Fender Rumble 100 Bass Amp Problems- Help, Please.

[BStringThumper]

[SIZE=4]Hi. Thank you for any help or suggestions.
This bass amp works fine until you turn the volume up over 1/4 setting. Then the speaker will oscillate in full negative and then out positive. With a meter on the speaker wires before the problem starts there is not -Vdc, when I turn the volume up the meter will go -24Vdc and then swing positive +20-24Vdc and then go negative again. Turning the volume down, unplugging the input jack, will not stop the oscillation. Interestingly, if the input jack is not plugged in when powering up the amp I can turn it full volume and the problem does not happen. Opamp rails are steady at -/+16 Vdc.. Output transistors are -/+41 Vdc before problem, then fluctuate from -/+ 33-41Vdc. Any thoughts would be greatly appreciated.

[Jazz P Bass]

With nothing plugged into the input jack, measure the Vac ripple on the + & - 41Vdc & the + & - 16 volt dc rails.

[BStringThumper]

Hi Jazz, thanks for the response.

No input-

Vac readings:
D30 31.8 Vac on cathode
D31 31.8 Vac on cathode
D32 91.0 Vac on cathode
D33 91.0 Vac on cathode
Was not able to probe D30-D33 anode with board on heatsink.
D34 35.1 Vac on cathode side
D34 0 Vac on anode
D35 0 Vac on cathode
D35 0 Vac on anode
-------------------------------------------------------
Vdc readings.
D30 0 Vdc on cathode
D31 0 Vdc on cathode
D32 41.7 Vdc on cathode
D33 41.7 Vdc on cathode
D34 16.3 Vdc cathode
D34 0 Vdc anode
D35 -16.7 Vdc anode
D35 0 Vdc cathode
Q17-19 -41.7 Vdc
Q16-18 41.7 Vdc
Q15 -41.7 Vdc
Q14 41.6 Vdc
Q10 - I need to get data sheet.
right leg 1.1 Vdc
center leg -1.1 Vdc
left .5 Vdc

Thanks for taking a look at these numbers.

[BStringThumper]

I'm sorry Jazz, you asked for ac ripple. I'll have to learn how to do it. Sorry.

[Jazz P Bass]

The capacitors on the + & - 41 volt rails are there to help smooth out the pulsating rectified dc voltage.
Set your meter to read Volts AC & probe the rails.
Black to ground. Red lead to the +41 & then the -41 rail.
If the caps are bad (which is what we are trying to decide) there will be an excess ac voltage component riding on the dc voltage. (a small amount is o/k)
Vac ripple.
The same idea applies to testing the opamp power supply voltages (+ & - 16Vdc).
Although for the opamps you don't want to see any Vac component.
That will drive the opamps nuts.

[BStringThumper]

Good morning, Jazz.
I did my voltage readings both ac & dc with the black probe to chassis ground and red proble on anode or cathode. I probed IC U6, pin #8 has 35.1 Vac on it, pin #4 has 0 Vac, pin#4 has -16.7 Vdc, pin#8 has 16.7 Vdc. So, it looks like I have a dc component on the positive side. That same voltage is at D34 cathode side. I will pull the the diodes and capacitors out of circuit and test them. I have an EICO cap tester. Thanks for the help, Jazz.

[BStringThumper]

AMP IS WORKING.................not sure why!

I pulled the capacitors out and tested them, they are fine. Diodes tested fine. Put everything back in and powered it up...........now it works. I've had it full blast into a 8 ohm 100 watt load resistor and can't get it to fail now. Confused.
Thanks.

[Jazz P Bass]

You had a bad solder joint.
Putting the parts back in & resoldering fixed it.
Maybe?

[BStringThumper]

I really hope so. I've never heard an amp do this. Fingers crossed.
Thanks again, Jazz.