Yes, that is what is happening. Something is causing TR10 to short between collector and emitter.

My best guess up to now is that something is shorting TR10.

My mistake, it is blowing fuses form cold and unloaded. Internal fuses that is, not mains.

Yes, the whack test didn't seem to have any effect once the DC noise kicked in or the fuse blew before I could try it. I also tried poking around with a wooden chopstick - same result. Temperature seems to be a factor - on a cold morning, with a fresh fuse, the amp will operate albeit with the popping sound I mentioned, but as soon as it gets warm... DC. Once it starts, that's when power cycling results in fuses blowing.

I know a young kid who would really appreciate this amp if I can get it working!

Thanks for the reply. No, it doesn't blow fuses with the speaker disconnected.

Hi All,

I was given one of these amps for free and was told it had 'a few problems'. Switched it on for the first time and it seemed fine, other than a few scratchy pots. Cleaned a few, replaced a few others, fired it up again and all seemed well until after about 5 to10 mins - POP...BURRRR... DC on the output confirmed by the speaker only moving in one direction when I cycled the power again. Let it cool down, powered up again and the same thing happened.

So, I disconnected the speaker, hooked the amp up to the function generator and 'scope and isolated the problem to the power amp as the signal as far as VR13 is fine. 48V rails are fine, 15V rails are fine. Re-soldered all power resistors - same fault. I had appropriate filter caps on hand, so I switched those as a matter of course. No change. Swapped out TR8, 9 & 10. No change. Swapped out TR4,5,6,7 - voila! No longer DC on output, however there was an intermittent popping sound, like arcing. Powered off, powered on again after a min or two and pop goes the fuse. Replaced the fuse and now back to square one.

I've built and repaired a fair few valve/tube amps, but this is the first time I've had a go at solid state. It's got me stumped and I'm reluctant to start randomly replacing components. HELP!