SVT3Pro power amp failure

[fredcapo]

Hi. I am working on an SVT3 Pro head that I was told just produced a buzz or hum. When I powered it up I intially heard the hum, then noticed
a burning smell. I quickly was able to verify that the preamp section was fine by patching out into another amp via the jacks.

Returning to the power output stage, I see that R27 is cooking, while its complement, R25 appears to be ok, at least visually.
My thought is keep the unit powered off and try to isolate any damaged or out of spec devices.
Is it safe to assume that one of the output FETs failed? If R27 is cooking I would assume that one or several of teh driver transistors is cooked.

My thought is to consider replacing all of the FET's and any other devices that appear to be out of spec.

I would greatly appreciate any guidance on this. Also if there are any recommended circuit upgrades I would appreciate suggestions (I know - buy a real SVT...)

I am experience in working with tube and transistor circuits and understand troubleshooting and safety techniques. However,
I have not previously attempted to troubleshoot this type of output stage.

Thanks for any assistance.

[mozwell]

Is there DC on the output.
Check all the power mosfets, drain to source, also measure each of the mosfet ballast resistors. Check Q6, Q5 and the components around them.
Q6 & Q5 can be temporarily removed during testing.

My guess is the power stage is blown.
Do not connect a speaker until the amp has no DC on the output

schematics attached, also a reliability mod from JM Fahey

[fredcapo]

Thanks for the info and the schematics. As I mentioned this is my first go at working on an amp like this. Pretty sure I cooked my test speaker.

[Jazz P Bass]

Not exactly the easiest amplifier to troubleshoot & repair.
They can be a bear.

[fredcapo]

Is there DC on the output.
Check all the power mosfets, drain to source, also measure each of the mosfet ballast resistors. Check Q6, Q5 and the components around them.
Q6 & Q5 can be temporarily removed during testing.

My guess is the power stage is blown.
Do not connect a speaker until the amp has no DC on the output

schematics attached, also a reliability mod from JM Fahey

Thanks again. What is the reliability mod?

Also, the owner of the amp was balking at the proposal of replacing all of the semiconductors. I told him I could not
warranty if not totally rebuilt. What are your thoughs as per rebuilding only the half of the complementary stage that is blown?