report. Thanks, I'll be looking for one.

Just got back from York, Maine from #1 sons 40th birthday bash. Im stuffed from lobster, steamed clams and mussels. Now to fall asleep trying to read a book.

Carl


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 Post subject:
PostPosted: Apr Sun 25, 2010 12:48 pm 
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Joined: Apr Sun 25, 2010 12:33 pm
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Hi everyone,

I own an HRO-500 which works very fine except for the fact that the audio is not clean.

In particular, when tuning an unmodulated carrier in SSB, I can distinctively hear two tones separated by some 200Hz (I determined the shift looking at the beat on an oscilloscope). It is like hearing a DTMF signal, though the unwanted tone is by far below the wanted tone than in the DTMF case.

Does anyone experience a similar problem? Could that be due to an amplitude modulation of the syntetizer output signal?

Thanks in advance for answer and 73.

Tony I0JX
Rome, Italy


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 Post subject:
PostPosted: Apr Mon 26, 2010 2:04 am 
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Location: Southern NH, 03076
When was the last time the radio was overhauled and aligned? That includes replacing all the original electrolytics at a minimum....there are a lot of them.

Carl


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 Post subject:
PostPosted: Apr Mon 26, 2010 5:00 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2814
Location: Mt. Airy, Maryland
There were issues with the product detector outlined in an old issue of Electric Radio as I recall. Don't remember the specifics but it had to do with a factory mis-winding of one of the IF transformers.

Also I had plenty of audio distortion in mine. Check all the resistors in the audio circuit. I replaced more than a dozen resistors in the unit I described above.


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 Post subject:
PostPosted: Apr Mon 26, 2010 5:38 pm 
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Thanks for answers. Problem is that the "twin tone" I hear is neither due to power supply hum (changing the electrolytics would then not help) nor to plain audio distortion.

It is probably due to a 200-Hz amplitude modulation of one of the conversion oscillator frequencies, that is equivalent to having two closely spaced beat frequencies instead of a single one.

Do you know whether the old issue of Electric Radio you mentioned is available on the web?

Thanks and 73.

Tony I0JX
Rome, Italy


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 Post subject:
PostPosted: Apr Mon 26, 2010 6:05 pm 
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Joined: Oct Sat 20, 2007 3:36 am
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Location: Southern NH, 03076
There are only 3 electrolytics in the PS section. Another 17-20 electrolytics (depending on the production run) are used just about everywhere, including coupling. The production Series are 75, 88, 88A, 102, and 122. The 75 had many problems.

There are several oscillators that MUST be precisely aligned with a counter. Those old transistors also develop leakage. I have an 88 on the bench now for a customer and it is giving me the usual grief :wink:

It is not an easy radio to do a proper overhaul on and a half way attempt results in poor performance.

The Electric Radio of March 1997 is the one you want and can be purchased thru www.ERmag.com. The earlier one was rather a useless article.

Carl
National Radio 1963-69


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 Post subject:
PostPosted: Apr Tue 27, 2010 1:08 am 
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Location: Beaver Falls, PA. USA
Did National have the same resistor problems by the time that they got around to building the NCX-1000?

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 Post subject:
PostPosted: Apr Tue 27, 2010 1:51 am 
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Location: Southern NH, 03076
We didnt have any resistor problems when they were built :lol:

The Series 88 HRO-500 Im finishing up only had two out of tolerance resistors and they were only at 20%. There were no bad disc caps or silver micas; just one shorted transistor, dirty switches, a bunch of those pesky 25V electrolytics and well out of alignment.

Carl


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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Sat 08, 2014 11:12 pm 
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Location: Southern NH, 03076
This is an old post I dug up and noticed an omission I made. The last HRO-500 run was the series 140. Plus the 88A were actually recalled or returned 75 series that received all the 88 updates plus all the black .1 CD discs were replaced. There may have been some unsold 75's in stock that were also refurbed. They were remarked 88A to let service people know it didnt have to be overhauled.

I dont remember how many I did before the quantity was overwhelming and other service was being delayed. Then a couple of the women on the production line changed all the caps and I just had the aligment to do.

Carl


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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Thu 13, 2014 3:23 am 
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Greetings to Carl and the Forum:

I heard somewhere that the HRO-500 series of receivers were full of germanium transistors which are no longer available, and that therefore the receiver is unrepairable and should be avoided. Since you are repairing them, it doesn't sound as though that information is accurate. Would you care to comment?

