Tek PS2520 (aka GW Instek PPT3615?) repair

[siggi]

Hey y'all,

I'm trying to fix a PS2520 I got for very little money on ePlay. So far I've been through a few distinct failures, and I wonder what the ... expletive ... is up with this thing.

The first failure was an 74LS244 shorted across the digital board's 5V rail, which apparently also took out the diode bridge for same - no worries. The second failure was a CD4093 on the CH1 analog board which was near-shorted from 5V to one input. Some of the opto couplers were apparently also getting tired to the point of not making the logic threshold voltages, so I switched all of them out.
After I fixed this, I then found that the AD7541 DAC on both CH1/2 was borked, effectively shorting many or all of the logic inputs to the output (and to each other). I only had one of those spare, but now at least one channel is getting near to working - still no output, but I'm seeing likely waveforms and voltages. By first approximation (diode check) the output and driver transistors look OK, so I expect I'll find a bit more logic damage somewhere...

This thing has a central digital control & display board whose ground is at mains earth, and then the three output channels are all galvanically isolated with opto-isolators. What I don't get is what sort of event might have caused this carnage across three or four supposedly isolated domains?
I'm I looking at a lightning strike victim, or some such?

The design of the thing is reasonably interesting - to me at least. There is a serial protocol from the controller to each channel, with a clock and a data signal, plus two distinct strobes. This is clocked into two shift registers, one is 16 bit and goes to the DAC, the other is 8 bits and seems to control readback and maybe relay control.
In the reverse direction there's one bit of read-back which is multiplexed to allow reading several different things back (I haven't sorted what's what there). The DAC on each channel is also multiplexed into at least three S&Hs for control voltages (voltage/current and over-voltage cutout), as well as for a software-SAR ADC for read-back.
I guess the design dates back to a time where 12bit DACs/ADCs were quite expensive .

Now, I'll muddle through, but it'd sure help if anyone has schematics for this thing. Anyone, anyone, Bueller?

Siggi