Hot Rod DeVille Volume and "feedback" problems

[Xico Pereira]

My Hot Rod DeVille amp is presenting two problems that may, not sure, whether caused by the same defect. Problem one: even with the volume control at full CCW (Normal Mode) the amp continues to produce sound. Problem 2: When I select the Drive Channel with the Treble control over 20% a noise like a feedback (high frequency) appears, moving the Bass Control CW the frequency decreases. The Middle control also produces the same changes in the frequency of the noise. If I put the three controls (Treble, Bass, Middle) at full CCW, even with Drive and Master controls turned CW, the noise disappears completely. Can someone guide me (or help) on what might be causing this or these problems. What components should I check etc..

[pontiacpete]

have you tried changing any of the pre amp tubes around? Sometimes the one on the input, the far right, can become microphonic.

[Xico Pereira]

have you tried changing any of the pre amp tubes around? Sometimes the one on the input, the far right, can become microphonic.

Yes I replaced all valves including 6L6 and the problem remains.

[pontiacpete]

Did you re-solder both boards? They usually have a lot of bad solder joints.

[Xico Pereira]

Yes I re-solder all of them including other points that seemed to have problems in other parts of the circuit.

[pontiacpete]

Unfortunately, I'm not one of the experts on this forum, but have you tried tapping the components around the board to see if any are microphonic?

[Xico Pereira]

Let me tell you something. I pulled out the Drive and Volume pots and the problem continues, inclusive in Normal mode the amp continues to produce sound.

[pontiacpete]

have you tried tracing a signal through the amp with a scope?

[Xico Pereira]

No, I do not have one. Don't You think the fact of the amp to produce sound (Normal Mode) even with the volume at zero can be a symptom of the problem of feedback.

[pdf64]

I suspect a power supply cap (eg with high ESR), probably on the pre amp supply node.
EDIT C36 http://www.webphix.com/schematic%20h...od_deville.pdf
That could allow signal to bypass a vol control, and a positive feedback loop, with sufficient gain and phase shift (from the tone controls).
Pete.

[Xico Pereira]

'll check all the power supply caps and inform the results when finished.

Thank you all!

[Xico Pereira]

I suspect a power supply cap (eg with high ESR), probably on the pre amp supply node.
EDIT C36 http://www.webphix.com/schematic%20h...od_deville.pdf
That could allow signal to bypass a vol control, and a positive feedback loop, with sufficient gain and phase shift (from the tone controls).
Pete.

Thanks pdf64, spot on advice. You're a life saver.
The problem was the C36 just as you suspected.

Thank you very, very much.

[pdf64]

As one elecrolytic has failed, it's best to replace them all. Not a good idea to wait for them to fail, as them can cause collateral damage when they do so.
Pete.

[Xico Pereira]

As one elecrolytic has failed, it's best to replace them all. Not a good idea to wait for them to fail, as them can cause collateral damage when they do so.
Pete.

All right, I'll do it.
Xico.