Thanks,
Jim T.
KB6GM

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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Thu 13, 2014 1:27 pm 
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Hi Jim,

Maybe Carl will chime in with a great source for germanium transistors :) I have a HRO-500 and restored one for a friend and both still are fine with their original transistors so it isn't like they are all going to go bad in mass. I recently picked up an Eddystone EC-10 and the germanium transistors in it are famous for developing "tin whisker" shorts to the case. Mine is working and I have read that in the UK where there are far more of them the typical fix is to use silicon and change the bias resistor values to work.

Bottom line is that some germanium transistors are still readily available and Soviet produced surplus shows up fairly often on ebay so some of those might be usable. It is also possible to slightly modify the circuits to use silicon transistors so if you really want a HRO-500 it is certainly possible to keep it running. I am sure Carl will provide more detail soon (or google) the production runs.

It is a very nice looking fun receiver and I had wanted one very since I came across the Popular Electronics featuring the HRO on the cover. I have mine set up with a NCX-5/VFO/NCL-2000 station and I believe this is one of the most attractive setups made by any of the manufacturers, certainly in the more modern era.

Rodger WQ9E


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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Thu 13, 2014 6:55 pm 
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rsingl wrote:
Hi Jim,

... I have read that in the UK where there are far more of them the typical fix is to use silicon and change the bias resistor values to work....


Rodger WQ9E


Oh my, this will destroy the acoustic ambiance.... :mrgreen:

Tony

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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Thu 13, 2014 7:31 pm 
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Location: Southern NH, 03076
Ive had good luck locating old US, European, or Japanese compatible stock and culling out the leakers from critical applications such as in the synthesizer. There are also some NTE replacements. Most anything works in the IF and slight differences in AGC action go unoticed in actual use. I do replace the product and AM detector diodes with Schottky 1N5711's.

Run as designed it is a superb radio and the extremely low phase noise makes it easy listening with good headphones or speaker; the audio bandwidth is out to beyond 5 KHz. Sensitivity is quite constant at under .2uV on AM for a 10dB SNR right to 30 MHz; the RF amp and first mixer are early generation silicon as are a few others, the germaniums further along on AGC are fine as is. Rebiasing those two devices with a 2N5109 as well as tweaking the AGC might be very beneficial; Ive never had the time or inclination to attack it.....yet. As with the competing 51S1, plus the various 51J series and variants, it does not like big (long and high) wire antennas; big yagis can also aggravate overload on bands where using the switched attenuator also puts the desired signal in the noise. Since the internal noise floor is so low Ive had no problem with a homebrew 90' T2FD BCB to 30 MHz. With the LF-10 and serious BCB use I use five 2 wire Beverages for a total of 10 directions.

What is critical is the PS voltages as well as those at the transistors but watch for unpublished differences between runs. For instance the 12V rail was changed to 13V, a simple zener change to compensate for tolerances in the circuits they feed directly. Ive also found several of the ancient 5.0 and 7.5V zeners down .5V or more which upsets a lot of things, new modern 5% take care of that for pennies.

DeOxit is to be used very sparingly with the tiniest on the HFO and buffer wafers and only on the rotors, switched thru all 5 positions multiple times, and then quickly flushed with disc brake cleaner. I use the D100L-25 needle dispenser but smaller amounts are available. It may take up to 1-2 days to stabilize the HFO locking since those cheap phenolic wafers are very hygroscopic, but even a tiny bit of silver sulfate can be troublesome at such low voltages. Some sets that come in here were almost ruined by idiotic bathing the wafers with a DeWrex-It spray or brush that takes multiple cleaner baths and several weeks of drying in total. This is one of the first steps I do so by the time Im thru replacing all the electrolytics, some other caps, out of tolerance resistors, test all transistors, etc, it is ready for alignment.

My own HRO-500 and LF-10 has been trouble free since being overhauled about 25 years ago; the R-390A hasnt been as reliable with several minor annoyances over the same time period.

Carl


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 Post subject: Re: HRO-500 challenges
PostPosted: Mar Thu 13, 2014 11:31 pm 
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Burnt Fingers wrote:
My own HRO-500 and LF-10 has been trouble free since being overhauled about 25 years ago; the R-390A hasnt been as reliable with several minor annoyances over the same time period.

Carl


Carl,

It has only been 9 years since I restored my HRO-500 but it has been trouble free and it gets a lot of run time. During the 5 years I have had my Galaxy R-530 I have had to touch up its synthesizer alignment twice since the initial repair and alignment. The HRO is a pretty unique receiver and it is one I wouldn't part with. If a transistor dies I will find a suitable replacement.

The only germanium I have replaced thus far was an IF transistor in my Heathkit Mohican. Certainly some of these transistors fail but I think the internet reports have blown it far out of proportion, sort of like "you can't use a 6146B in that radio" ;)

Rodger WQ9E


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