Help with a markIII - very low output

I have a pair of rebuilt MarkIIIs and one has developed a problem. The amps use the VTA octal driver board and the SDS cap board and were working well and sounded great.

One amp has lost almost all output. At full input gain, sound can be slightly heard from the speaker. Another amp works in the system, so it's nothing upstream or the speaker. I swapped all tubes between the two amps with no change. The tubes from the "bad" amp function in the "good" amp, so it seems like an issue within the amp.

When the problem began, I had been listening to the system and with nothing playing I heard a very soft thump or pop from the speaker connected to the "bad" amp. Very reduced output from that time forward either in the system or on the bench.


With tubes in place and a line voltage of 114vAC, I measure voltages as:

B+: 438vDC

Bias Inlet: 52.8vAC

Rectifier:
Pin 2 507vDC
Pin 4 398vAC
Pin 6 398vAC
Pin 8 504vDC
Pin 2 - 8 4.9vAC

Output tube:
Pin 3 496vDC
Pin 4 498vDC
Pin 5 -62.8vDC
Pin 6 -62.9vDC
Pin 8 biased at .5v
Pin 2 - 7 6.2vAC

My uneducated guess is that there is an issue with a component on the VTA driver board. The resistors which can be measured in place measure to spec.. Since I had some on hand, I replaced the C6 and C8 .22 400v coupling caps and there was no change.

Can someone suggest how or what to check next?

Thanks for any suggestions,
E.

Has the ground come off the RCA Socket where it meets the board? Believe it or not when I bought my MK4s this happened on one side. Mine was scratchy and weak but the ground was just resting against the circuit board.

Your voltages look fine. You say you have a bench and you have taken DC voltage readings so you are somewhat proficient with testing. If you have an O scope then test both amps on the bench at the same time. Same test signal to both with each having a dummy load. Put a signal to the input and measure it. Maybe .1 -.3 VAC. Test back and forth between amps as you proceed thru the schematic from input to output. If that is not something you have or can do then at least put the bad amp in place and try a few of the next steps. You have a volt meter in lieu of the scope. It will work on volts AC..

Put a signal to the input and measure it. Maybe .1 -.3 VAC. If not a signal generator then a tuner or CD player playing at that level. Then follow thru to the input tube grid pin. Same VAC? It should be. Then check at the output plate of that section and move on to the next tube. The AC signal should get larger with each section. Somewhere long the path it will not be there or be vary small. When that happens the problem is in or near there.

Sorry – you do have the correct schematic. Right? The original circuit was very simple but the VTA and SDS boards complicate matters. You have to have the correct schematic to follow this thru. Still not hard but adds a little more to the effort.

Thanks. I don't have a scope, but I do have all the correct schematics. I'll check signal path voltages.

E.

95% chance a bad soldering connection somewhere.

The amp is fixed. It was the CCS LM334.

Thanks everyone.