Thanks all - yes i have been using the diode test on transistors and nothing obviously wrong on readings (i am comparing one board against the other? though I guess when powered up one may break down? I have some spare power transistors both RCA vintage and some new made in Mexico! Going to start with on board ones - at least only chasing fault in channel. When the amp is working no noises at all and sounds fine - just this annoying pop/crack when i turn it on. Just worried for my aging BC1's
I did find one resistor 4.7k which was reading a little low and replaced this but no effect.