[R-390] Intermittent

[Barry]

On 5/1/2010 6:49 AM, Steve Hobensack wrote:
> I have been troubled with an audio intermittent for a quite a while. The audio will jump to a
> low level and snap back a minute or so later. This will happen many times a day. The carrier level is not affected. I have traced
> the problem to IF transformer t-503. It is the transformer that couples the last IF tube (6ak6) to the detector (5814a). When the audio is low, pulling the cover from t-503 will increase the volume. I can momentarily touch a clip lead across the primary, and the radio will snap back to
> normal and maybe stay that way the rest of the day. There are three components shunted
> across the primary. Those are a Q swamping resistor, a pink colored square mica cap, and
> a brown encapsulated tubular ceramic cap. Before I do micro surgery, which one of  those components might be the culprit? What are the values of those capacitors?
> Thanks
> Steve N8YE
>   		 	   		
>
>    
Steve,

I would assume that a resistor would not behave that way.  It sounds 
more like capacitor issues. I know the silver mica caps in the 
older-style IF cans can be a problem. I had one go bad and when I opened 
it up, it was obvious the silver had "migrated" (I believe that's the 
term).  Replacing it with a dipped silver mica fixed the problem for me.

As for values, I'm sure they vary, but I *believe* they're around 
100pF.  Perhaps someone else on the list can confirm.

Good luck with it,

Barry - N4BUQ