37-650 fading performance - Printable Version

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+--- Thread: 37-650 fading performance (/showthread.php?tid=17272)

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37-650 fading performance - Tgace459 - 09-15-2017

The 37-650 I have starts up fine and strong but fairly soon (5 min or less) starts to "fade out" to low volume. I have to turn the volume pot to almost max to get "normal volume".

If I touch or even get close to the grid cap of the 6K7 tube that attaches to the tuning cap I get full volume to return. I've swapped out the 6K7 tube but get the same results.

Any suggestions on where to start looking? Output tubes getting gassy as they heat up?

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RE: 37-650 fading performance - Mondial - 09-15-2017

I doubt its the output tubes. If the volume returns when you touch the cap of the RF amp 6K7, then it probably not related to the output stage. Does the B+ power supply voltage change when the problem occurs? Gassy outputs would make the B+ voltage drop noticeably.

Sounds like it might be related to the AVC circuit which controls the negative DC bias voltage on the 6K7 grid. This AVC circuit is also connected to the 6A8 and the 6K7 IF amp, so I would also try replacing these with known good ones.

You can also check the resistors marked 7 and 34 on the schematic and see if they have increased significantly in resistance.

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RE: 37-650 fading performance - robsradioactive - 09-18-2017

What's your oscillator voltage doing? Is it going down until you touch the grid cap?
(I am assuming it's not de-tuning itself...that is, turning the tuning knob does not bring back full volume.)

Rob

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RE: 37-650 fading performance - morzh - 09-18-2017

I had the same problem, happened to be the 75 tube.
Can be any tube, can be other reasons, but I would replace the tubes one by one.

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RE: 37-650 fading performance - Tgace459 - 09-19-2017

(09-18-2017, 06:44 PM)robsradioactive Wrote:  What's your oscillator voltage doing? Is it going down until you touch the grid cap?
(I am assuming it's not de-tuning itself...that is, turning the tuning knob does not bring back full volume.)

Rob

Actually. If I could stand there with my finger on the grid cap all day I'd have a perfect playing set.

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RE: 37-650 fading performance - morzh - 09-19-2017

(09-19-2017, 09:08 AM)Tgace459 Wrote:  Actually. If I could stand there with my finger on the grid cap all day I'd have a perfect playing set.

Might be worth it

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RE: 37-650 fading performance - NostalgiaRadioTime - 09-19-2017

Clip a gator wire from the grid cap to your long wire antenna and you can get your finger back!

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RE: 37-650 fading performance - robsradioactive - 09-21-2017

heehee

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RE: 37-650 fading performance - Tgace459 - 10-24-2017

Hmmm...AM was fading but AUX input was working fine. Now even the AUX is faded out to almost static.

I'm gonna have to pull the chassis and take a look.

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