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R-390 HUM on LOUD signal symptom - HELP!




 
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October 23, 2018, 02:04:52 AM *
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Author Topic: R-390 HUM on LOUD signal symptom - HELP!  (Read 698 times)
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WBear2GCR
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« on: March 03, 2018, 08:47:12 PM »


R-390 (non "a").
Symptom: With strong RF, AM signal, any band, a hum appears with signal, loud, noticeable.
Either with AVC or MVC.
Reducing RF Gain control, hum reduces...
...with strong signal increasing RF gain, hum/distortion goes up.

Tweaking ANT tune, if the signal increases - hum/distortion increases.

Stumped at the moment.

Changed out the IF strip module and the AF/regulator module, swapped out the 1st RF and 2nd RF tube.
Unplugged the CAL BNC cable.

Unlikely that the same problem shows up on 2 modules, but I suppose it is possible.
There still are tubes that could be swapped on the RF deck, I guess... (I'll do that, I suppose, just
to be sure)

Anyone got an idea here?? I don't have a theory on this one.

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KA2DZT
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« Reply #1 on: March 03, 2018, 08:57:52 PM »

Sure you don't have a power line near or touching your antennas
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WBear2GCR
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« Reply #2 on: March 03, 2018, 08:59:53 PM »

Doesn't do this on my R-388... but... I suppose I can look at my coax going up in the air...
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nq5t
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« Reply #3 on: March 03, 2018, 09:17:39 PM »

I think there may have been some discussion of this here .. or possibly on the boatanchors list .. some time in the past.
Could be related to this situation involving hum modulation something in the shack, or perhaps in the R-390 power supply.

http://home.computer.net/~pritch/shortwav.htm

There are also some related threads on the Antique Radio forum and other places, such as this one:

http://www.antiqueradios.com/forums/viewtopic.php?f=2&t=210176
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WBear2GCR
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« Reply #4 on: March 03, 2018, 10:01:16 PM »

cathode to heater leakage was what I was hoping to find, but there are only a couple of tubes left to swap out... Sad
maybe a miracle and it will be one of them... <sigh>

It was not doing this a few weeks ago, so something changed.
Just haven't spotted it yet.
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WBear2GCR
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« Reply #5 on: March 04, 2018, 04:15:53 PM »

As of now, I'll say that I found the problem.

Looks like ONE of the two 26Z5W rectifiers was NG or very weak.
My cheezy tube tester doesn't know what they are.

The ~180vdc test point looked good, which is after the regulator.
Did not see a lot of ripple using the DVM.
I was going to haul out the scope to check.

But both of the Regulator/Audio modules exhibited the same symptom, which was the
tip-off. Both of the gas regulator tubes "blinked" when the RF gain was turned up on
strong signals. And the distortion coincided with that event.

Last night it occured to me when looking at the schematic, that it might be a lack of sufficient
current availability, and the one thing that might cause that is one rectifier not working
in whole or part...

Of course, if it comes back when the rig gets moved, that wasn't it... but I think it was.


* R-390 PS + RECTIFIERS.pdf (65.03 KB - downloaded 26 times.)
* R-390 VREG SCHEMATIC.pdf (50.05 KB - downloaded 21 times.)
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