The wreck, a Marshall Origin 50 on the workbench.
Found that the EL34 power tubes had given up for some reason.
Had the tubes replaced, but the amp did not act as it should. Only produces about 2W in any of the power settings. Low, mid and high.
Setting the power scaling switch in low and mid position gives the bias circuit for the output tubes fixed voltages, in the high pos it’s adjusted with a bias trimmer.
Measuring the voltage over the 1 ohm bias resistor indicated it hardly pass any current. Voltage reading on the screens gave a hint of what’s wrong. At the high setting the screen voltage was about 140V lower than the plate voltage.
Now both the bias voltage and the screen voltage are modulated by a LTV826 Optocoupler, the screen voltage also uses some power transistors. Checking the 1M resistor feeding B+ to pin 6 of of the LTV826 solved the problem, it had an infinity reading.
Replacing the 1M resistor made all the difference. The new tubes are now rebiased and all three settings produce the wattage they’re meant to. The amp is fully functional again.
Chart for the voltage readings when everything is fine, bias is messured via the joint 1 ohm resitor for the cathodes. Hopefully it could help someone.