Sunn Beta Lead Power Amp Problem

[jeff1058ha]

I just restored a broken Sunn Beta Lead head and everything is working perfect except for the power amp. This is an odd problem. When I remove the op amp used in the feedback loop error detection circuit (4558) the power amp sounds fine and amplifys the signal to normal levels. The only problem is that the output has -8 volts DC on it when in idle state and the negative power transistors run hot. Not hot enough to ruin anything immediately but much hotter than the 2n3055"s which are cool. I put IC sockets on the board for all chips so that I can change again if necessary. As soon as I plug the feedback chip back in, the negative rail power transistors cool off and the negative voltage is removed from the output but now the amp doesn't ampflify the signal correctly. If I strum the strings really hard they will make a harsh sound through the speaker for half a second and then the sound abruptly cuts out. If I strum the strings again I get the same thing. If I touch nothing else on the amp but the feedback chip and remove it from the circuit, the amp amplifiys the signal to normal levels and sounds fine. All voltages look good surrounding all transistors when the chip is removed and when it is installed. I have spent close to 50 hours working on this thing and I am stuck. Any help would be most appreciated.

[Jazz P Bass]

What dc voltage do you have on IC #1, pin 7?
With the IC in & with it out.
Did you replace C17?
Edit: C12 across Ic#1

[jeff1058ha]

Respone for Jazz P Bass

Thanks for you help, here are my findings. When IC#1 is inserted in the circuit I read 320mV on pin #7 and it keeps climbing slowly like a cap is charging. When I removed IC#1 and measured pin #7 it read .8mV . I put a scope on both large filter caps #16 and #17. Cap #16 had a smooth DC voltage. Cap #17 had good amount of visible ripple. When IC #1 was in the circuit the ripple on Cap#17 was 200 millivolts Peak to Peak. When IC #1 was not in the circuit the ripple was 3 volts Peak to Peak. It looks like I need to surely change Cap 17 for starters. I will check the lab and see if I have a close replacement. I would have expected to hear a loud hum with that type of ripple. That's why I didn't even expect that. I will let you know what happens after I substitue the cap.
Thank you so much for your help.

[Jazz P Bass]

Terribly sorry, but I meant C12. Across IC#1 pin 6 & pin 7.
C12: 10uf/25V

[jeff1058ha]

No problem. I just changed cap #12 and the amp still has the exact same problem. I just noticed that the power transistors are different from the callout. They are the same type npn / pnp but actually have a higher rated wattage, can handle more current, and have a higher VEO rating. They all test perfect so I don't think that is causing the problem cause the amp sounds great and every feature works as it should when IC #1 is removed with the exception of the negative 8 Volts on the output. Puzzling. Do you think the different power transistors could do this. The npn's are 2N5630's and the pnp's are 2N6030. Thanks again.

[Jazz P Bass]

How about we do a few voltage measurements.
One with IC #1 in & one with it out.
If the output is at -8Vdc someting in the output section has to show it doing that.
If checking all the transistors shows no problem, a voltage that is askew may help pinpoint it.
I would remove & check Q2 & Q3. The LTP.

[Bernardduur]

I had the same problem next to that I couldn't bias the unit!

Turned out all caps were shot and leaking and most diodes were bad. After a diode and cap job the unit sounds pretty fine!