Dangers of 50 year old amp

kappclark

I have a friend who has recently acquired a Macintosh MC-250. It is in beautiful condition. Hasn't been fired up in probably 30 - 40 years..Just not sure if any precautions need to be taken before he connects the thing up to a source and some speakers... comments ?

heiney9

Use a variac.

tonyb

heiney9 wrote: »

Use a variac.

Bingo....

....and don't test it out on the good gear first. Hook up some garage speakers just in case. Always a good idea to have a tech go over vintage gear before slapping it on the good stuff at home.

mhardy6647

One good thing -- the MC-250 is autoformer-coupled -- although, since it's an autoformer as opposed to a transformer on each channel's output, I suppose it could pass DC to the loudspeakers (which is the prime concern with old solid state equipment).
Transformers inductively couple the signal from the power output devices (transistors, in this case) to the loudspeaker via two separate sets of windings; the "primary" and "secondary" windings in an autoformer are contiguous (i.e., DC-coupled). An output transformer can't pass DC to the speakers, but an output autoformer probably can.

The 250 and its close cousin the MC-2100 are pretty notorious for needing a fair amount of remediation at this late date. There's a fair amount of literature on rehabbing these amps; including a good article on the MC-2100 in AudioXpress magazine some years back. As an MC-2100 owner, I do still have a PDF of that article someplace. If the OP's interested in it, send me a PM.

At minimum, the electrolytic capacitors should probably all be replaced. Driver and output transistors may be suspect, too. Probably also worth checking that resistors are at least reasonably close to their specified (nominal) values; there isn't all that much stuff inside these amplifiers.

Now, all this being said, I can't resist taking my usual shot at the MC-250 and MC-2100. I find them to be terribly solid-statey sounding amplifiers, in the worst possible sense of that word (and autoformer coupling notwithstanding). Many folks do like them, though... but, then again, many folks like the New York Yankees, too.

;-)

kappclark

Phillies fan here (yes, and Eagles as well, so call me a **** ..) Thank you for the warnings, and for the thorough explanation of the output autoformer .. he still has it at work, and I will advise him to a) take it to a tech for safety eval or b) find a sacrificial pair of speakers to do first test ... I believe this was one of their first efforts in solid state ? I checked EBay, and the restored ones are certainly out of my price range ! I bet he goes that route, and if he does, I will reach out again to this fine community for recommendations..