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Mystery AVC issue on 38-9 - Printable Version

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Mystery AVC issue on 38-9 - spierce328 - 03-01-2020

Hello all. I can use some help on another mystery problem, this time on a Philco 38-9

http://www.nostalgiaair.org/Resources/262/M0013262.htm

This is a customer radio that will drop the volume after about 10 min playing. Low to moderately strong stations drop out completely, while stronger stations are still at a listenable level, although equally diminished.

Measuring the AVC voltage tuned to a moderately strong station, at the junction of resistor 15 and cap 3, I see that the voltage starts at about -5 VDC then slowly rises to about -.5 VDC, where the radio abruptly loses volume. Turn is off and let if cool, for a few minutes and and cycle repeats.

I have replace all Electrolytics (twice), all other caps (including the two micas ) and most resistors including 43
I have swapped tubes multiple times
The power transformer voltages are stable
The voltages on all tubes are within tolerances

I have spent hours and hours on this one. I would appreciate any suggestions where I might look as to the source of the disappearing AVC voltage.

Thanks Steve

RE: Mystery AVC issue on 38-9 - mikethedruid - 03-01-2020

You haven't said whether you have replaced the paper condensers, I would start there. I suspect the problem is in #23, or #19a, 19b, or 19c. You also may have a resistor which is changing in value as it heats up, especially #20 or #21. It could also be #28, #31, #34, or #36. First step is to replace ALL the paper condensers if you haven't already. If one is sliding in value as it heats up, it means that ALL of them are not reliable any more. I always replace ALL the paper condensers as well as the electrolytics before I do anything else, even before I turn on a set of this age.

RE: Mystery AVC issue on 38-9 - Steve Davis - 03-01-2020

The AVC voltage is not the problem, it is a response to the problem. Something in the front end or the IF is causing the signal to drop, the AVC is trying to compensate. You might try a can of circuit cooler to locate the problem. 

Steve

RE: Mystery AVC issue on 38-9 - spierce328 - 03-01-2020

Yes, all the paper capacitors have been replaced. Steve

RE: Mystery AVC issue on 38-9 - spierce328 - 03-01-2020

Thanks Steve, I totally agree. I need to find root cause.  I'm not familiar with circuit cooler or how it's used. Can you fill me in.

RE: Mystery AVC issue on 38-9 - radiodale - 03-01-2020

Here is a source:

https://www.govets.com/maintenance-repair-and-operations/welding/soldering/soldering-accessories/circuit-cooler-aerosol-el-tm-2003-12-oz-en?gclid=Cj0KCQiA1-3yBRCmARIsAN7B4H23CD5EVYZSPnCfM5J7q-R8JypVlWBgTCJoAT213ZvzIgiVLAE0e7YaAvVNEALw_wcB

Do not keep it on one component very long. Quick spray is enough.

RE: Mystery AVC issue on 38-9 - spierce328 - 03-01-2020

Good stuff. Have you ever tried cooling the major components: transformers, variable caps, tubes, etc ?

RE: Mystery AVC issue on 38-9 - morzh - 03-01-2020

Canned air (duster) when the can is inverted act as a cooler.

RE: Mystery AVC issue on 38-9 - spierce328 - 03-03-2020

It was the 6K7G. As you all know, Philco radios, with the heat shield socket in tact it will only allow 6K7Gs. 6K7s or 6k7GTs wont fit, at least not without some encouragement from Mr. vice grips. I had 3, count 'em, 3 6K7G's test ok, but all would result the exact same symptoms in the radio. Out of desperation i ground off the rivets holding the socket and shield and put in a metal 6K7, ..... Bingo.

Thanks to all for the help. There are alot of great ideas here I can use next time. And one thing is for sure, there will always be a next time.

Steve