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McIntosh MC-75 in unknown condition, what do I do?

OK, I hope people don't hate me for this, I found a McIntosh MC-75 that was getting thrown out with a bunch of old electrical test equipment. I have no idea why it was getting dumped but it was there and I am attempting to recycle it.

Of course this isn't a process with out a few hang ups. First, I only have one amp and these are mono-blocks, normally sold as a pair. As such I don't know how much I should expect to pay for 1 additional amp so I can have a pair of the things. Heck, I'm not sure I even know where to find a single MC-75! Well aside from in a pile of junk test equipment.

Second, I haven't even plugged the thing in yet. I don't want to find out that something is wrong with it and I did more damage by powering it up without checking first. Any suggestions as to what I might check before I plug it in and what I might do after I plug it in?

Finally, any suggested resources for this amp? I admit I haven't searched much but if anyone has any recommendations off the top of the head I would appreciate it.
nikkidanjo
almarg's avatar
almarg
9,670 posts
 
That is a valuable and highly prized collectable that you rescued!

The value is highly sensitive to cosmetic condition, however. A stereo pair of these amps with near-mint cosmetics and excellent chrome would easily command several thousand dollars, and possibly much more. However, poor chrome reduces the value drastically. And one amp is worth considerably less than one-half the value of a pair, for obvious reasons which you alluded to.

Obviously finding another one, especially at a reasonable price, will be difficult and require patience. All I can suggest is watching the offerings here, at eBay, etc.

You are correct to be cautious about powering it up. Vintage equipment, especially equipment which might not have been previously powered up in a long time, should have its ac power brought up slowly, over a period of several hours or more, by means of a variac or other variable ac power supply. That will allow the electrolytic capacitors to "reform," and will allow overheating or other problems to be detected before more serious secondary damage occurs.

Here is a little bit of info on it. A little Googling will turn up much more.

http://www.roger-russell.com/amplif1.htm#mc75

Best of luck with the great find!

-- Al
almarg's avatar
almarg
9,670 posts
 
Also, I'd suggest not operating it without a speaker load. That might result in higher than normal internal operating voltages, which could overstress capacitors or other internal components. I use a $10 or so Radio Shack speaker driver when I do initial test and troubleshooting of older amplifiers.

Regards,
-- Al
nikkidanjo OP
26 posts
 
Almarg,

Thanks for the info. Is there a reliable way to check tubes or is this something that really requires a tube checker? The tubes appear to be originals. I suspect the lab bought the amp for some long forgotten experiment then allowed it to sit for decades. The tubes are 3 Telefunkens, 1 McIntosh by RCA and 2 Genelex KT-88s.

The amp has a reasonable amount of surface rust. Nothing that keeps you from reading the labels but far from clean. Any suggestions for cleaning the chrome?

Oddly enough I have a Radio Shack driver from the same pile! It's been modified such that the air displaced by the cone is pumped into a 1" pipe. I'm not at all sure if the amp and the driver were ever used together.
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