Philco squeals on weak stations

[andrefranc]

After changing filters the radio squealed at just some volume settings. I recapped and repaired all block capacitors and now it will squeall on shortwave stations in the same manner but BC band seems to be OK now.
when squealing I can turn the volume control slightly and this will usually stop the squealing. I tried a new volume control with the same results. Using one setting of the tone control signal is suppressed but doesn't seem to ever squeal. I disconnected the tone circuits but stilll had squealing.

[Reece]

What model set is this? Something is oscillating that shouldn't be, probably in the RF or IF section. Are all tube shields in place and making contact with their grounding bases? Try this: with chassis upside down and radio squealing, carefully move wires and components slightly in the RF and IF sections with a plastic or wooden stick to see if squeal stops. Sometimes improper "lead dress" can cause oscillation, when plate circuit parts are too close to grid circuit parts.

Reece

[andrefranc]

Philco Model 635. ON strong stations when I turn up the volume real loud you can hear the shreeking in the background and on weak stations there is shreaking but if you turn the tone control fully clockwise it will go away.

[toxcrusadr]

There could be a cap somewhere that is supposed to shunt that crap to ground at a certain point, that is either open, not connected to ground, etc. Also second the lead dress theory.

[jonboy55]

I just repaired a 1936 Emerson with the same problem.

Though its a long shot, I found it was the 6A7 tube. It tested good but in the circuit it didn't work. You might want to check your 6A7 or 78's.

Good luck.

[andrefranc]

Thanks for the tip but I think I tried that once. Will check it again.

[wa2ise]

There's schematics and info for that model at http://www.nostalgiaair.org/Resources/814/M0013814.htm

[andrefranc]

Found the problem. The grid wire to the 75 was not run inside the shield can. It was outside the shield and next to the 42 tube giving feedback at times.

[toxcrusadr]

Excellent! Amazing how something that simple can make all the difference. Glad you got it fixed.