T3 pin 1 goes to C4
T3 pin 3 goes to R7
So that checks out as well. It would sure be handy if Marshall gave us some voltages and a board layout.

D4 cathode goes to pin 2 of T5
R42 goes to pin 3 of T5
That checks out according to the schematic.

Yes, any transistors without plastic backs that are mounted on the heatsink need an insulator.
If the Q9 collector was in contact with the heatsink (ground), that probably explains the excess current through Q4 (via R97).

The heatsink is a large aluminum fin style with Q7, Q8, and Q9 attached. When I received the amplifier for repair it had what looked like white lithium grease between the transistors and the heatsink and no mica insulators. I cleaned and replaced the grease with Arctic MX-6 Thermal Compound when I replaced the transistors. Could that be the problem? Does Q9 need to be isolated?
Small update: The version of the Q9 D600K that I used has a metal backplate that is connected to pin-2 the Collector (the original did not have an exposed backplate). I'm going to make an insulating wafer and will update soon.

Quote from: Jesse on May 07, 2024, 04:01:33 PMQ9 has fresh thermal paste and seems to be tight to the heatsink.

What about it's insulator?  Don't it and the output transistors that are mounted on the heatsink have mica insulators or similar?

ah!, you are correct, I swapped R97 and R98 while typing.
Q9 has fresh thermal paste and seems to be tight to the heatsink.

I think you got those R97 and R98 voltages reversed, but voltage across R41 seems normal, and 0V across R98 would be good.
So there seems to be about 50mA current through Q4, which is too much.
Is it possible Q9 is no longer insulated from the heatsink?

R97 is 100 ohms
When voltage across R98 is 5VDC then R97 is 0VDC and the voltage across R41 is 7.4VDC

Quote from: markorock37 on May 07, 2024, 12:59:57 AMIs it possible the new ones dont have the same pinout as the originals?

Yes it is possible.  Measure resistance from D4 cathode to T5.  One of the T5 pins should read zero ohms to D4 cathode.  Which pin is it, pin 1,2, or 3?
Which T5 pin reads short to R42?

Can you verifiy that R97 is still 100 ohms?
Also measure voltage across R98 when you have the 5V across R97.
And also voltage across R41.