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.
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.

Best of luck with the great find!

-- Al
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.

-- Al

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.
Nikki, with all due respect to Almarg, you don't want to operate it AT ALL! The electrolytic caps are probably shot, and even if they aren't, they have to be brought up to power slowly with a Variac. Don't even worry about the tubes at this point!

The best person to put this little gem back in shape for you is Terry DeWick in Knoxville TN.
Terry is a McIntosh wizard and does most of the important McIntosh repair and/or restoration for owners all over the country (including me!)

As for another one, singles often come up here on Agon, on eBay, and you can ask Terry as well. He can give you tips on cleaning the chrome and other info, so don't hesitate to call him. Also, if your amp has a pair of ORIGINAL KT-88's (made in the UK) and they have good getters (still fairly silvery on the top and both sides of the tube) those alone are worth $200 to $500 a pair!

Definitely a keeper ;-)
It would obviously be desirable to run the tubes across a tube tester, but if you don't have access to one I think it should be safe to try the amp without first testing the tubes, PROVIDED that you use a variac or equivalent to bring up the voltage very slowly, as I described. While you do that, look for any reddening of the plates in the tubes, as well as excessive warming of the transformers, and of any electrolytic capacitors which may be on top of the chassis. Obviously, don't go underneath the chassis unless you know exactly what you are doing, or there is an electrocution risk, even for some time after power is removed.

To help the chrome on similar vintage components, I've simply used commonly available metal polishes which list chrome as one of the metals they are suitable for. Such as Noxon, which you can find at supermarkets and hardware stores. There are also chrome-specific polishes, used for automotive and motorcycle purposes, but I'm not particularly familiar with them.

Apply the polish with cloths or cotton swabs, being very careful to get any of it on the lettering, and being careful not to get it into crevices from which you may not be able to wipe off the residue. Then wipe it off as directed.

-- Al

Of course I can't say with certainty that the tubes are the original units. I can say with certainty that they are Genelex KT-88s made in the UK (as stated on the tubes). The tops appear quite silvery. The sides look to be somewhat brown. I would also like to clean the tubes as it's clear people have touched them over the years and left some finger prints.

The caps for this model are all inside the base. Any suggestions for testing the caps?

I do appreciate the cautionary comments. I have a reasonable amount of experience working with electronics and some higher voltage stuff. However, my classes seemed to have skipped the section on tube based circuits ;)

I'm not sure I have easy access to a variac but I will check if I decide to plug it in. I also will give Terry a call.

The cleaning/ polish plan sounds like what I had in mind. Brasso and lots of TLC.
Oops! The first sentence of the last paragraph of my preceding post should have read:

Apply the polish with cloths or cotton swabs, being very careful NOT to get any of it on the lettering, and being careful not to get it into crevices from which you may not be able to wipe off the residue

-- Al
Here is another alternative. These guys are big on Mac gear.
Nikki, for cleaning tubes I always use Ronsonal lighter fluid (naptha - the same solvent drycleaners use.) It will amazingly NOT hurt even the fragile white chalky printing on a lot of European tubes like Telefunkens! Whereas anything with water in it will take the labels right off! The labels on the KT-88's are fairly durable. They are probably Gold Lions, correct? -- that's what McIntosh used quite a lot as their OEM KT88's back in the day. Though used, they're probably still stronger than current KT88's are new -- I have a few and they all test better than the new stuff!

If the lettering on the chrome chassis is 80% intact or more, then it's worth bending over backward to save; so I'd caution against using any kind of abrasive cleaner or pad because it will remove the lettering if it's even in the same room! ;-) Terry recommends something that comes as cotton wad impregnated with some chemical that won't wipe the lettering off -- you'll have to ask him.

If you go to his site, you'll see some unbelievable restoration jobs -- amps that have been through fires, etc.

Thanks for the tips! I don't recall seeing Gold Lion on them. I'll take another look when I'm back in the lab tomorrow.

The chassis chrome is generally good. The top could easily be cleaned without ever touching the printing. The worst parts are the bends in the metal and the back where all the specs are printed. Everything is still 100% readable but hardly clean.

The hardest part of this whole thing is resisting the urge just to try the amp!

I've given the amp some TLC over the last few days. I took the lighter fluid advice and cleaned the tubes themselves. I've been carefully using brasso to clean the chrome surfaces. The rust pits are their for good but it's MUCH better. The guts looked good so slowly powered the amp up. It does in fact work. The RCA plug certainly could stand to be cleaned. It's connection to the cheap Y-cable from my mp3 player was iffy at best. The amp drove my MMG to a reasonable level.

I can't say anything about true sound quality at this point as I never feed it a good signal and really, I just have one.

I sure is a pretty thing with some cleanup. It makes me wish I could find a second dumpster 75! I haven't actually figured out what to do with this one. I would like to keep it but it's not well suited for either my MMGs nor my NHT 2.5i's. The cost of a second amp is questionable budget wise, adding new speakers on top of that is a non-starter. It's a bit like being given a Ferrari with a bad transmission. You know it's capable of so much but until you sink the cost of a nice car into the transmission you have nothing but a beautiful garage queen.

Thanks for the help all!
Nikki, it wont't cost you anything but patience and vigilance before you come up with a second one at a reasonable cost (or no cost!) It's just a matter of time and watchfulness. At that point (and it WILL happen) I bet your speculation about what to do will take on a whole new complexion! In the meantime, remember, there's someone out there in exactly the same situation as you -- just waiting for YOU to give up first !!

It's a bit like being given a Ferrari with a bad transmission.

In the meantime, remember, there's someone out there in exactly the same situation as you -- just waiting for YOU to give up first !!

Very well put, in both cases! :)

-- Al