Music Man HD-130 oscillation

[Apexelectric]

Got a good one here

First off, solid state amps are not my strong point and this circuit is a bit foreign to me so bear with me. While this one is a hybrid, it is mostly solid state. This one definitely has me scratching my head a bit.

Customer brought me the amp with no output and said it quit on him while playing. Lights up but no sound. Found two issues right away, bad phase inverter op amp and one bad tube in the quad of EL-34s. For grins I replaced the filter caps since they were the original ones. Amp fired up and came back to life. I tested it with only two of the good original output tubes just to see if it all worked before I put in the new quad that I had coming. Worked fine except a rather annoying volume tapering, high pitched, squeal upon powering the amp off. Otherwise the amp functioned just fine. Then I put in the new quad and it the amp powers up and when the output tubes start conduction full current then one side crashes, the other side rises and the amp distorts. Works fine on two tubes in any of the sockets whether inside pair or outside pair but with the full quad it crashes every time.

Here are the items I checked and replaced

Bias to factory recommended 25mv voltage drop across 3.9R resistors, both sides are within a few MV

New main filter caps and bias caps and a couple bypass caps

New 1458N phase inverter op amp

New output tubes

Checked component values is bias circuit, snubber caps and diodes on the output tubes

Resistor values throughout bias and output section

Checked voltages throughout - 16 volt supply is slightly imbalanced between the positive and negative sides, about a half volt more on one side and a half volt less on the other. 2W 620R resistors seem within tolerances.


The preamp, including the tremolo seems to work just fine so I haven’t really bothered to delve too deep there. The amp came to me in original and rather excellent condition so there isn’t anything needing to be reversed or fixed from a previous repair.

I’ve spent some time searching around for similar issues and couldn’t find anything too definitive that would point me in the right direction so I’m hoping that someone here might have a little insight on this amp.

Thanks!

[shooter]

full quad it crashes every time.

Q1 n 2 source the current for the tubes, they might be iffy enough to handle 2 but not 4 tubes
D7 n 8 could drag down the PA tubes


what's the PCB like?  does it look intact or many hacks from former techs and their soldering

[Apexelectric]

full quad it crashes every time.

Q1 n 2 source the current for the tubes, they might be iffy enough to handle 2 but not 4 tubes
D7 n 8 could drag down the PA tubes


what's the PCB like?  does it look intact or many hacks from former techs and their soldering

The amp came to me in great shape shy of the problems noted. No previous work had been done.

Another odd behavior is that I can get  either side to crash/run away by swapping the tubes around even though they’re new, good and matched. The opamp was bad, does it make sense that whatever event took out the opamp also compromised both Q1 and Q2?

[EL34]

Please do not create multiple post

[Apexelectric]

Sorry bout that😞