It’s a shame when I can’t help…. this is one of those times.
I don’t have any circuit information for the AD62/3, this was a mixer that we made just before the 828.
Certainly the problem is a power supply one; from memory that mixer used a single rail 24 volt power supply. I would suggest to your tech that he replaces the whole power supply with a modern switch-mode type; these are readily available now and not expensive.