Hi AVG
Firstly I'm sorry for the silence .. .. I'm disabled and the current heatwave is giving me a few problems.
I think I have made some progress .. .. .. I used a DC - DC module to provide an independent + & - 15vdc, along with a 5vdc from my bench PSU but got the same problem of +15v collapsing; There are 3 x 470uf caps right next to the connector which I had replaced, but I removed them again so that I could see the circuit clearly, found no problem so I put them back - +15v is now stable ! Don't ask 'cos I don't know !
It made no difference to the screen display however .. .. .so I did some circuit tracking and realised that apart from the power connections to the screen board most of the other tracks go to an 18 pin IC marked AD7541AKN which the datasheet describes as a DAC which uses a V. Ref from a TL431 nearby.
There is a ferrite toroid in the vicinity which had been allowed to bounce, smashing some of the decoupling caps and it had flattened that TL431, shorting it out, so I replaced it; maybe the shorting has damaged the DAC too .. .. .. .
Anyway, I await the arrival of a replacement AD7541A and then I can answer that one !
Thanks
MM