McIntosh MC-240 Restoration

Well, here is the subject of my next restoration --- a McIntosh MC-240 amplifier. Since rebuilding my MX-110 recently, I had been on the lookout for a good mate for it. Possibilities were a pair of MC-30's, a MC-225, or maybe a MC-240? It would have to be a fixer-upper to stay in my meager budget. Started watching eBay and other sources and found that all of these amps command a high price when they have very nice cosmetics... McIntosh amps with pitted chrome and missing silkscreening were much more affordable. Then I read in an old thread that someone is reproducing the MC-240 chrome chassis. :scratch2: Perhaps my best be would be to find a complete MC-240 with poor cosmetics, and rebuild it onto one of these repro chassis?

Here is what I ended up with:


I took a bit of a chance on this one... It was sold to me as-is and untested. Upon first inspection, I see that the chrome is pitted, rusty, and much of the silkscreening is gone or damaged. The knob on the three position switch is missing. One of the 12AX7's and one of the 12BH7's is missing. One of the 6L6GC's is different than the other three. But, I notice several good things as well --- Three of the 6L6's are original McIntosh branded RCA's. The two remaining 12AX7's are Telefunken's. The transformers look pretty decent with no dents and the stickers intact. The base and MC-240 emblem look good. Plus, I am already impressed with the overall look and feel of the amplifier. It is big and heavy! :thmbsp:

Took a quick peek inside and found that everything under the hood looks very nice, clean, and all original. Replaced the missing tubes, hooked to my variac, and soon the MC-240 was making music. Sounds pretty good!

Started checking on the reproduction chassis, and found a seller with the help of dewickt. Thanks Terry! One slight problem with this... The repro chassis is built for the later MC-240 and does not have provisions for the preamp power socket on my amp. :sigh: Nothing's that easy for me! Shouldn't be too hard to retrofit....

So, my plans are to remove all of the components from the old chassis and rebuild on the new. I also want to restore anything necessary electronically to make the amp safe for frequent operation.

First update will be to replace the selenium rectifier and capacitor in the bias circuit.

Should I plan to replace any of the other electrolytics?
Are there any other known problem areas that need to be addressed?

If y'all are interested, I can post step-by-step pictures as I make progress.

Thanks for any tips or hints for this project!

Wes Carroll

Please post step-by-step pictures. Very much appreciated.

Wes,
I've recapped one of the Mc-240s that I have. I replaced every electrolytic in the amp including the power cans. TerryD recommended illinois capacitors and that's what I went with. The caps and cans are available at the antiques electronics supply (as I remember). If you need any other partics then shoot me a PM and I'll share what I have. The chassis' on my Mcs could stand replacing so I'd be interested in what you find out when you do that evolution.

I have the knob (sevral hundred) for the 3 position switch, also know where to find a new switch if you need one.

Started stripping the MC240 over the past few evenings. One of the first things I notice is that this amp is pretty complex. To make things as simple as possible when it comes time to reassemble, I took many, many digital pictures along the way and also drew as many pictures, diagrams, and other notes as possible.

Started by removing the power cord and then the varistor:



I barely touched the thermistor and it broke.... Will need to find a replacement for this.

Then removed each of the output transformers and then the power transformer. I labeled each lead with a stick-on wire marker and made notes as to where each wire connects:



With the transformers out of the way, I removed the bias supply board and the can capacitors / mounting bracket. You can see a crack down the length of the "bumble bee" cap:




Here is a picture of the top of the old rusty and worn original chassis:



And, the transformers with each lead labeled:



Looks like it's time to get out my drill to start removing the rivets. :thmbsp:

Thanks again go to dewickt for helping me with a replacement knob for the 3-position switch. :yes:

Stay tuned for step two......

Nice pics Wes. Your work area is a lot neater than mine. :yes:

Speaking or rivets, I got this with the idea of mounting both sides of the rivet sets in it. For perspective, it's 13" tall, 10.5" deep, 5" wide and weighs 35 lbs. The throat depth is 3.75" and was $65 delivered.

This is still in the figuring out stage but I think it'll work. Just have to drill a 3/16" hole in the ram and base to mount the rivet sets. Maybe trim the business end of the ram a little.

Thanks for posting the pics and keep 'em coming!

Murray

This thread is so interesting and involving :thmbsp: one could say sex is half as much fun....one could say that.....one COULD..... :scratch2:

Great photos! I was just getting ready to bring this thread back up to see what was going on. What are you using for desoldering? Looks like very neat work(or unwork).

Finished stripping the MC-240 tonight. :yes: I used a hand drill and a 1/8" bit to remove the rivets. Not too bad, but had to be real careful when drilling around the circuit board. Some of the rivets were a bit challenging to reach with the drill.... And some would spin in the chassis which was frustrating. Anyway, they are all out!


Here's the "naked" circuit that lifted out of the chassis once the rivets were gone:


Unfortunately, the first 12AX7 socket is shielded and will not drop out of the bottom with the rest of the circuit. It had to be desoldered and removed from the top. (scott0527, I use the Edsyn DS017 desoldering pump. You can see it in the picture of the bare chassis. Works great.)


