Sort of what I was thinking. I was able to find the ouput portion had it's own DC bus off of a large filter cap. I disconnected and the issue went away.
I'm not exactly sure how it works, but if I trace back from the base of the failed Darlington I see this:
Base links to the striped side of a diode (black plastic like rectifier diode), the other side links to a 51 ohm 2-watt resistor. The other end of the resistor links to the collector of a 2N2907 K8619 transistor. The base of this transistor connects to 2 small 1K ohm resistors one connects to pin 4, the other pin 6 of a GE H11L1 opto isolator.
Seems to me a circuit like that you should be able to throw any proper voltage, current, frequency rated transistor in and have it work.
I think it may just be junk as it 30 or more yrs old while I don't see any sign of it there could be some bad caps on it also. This unit was supposed to be a PLC trainer for a school. All of the parameters are set with potentiometers. I have a small AB 120/240 3-4A PLC with LCD screen, lots more features I could use instead.
The only real reason I desire to fix this is to have something I can use to test transformers or get a variable frequency signal from. The first one I can't do without some larger transistors. The later I could probably just tap off on the base of one of the old darlington transistors and get a small voltage variable frequency signal.