It's not the analogue board, because this faulty logic board started off in another Plus. I swapped the boards over and the fault moved with it. The Plus that this board came out of is now working perfectly.

The analogue board in this one needs some work anyway, because of the pulsing video problem. C1 has expanded, there is also another capacitor looks suspicious. I'd like to fix this up, but there's no point when it doesn't boot!

I've repaired BBC computers to component level, but they're a bit simpler and I had a copy of the service manual.

Otherwise, I'll keep an eye out on eBay for Plus logic boards, preferably in the UK.