Marshall bass 100 solid state transmitter

[ruteng]

Hi all,
I have this marshall ss amp on the bench that oscillates on the negative rail at around 20k. I have replaced all the transistors, caps, bias pot in the power amp section. It draws so much current, (2.4A @8vdc thru the .33 ohm mj2501 emitter resistor) I cannot power it at more than 8vdc using the variac. If I remove the wire to the base of the MJ2501, it stops oscillating and I can bring it up to 40vdc @120vac. I have another 2501 ordered although it tests good.
If anyone can help me get this thing off my bench and back to the very patient customer, I will be very grateful. See attached diag.
Thanks, Rudy

 

[JonSnell Electronic]

It cannot drop that sort of voltage across the resistor as the protection transistor will turn on, stopping the drive. Check the components used and their orientation.

 

[JonSnell Electronic]

Is the MPSA06 current supply working correctly or the wrong component? Always use original type components!

 

[JMFahey]

**disconnect** that output transformer, replace missing transistors and recheck.