[Radio Wrangler]

Yep, that sounds like it's the synth that's misbehaving, not the display.

I don't have the RA1772 synth block diagram in my head, but it's a multi-loop synthesiser. There should be a decent fault-finding section in the manual.

It has a few phase locked loops which each synthesise a signal in frequency steps. The steps are quite large so that it settles quickly and its noise performance is good. The signals from these loops are then added together.

Don't start at the end of the combining chain and try to logic your way backwards, that can get you in a right muddle.

Find the test points for each loop and check it covers the range the manual says, and hat the relationship between the tuning display number and the measured frequency is right. Only if these are right, move on into checking the summing arrangements. You are going with the flow of the signals in the synthesiser, not trying to go backwards, guessing which tributary must be wrong.

I had the job of designing a five-loop synth many years ago and had to debug prototypes which had never worked before, so I didn't have the help of knowing it once worked. It's manageable.

Each loop has a tuning voltage. It is very likely that a bad loop can be spotted by its tuning voltage being out of range. You can get fair bit of value from a basic scope in these tests.

It looks complicated, overall, and it is. But it is really an array of many fairly basic circuits flying in close formation. Debug at system level without worrying about details until you find a bad block of circuitry, then you only have to delve into that block in detail.

David