Now I need to order some parts and have a few questions for the MC-240 experts out there:

My schematic does not have a parts list. Does anyone have a list that shows the capacitor values and voltages?

Will a CL80 work in place of the broken thermistor?

If I replace the selenium with a 1N4007, will I have to insert a resistor to lower the bias voltage? If so, a ballpark resistance value would be great to know! :thmbsp:

Also, could use some opinions on further cosmetic restoration of this amp... The transformers look pretty good as-is and will respond well to cleaning and waxing. There are some scratches here and there. The base and emblem look good as well, but not perfect. On one hand, I kind of like the character of the parts as-is. On the other hand, I could repaint the transformers and base, which may end up looking like just that --- repainted. What would you do? :scratch2:

Excellent restoration!
Reminds me of a transformer "transplant" I performed on a MC275 a decade ago. The "donor" was a rusty parts unit. Back then I also used sketches and detailed notes to hook the thing back together. Terry, then on USENET's rec.audio.tubes, also provided invaluable help on things like 1N4007 replacements and correct bias.

Yes, you will need to add a series resistor to compensate for the lower voltage drop of the silicon diode. Perhaps Terry could suggest an approx. value.

If the original transformer paint is not too bad, I would not repaint them. You can always use things like car or black shoe polish to enhance them or even (God forbid!) small touch-ups with a sharpie pen. That's what I did to my restored MC275 and it looked like a million bucks, IMHO.

I am sure your finished MC240 is going to be a winner! :thmbsp:

Now that I remember, I even managed to restore lost lettering on mine by using a generic silkscreen transfer kit. I remember it was very time consuming to recreate the font and everything. Last shots I took of the unit was in 2000. Resolution is not great unfortunately.

Looks good Wes, you too Crooner.

The issue of the thermistor came up before and I don't think there was any resolution. I have an Mc "crossover" book but I don't know what the 240 pn is. Dunno what the value is for the bias resistor when swiching to silicon diode.

Murray

I use the CL-50, not quite the start resistance but rated 5A.
The bias should trim out at 470 to 510 ohms for the series resistor.
Feel like an old man when rec.audio.tubes gets mentioned, the old hang out........

dewickt said:

I use the CL-50, not quite the start resistance but rated 5A.
The bias should trim out at 470 to 510 ohms for the series resistor.
Feel like an old man when rec.audio.tubes gets mentioned, the old hang out........

Click to expand...

Please keep the picts comming.I would think you could take the chassie to any crome shop that doe's crome for motorcycles or the custom auto industry.That is if the silk screening is already gone.

crooner said:

Now that I remember, I even managed to restore lost lettering on mine by using a generic silkscreen transfer kit. I remember it was very time consuming to recreate the font and everything. Last shots I took of the unit was in 2000. Resolution is not great unfortunately.

Click to expand...

Generic silkscreen transfer kit? Have anymore details on that kit? Your job looks very nice.

scott0527 said:

Generic silkscreen transfer kit? Have anymore details on that kit? Your job looks very nice.

Click to expand...

Thanks! It was not perfect, obviously. At a close glance you could see the imperfections. However, it was much better than the worn lettering originally on the unit.

Let's see if my memory doesn't fail me...

The silkscreen was recreated by means of a letter transfering set I bought at an arts store. It was made in Germany by Staedtler, IIRC.

The fonts were obviously different so I had to build my own by cutting and pasting parts from the set. Very time consuming..

The end result was very good but quite fragile. It could only stand moderate wiping and no cleaning products of any kind.

Before the MC275 I tried this approach on a MA5100 integrated amp recreating most of the lettering on the front panel....

Back in the old days I used to devote a lot of time in painstainkly restoring McIntosh models. I even devised a method to eliminate bubbles in the front panel glass!

While I wait for my rivets to come in, I went ahead and changed some of the components on the small circuit board. Here's a picture of the board with the old components removed. Ever wonder what the inside of a bumble bee cap looks like? This one finished falling apart during removal:



Heres what the board looks like with the new parts. The diode is a 1N4007. Also added another terminal in the existing hole of the pc board to tie in the 480 ohm dropping resistor (as recommended above). This was easy to do... just uncrimped it from a spare terminal strip and recrimped it into the hole:



Not too exciting, but things should start rolling along when the rivets get here. :yes:

Trying to decide if I should go ahead and replace all of the other bumble bee's? :scratch2:

Yes, a bumble bee cap is almost allways bad, if they don't have leakage now they will afer some temperature cycles.

Very nice! :thmbsp:

I remember when I bought a MC30 back in 1992 or so, the bumble bees were falling apart. Needless to say, the unit was not working properly. I later sold it as a parts unit. I wish I had it back to restore! :bash:

This looks great, it's good to hear about bumble bee caps, I think all my 240's have all original parts.

I've had good luck cleaning mildly pitted chassis with automotive wax, this cleans oxidation off of autoformers too.

Two things I've found;

1) Windex removes the "Made for Mcintosh" lettering on tubes.
and it's also good for:
2) removing all the silkscreened lettering on early run C-20 glass. (Guess how I found that out.)

Mike