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What’s wrong with these McIntosh Monoblocks?

Evening, got a question about these two McIntosh MC40 amplifiers. I got them from garage type environment, humidity was present as they have some surface rust, but absolutely no damage or dents. Opened them up and did a visual inspection, nothing looks burnt out or bad. However, I replaced the fuses as they were both burnt out with ones that were identical, ensured the voltage setting was correct, plugged the amps in(no speakers connected), and nothing. Removed the fuses to find them both burnt out again. What could be causing this? Would be eternally grateful to anyone who can help.

polak
testpilot
896 posts
 

Most tube amplifiers require a speaker load present, therefore, if the amp is ran unloaded, the fuses will fail to protect the output transformer.  

roxy54's avatar
roxy54
7,693 posts
 

Test pilot is correct. Hooking them up without a load is a no no.

aball
2,016 posts
 

I am an electrical engineer who restores vintage audio equipment as a hobby.  The previous posts are not really correct - the MC40 does not need a load connected to verify the DC circuit voltages, as the circuit is perfectly stable at idle.  Just do not connect a signal at the input.

But the real issue here is that you plugged in ancient amps into the wall outlet!  YIKES!  You are lucky they are well fused or you might have gotten a fire on your hands.  Thought I suspect previous people made the same mistake.  You needed a variac at a minimum.

I can tell you all the caps are shot.  The electrolytics are probably shorted.  Hopefully the output transformers are ok - that's the most worrisome part.  Replacing all the caps is a huge job (I've done it 5 times and don't want to do it again lol).  But it would be worth it.

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