The computer would display garbage characters on screen, which is a good sign. This means power, clock, CPU and video are just fine. This usually indicates some RAM problems.
I tried to use a divMMC but it wouldn't boot and just show a black screen instead. I kind of remembered it would work in other cases where the RAM is faulty, but I could be wrong (or the RAM is very damaged). I decided to burn an EPROM with the Retroleum diagnostic ROM v59. As I didn't have any 16KB chip, I had to use a W27C512 (64KB) which is pin compatible with the exception of 2 additional address lines. As such I duplicated the ROM content 4 times to accommodate the extra space.
The diag ROM would actually report no error, but also only 16KB instead of the expected 128KB. Weird! Might not be the RAM after all...
Upon visual inspection I found some rather discolored traces on the PCB but no obvious traces of oxydation or leaks. Testing those traces, however, revealed that at least one of them was cut...
One bodge wire later, the machine is up & working again!
Now time to change the drive belt and fix the audio distorsion :-)
1 From Cèbe -
Audio fix: https://spectrumforeveryone.com/technical/fixing-audio-distortion-spectrum-3/
Below is a copy just in case:
Firstly, replace the following resistors on the +3 PCB with the given values below (all 0.25W types):
R42: 1.2kOhm (original: 1.8kΩ)
R43: 5.6kOhm (original: 3.3kΩ)
R62: 1.5kOhm (original: 1.5kΩ)
R63: 2.2kOhm (original: 4.7kΩ)
R64: 6.8kOhm (original: 4.7kΩ)
R67: 15kOhm (original: 47kΩ)
R72: 820 ohm (original: 2.2kΩ)
The redesigned circuit on the +2B added some extra resistors for which there is no location on the +3 PCB. These need to be added as follows.
R204: 10kOhm – install between the base of TR5 and ground (or between adjacent legs of R50 and R67).
R206: 390 ohm between the emitter of TR5 and 0v. This is best done by removing TR5, bend up the emitter leg and install R206 in the through hole formerly occupied by TR5’s emitter. Then replace TR5 and solder the emitter leg to the resistor.
Finally for best results, remove ceramic capacitor C37, next to the modulator.
And indeed, does it sound good now!