Hi
I started by isolating the power supply. With an open output, the voltages are all good. The LM395 transistors on the power supply limit current to about 2.2A. I can safely connect the power supply to a dead short. Good for testing purposes. The power supply is fully operational.
I then isolated the fault to the main board. When the main board is connected, the +/-18V supplies are near zero. As a result the outputs of the +/-15V regulators on the main board are also near zero. Something on the main board is taking about 2A. None of the chips have a high enough supply voltage to operate, so there is no signal to trace and no function to check.
The 5V supply on the main board is OK so all of the digital logic devices are also OK. None are exposed to the +/-18V supply.
Temperature measurements on the power supply shows that the +18V regulator is running hotter than the -18V regulator. This could simply be because of the design loading on the +18V circuits is more than the -18V circuits.
There are several hundred parts on the main board so randomly lifting parts would be the option of last resort.
I have started by marking the components supplied from the +/-18V including the +/-15V regulators and attached parts. Without the 2 missing schematics, this is difficult to finish.
Ideally I would have a thermal imaging camera to follow the current along the heated tracks to the faulty component, but I don't have one. I am considering converting an old webcam to infrared.
Today I am going to try and trace voltage gradients along the PCB tracks, but I don't have the PCB drawing or a really sensitive volt meter. I will pay particular attention to capacitors. There are no electrolytics on the main board.
Dazz