Fendet twin reverb osillating when the reverb depth is increased

[Jay D]

Hello forum members:
I have a silver face Fender Twin Reverb amp that works normal until you increase the reverb depth, and then the amp now breaks into oscillation. The frequency measures 850 Hz at minium reverb intensity (around 2), to 2500 Hz AT Maximum reverb intensity (10). I have checked and substituted all components in the reverb circuit, and also new tubes 12AT7, 12AX7, and new reverb unit.
The driver transformer primary measures 1,000 ohms DC resistance, and 0.7 ohms DC resistance on the secondary. I recently purchased a new driver transformer, and the DC resistances are within 10% of the new transformer.
I have also replaced reverb foot pedal RCA jack, and the associated 220K resistor, and still no results.
Does anyone have suggestions on further troubleshooting this amp for the above problem?

[Jazz P Bass]

Where are you measuring the oscillation?
At the output of the amp.
Try to narrow down the issue.
You have already replaced everything in the reverb driver stage.
How about looking at the recovery & the mixer stage?

[pdf64]

Using the TRRI schematic as a reference
http://support.fender.com/schematics..._schematic.pdf
because it's got part # references, C15 the cathode bypass cap on V4 is your most likely suspect. If that develops high ESR, then a positive feedback loop is created when the rev intensity is turned up. Pete.

[Enzo]

Take the screws out and move the reverb pan outside the amp cabinet. Lay it nearby. Does it still do it?

In fact before that, set the amp up so it is doing this, then reach in the back and grasp the reverb pan tightly. Leave it in the bag, just grab it hard to prevent vibration. ANy help?

Is the pan sitting in there loose in the bag? You don;t have the pan screwed down tight do you?

[Jay D]

Hello:
The reverb pan is not connected to the amp, and the oscillations occure.
The suspicion is now elseware, as I have checked both the reverb driver and the recovery stage components.
I have also substituted a good new reverb transformer, and still oscillates.
I m checking all cathode bypass caps, etc. looking for any high impedance paths which would give the feedback necessary.

[Jay D]

Hello:
The oscillations were measured at the output speaker terminal, into a non-inductive load.
Also, measured at the driver stage transfomer, and at the plate resistor of the recovery stage 12AX7A.
I have been checking all components of both the driver and recovery stage, and the mixer stage components.
I am now looking into capacitors, and any other source of high impedance to help sustain the oscillations.

[pdf64]

Have you checked C15 (just tack a good cap in parallel with it)?
If not that then C34 on power supply node X.
IF not that then try a different tube in V4. Pete.

[Jay D]

Hi Pete:
Thanks for the tip. The schematic is the AB763, and the problem is related two components. First, the bypass cap 25uF paralleled to the 820 ohm resistor in the reverb recover amp circuit.
The cap was measuring infinite on my fluke meter, and the second source is the power supply filter capacitor, 20uF/525VDC at voltage point D. I had previously changed the caps and resistors in the power supply, and believe it or not, I had a spague atom cap go bad. Replaced both caps, and the oscillations ceased.

[Jay D]

Fender Twin Reverb Oscillating

Take the screws out and move the reverb pan outside the amp cabinet. Lay it nearby. Does it still do it?

In fact before that, set the amp up so it is doing this, then reach in the back and grasp the reverb pan tightly. Leave it in the bag, just grab it hard to prevent vibration. ANy help?

Is the pan sitting in there loose in the bag? You don;t have the pan screwed down tight do you?

Hello Enzo:
Thank You for your trouble shooting information.
The trouble wound up being an open bypass capacitor, and a power supply capacitor went bad.

Again;
Thank you for your innput.

Regards,

Jay D