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I need help with receiver restoration




 
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Author Topic: I need help with receiver restoration  (Read 1344 times)
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W8ACR
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« on: April 03, 2022, 02:10:26 PM »

I've never been very good at receiver repair, so I thought I'd try to repair something simple, a Heathkit HR-10B. When I started, the receiver lit up, and that was about it. I found a few cold solder joints, and replaced one IF transformer and a few other components, but I have hit a snag that I can't seem to overcome.

When I apply power and turn up both the AF and RF gain, no antenna attached, I should hear white noise, or static type noise, right? All I hear is a very low level hum, barely audible, and not changed much by rotating the AF gain control. If I touch the grid pin of the audio input or audio output tube, I get a much louder hum. If I inject a modulated IF signal at the detector input, I get the appropriate tone out of the speaker that is properly controlled by the AF gain control.

My interpretation is that the audio sections are functioning properly, but obviously something else is not. Grounding the AVC line makes no difference. All tubes have been tested on a Hickok tube tester. DC voltages are uniformly about 20% higher than what is shown on the schematic. Equipment at my disposal includes a VTVM, signal generator capable of 400Hz and 1000Hz modulation, oscilloscope, and frequency counter.

Any ideas of what to do next?

Thanks, Ron
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Pete, WA2CWA
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« Reply #1 on: April 03, 2022, 04:23:20 PM »

Take your RF signal generator and insert an IF frequency at the output of the mixer\input to the IF chain) and, using your scope see if you have gain, hum, or whatever.  You can also pick a frequency, say 3 MHz, and input it at the antenna terminals to see if it's getting through to the mixer.

If you like to watch Youtube videos and/or 1001 ways to fix a tube radio, go here where there's lots of info:
https://www.google.com/search?client=firefox-b-1-d&q=how+to+repair+a+tube+radio
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« Reply #2 on: April 03, 2022, 10:57:25 PM »

Hi Doc,

Lots of deaf receivers suffer from the RF input coil being open from a surge in the antenna line.

Ohm it out, it might be the problem.

73DG
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WD8BIL
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« Reply #3 on: April 04, 2022, 11:33:04 AM »

Also check the local oscillator. If it ain't oscillating you won't have an I.F. signal.
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W8ACR
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« Reply #4 on: April 04, 2022, 02:18:33 PM »

Thanks everyone for the replies. I have watched the youtube video that Mike WU2D did on the HR-10. This was very helpful. I'm pretty sure the RF section, local oscillator and IF section are working well. Injection of signals into the antenna jack and IF strip and observing waveforms on the o'scope seems to indicate that these sections work properly. The LO likewise has good output of the correct frequencies.

The next thing I am going to check is the phone jack on the front panel. I'm thinking the poor audio output may be as simple as a dirty contact. I'm also not convinced the AVC voltage is correct. The S-meter pegs quickly when the RF gain is turned down from full clockwise rotation if AVC is turned on.

I have shifted gears, and now think that the problem is in the audio section, and perhaps an AVC issue.

Thanks again, Ron
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« Reply #5 on: April 05, 2022, 02:14:08 AM »

That unit is pretty old, have you checked all the dropping resistors values and checked for leaky/shorting decoupling caps for the screen grids of the tubes? I had an intermittently opening G2 resistor on a different set and it drove me nuts. As usual it was the last thing I checked!
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« Reply #6 on: April 05, 2022, 04:25:20 PM »

Ron,

I believe that is normal for the HR-10 with the S meter behavior.   It has been a long time since I did any work in an HR-10 though.  The IF stage has a lot of gain and depending on how you tune the IF cans, you can actually make it oscillate.  Slightly de-tuning the cans usually fixes that problem without altering the shape factor of the IF chain.  The BFO tends to pull on strong signals.  There are many good mods out there to fix that issue. One of these days, I want to go through an HR-10 I have here and redesign the sections that have some issues.  I think with some relatively simply mods, it can be a better receiver.  It will never be a great receiver, but a better one!  There are some AGC mods out there as well. 

Joe
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W8ACR
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« Reply #7 on: April 26, 2022, 12:17:44 AM »

OK, I've been plugging along working on the HR-10. I replaced all the components associated with the Detector/AVC tube, and I also replaced the RF gain pot since it was noisy and erratic. IF alignment went well, RF alignment a bit more difficult, but after many practice runs, it seems that the front end is also properly tuned up. The receiver now receives well, and is usable, but not perfect. There is still an AVC issue.

When the AVC is off, I can open up the audio gain and adjust signal strength with the RF gain - as expected. The receiver actually works pretty well in this fashion. If I turn the AVC to ON, I can tell that the AVC circuit does work to a certain degree. With the RF gain at say at 75-80% max, turning the AVC on and off definitely affects the signal strength. With the RF gain at full open however, the signals are loud and distorted, and sound the same whether AVC is turned ON or OFF.

It seems that I need a more negative AVC voltage, or maybe less IF gain when the RF gain control is wide open. There are two AVC controlled tubes - the RF amplifier, a 6BZ6, and the second IF amp, a 6EA8. The AVC voltage is generated by a 6BJ7 diode, and is dropped by a 1 meg resistor at the beginning of the AVC line. I was thinking of maybe reducing the value of the 1 meg resistor, or detuning the IF transformers slightly to reduce the IF gain.

Any thoughts? Thanks, Ron

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