I don't know if this radio is new to you and it came this way or not, but...........
Since you have the radio with the problems that can be caused by just about anything, or something very simple like a harness that needs cleaning, or just needs all the capacitors replaced. (Nothing like recapping and it still doesn't work.)
You'll have to start by having the service manual and schematic to read and study. Once you get familiar with all of the different circuits, their connections, and how they're laid out and work with each other. You might be able to find a problem area to start looking at. I can't stress "Read the service manual" enough.
Once you have an idea of what doesn't work and where it's located in the radio. It would be the time to open up the radio. (I'm sure you opened it a long time ago.)
You'll want to look for any bad traces, bad-looking capacitors, resistors, or even diodes ...... anything that just doesn't look right. It may be a harness not making a good connection. If it works after that then you're done,
if not .......................
It's time to trace for voltages, that should be there. If there is voltage (or signal) missing it could be from a bad capacitor, a bad regulator that supplies voltage to other circuits, or something else ....... like a bad solder connection.
The answer you don't want to find out is that one of the CPUs is bad or parts are no longer available. Many people scrap radios and sell parts on flea-bay.
Wish I had the answer you wanted, but that's why techs make the big bucks, they either know from experience or troubleshoot .... Some techs just send it back saying they fixed it!
Please, let us know how it turns out.
Hope you had a good 4th of July,
73
