Okay - no more help required. Unfortunately I've just fried the amp.
Working on it live, prised out the amplifier board and something shorted. Sparks and crackles ensued, and now i have minuscule output on all channels. I'm now abandoning all hope of repair and have sadly consigned the BM 6000 to the cupboard.
To be fair, the amp has always been the weak link in my set up. I think it's been on the way out ever since i've been using it. Muddy sounding, and lacking in clarity.
I still have my Beogram 6000, and Beovox 5700 speakers - all of which I've just hooked up to a modern amplifier (RIAA built in, so no phono preamp needed) and it actually sounds better than it ever has. Doesn't look as nice as before, but I think the best bits of the setup are now able to perform properly.
And at least finally I can plug things in with nice, normal RCA connectors instead of faffing around with horrible DIN plugs.... A feint silver lining maybe, but I guess that's that.
Thanks for the help.
Anyone want to buy a blown BM 6000?!
Owen