Somewhere on one of your cards some component has had all of its smoke released. You're trying to resuscitate what is probably at least 40 years old. That's outside even Iwatsu's Mean Time Between Failures stat and why your tech is unfamiliar with this system. I may be the only guy in Western NY that's even seen one of these babies and isn't afraid to admit it.
Trial and error is going to be about the only way you can resolve this, but by the time you've rounded up enough spares to start testing, you'll have probably wasted enough time to make just getting a new system more worth your while.
As I recall, system tones. like DT and ring back all come from a module on the processor card, so all your power troubles have probably damaged something on the processor. If the CPU card isn't working properly, nothing else will.
Also, the power supply puts out more than one DC voltage, (5 and 24 VDC IIRC)so that may be part of your problem as well.
If you can locate another CPU and power supply, that would be the place to start. Worry about programming when you have all your extensions working...
