[R-390] Audio HUM WAS: B+ short in RF deck

[Charles Steinmetz]

Dennis wrote:

>An update to the audio him issue.  Received replacement caps for
>C603 and 606 and replaced the 20 year old caps on the headers I made back
>when I got the receiver.  Audio hum remains at previous levels.
>
>Placing a 47 mf cap across the B+ reduces the hum drastically, as
>before, and another 47 mf cap in parallel drops the him down to about 160
>mv on a 195 volt B+ line.
>
>Obviously, something isn't working right.

When you eliminate the impossible, whatever remains, however improbable....

It sounds like C606A/B are not actually in circuit due to a bad 
ground, broken wires, bad socket connections, bad solder joints, or 
whatever.  If C606A/B are each ~45uF (per the schematic), then adding 
another 47uF should only drop the AC hum voltage 50%.  Since it drops 
the hum drastically, one must conclude that C606A/B are not really 
connected.  Are you sure you didn't get the header pins mixed up when 
you built your plug-in caps (sometimes people get confused with pin 
assignments as viewed from above vs. below)?

Here's a test -- pull C606 out.  Does the hum get worse, or stay the 
same (as measured w/ an oscilloscope at the nodes where C606A and 
C606B should connect)?  From what you said above, I'm guessing it 
stays the same.

If all else fails, pull C606, throw it away, and replace it with two 
47uF capacitors soldered in under the chassis.

(I didn't mention C603, because it isn't contributing to THIS 
problem.  But it may have the same problem as C606.)

Best regards,

Charles