fender stage se112 - warble at low volume

[jbmdisco]

I have the above in at the moment. It came in with to start off with a blown speaker. Closer inspection revealed that a crossover had been fitted with a small horn ??? I removed this 'mod' and had a look at the board. 4 different types of output chips, now replaced, all output resistors blown, now replaced. Another look over has revealed no other 'mods'. After powering up at low volume - all channels, there is a warbling sound which disappears after the volumes go above 2-3. I believe there may be a grounding fault. With a signal to the input and scoping with the heat sink as the scope ground the warble shows at the input jack. Using the jack as the ground the signal is reasonable until we reach the output stages (my scope is an aging tool. I usually use the pc scope but this week it has issues)
Any ideas?
Cheers
Jbm

[Jazz P Bass]

I would suggest that the first thing that you would want to monitor is the power supply.
With no signal, the lower the Vac ripple on the main rail caps, the better.
There should (ideally) be zero volts ripple on the opamp supply.

[jbmdisco]

Thanks.
I did have a suspicion regarding the power supply and caps which now, with some confirmation, I will investigate more tomorrow. I have probably fallen into the trap where the main caps are relatively new spragues but given the previous state of the previous 'mods'/repairs there is every chance they may be an incorrect fitment. Unfortunately I keep all my schematics on my pc which has decided to play silly buggers this week but from memory I think they should be 6800uf at 100v. The other problem I had with this when it came in was I was handed the cab, the amp then the tranny Kit form repair I believe it to be known as.
Regards

[g1]

This print is showing the main filters as 3300uF

[gui_tarzan]

I have that amp. I played it for a long time in our old church praise team and it has a gorgeous clean sound.

[jbmdisco]

Thanks for the values G-one.
G-T, funny you should mention your use of the amp because the owner uses it in his local Church for his music group. This is the 2nd one I've had in within 6 months and have to agree about the clear sound, with plenty of power.

[J M Fahey]

1 small detail which may confuse Techs: the output rail is grounded, so you will never be able to measure offset at it

The proper measurement is from (chassis) ground to big filter caps center point (labelled as C.T. in the schematic).

Yes, you got it, I also got burnt by that

Of course, if you measure across the speaker out terminals (CP13/14) it's fine , but not from ground to hot.