Very interesting. I've never seen an ISCU with any other failure than the
high current transistors. I have seen a rare case where the diode check
came out good...on a bad transistor.

In another message, I'll send on some research about suitable replacement
transistors.

Also, with electronic items of that age, check out the electrolytic
capacitors (or, simply replace them all...sometimes that is just as easy).

Jon