rusty MC250 rebuild

eedork

Super Member
I just picked up an early MC250 in fairly rough but original condition. It appears to be untouched and I was told that it does indeed work. The biggest issue is the rust on the chassis which, unfortunately, is not easily fixed.

Assuming it works as the seller stated, I'd like to replace all of the caps on the input and power amp boards. The main filters and multicaps will probably stay for now, although eventually I'd like to replace the multicaps as well. I'm not planning on replacing any other components unless they test way off value. I'll probably also clean and re-grease the outputs and drivers.

Looking at the schematic I see two pots on the input board - R3 and R4 - it looks like these are just used to adjust the DC voltage on the emitters of Q5 and Q6, respectively. Is this correct?

Thanks!
-Matt
 
Last edited:
For the old McIntosh chrome, nothing works better than Wenol - buff (repeat Wenol/buff if needed) - Carnauba wax - buff. Make sure to use microfiber cloths, and don't overuse them - consider exchanging for new cloth every cycle, and definitely when switching from Wenol to wax. You'll be surprised how much comes off.

Sounds like a reasonable approach. I've only worked with the tube amps of slightly earlier era, myself, but the one thing I can tell you from experience is that I've never regretted doing a full restore exactly one time. If the multi-caps or the filter caps are even slightly inconvenient to access, you may thank yourself for taking care of those with the rest of the restore. Also, if/when you need to troubleshoot you don't have to worry about half-century old electrolytics.

I'll let others handle the rest - good luck!
 
The chassis on my MC40 monoblocks has a lot of pitting. I just ordered a tube of the Wenol blue compound. I take it that it's best to stay away from any silkscreening or that will be removed as well. Should be here in about a week.

eedork, how about some pics of your MC250, I always liked the looks of those amps.

BillWojo
 
Sorry if I wasn't specific - the Wenol you want is actually their Metal Polish, which comes in a red tube. I didn't realize they made a blue tube until I just looked.

It has an abrasive in it and will clean up pitting as much as can feasibly be accomplished. Not perfect, but a huge improvement in nearly all cases, and it'll shine up the underlying chrome. It does NOT remove the silkscreening even with a fairly firm hand. I used it on my MC60s which have all the original lettering intact, and they still do. No idea how it can spare the lettering while still working on the chrome/pitting, I was extremely careful and skeptical when I first tried it, but it works.
 
When I did some research on it, the blue was recommended for plated metal. I'll give it a try, if I'm not happy I'll get some of the red.

BillWojo
 
Here are some some pictures. I've already started cleaning it, and most of the rust is just on the surface. However, the chrome underneath is quite pitted. It will never look perfect, but it shouldn't be too bad once I get all of the surface rust off.

-Matt
 

Attachments

  • IMG_0473.JPG
    IMG_0473.JPG
    66.4 KB · Views: 165
  • IMG_0474.JPG
    IMG_0474.JPG
    80.5 KB · Views: 162
  • IMG_0475.JPG
    IMG_0475.JPG
    73.3 KB · Views: 156
  • IMG_0476.JPG
    IMG_0476.JPG
    104.9 KB · Views: 158
  • IMG_0477.JPG
    IMG_0477.JPG
    68.2 KB · Views: 154
  • IMG_0478.JPG
    IMG_0478.JPG
    63.8 KB · Views: 155
Be sure to include some after pics. I've seen worse on here, it's a shame they didn't use a heavier plating on them. My MC40's sat in a unheated garage for over 20 years but fared a lot better than yours. They will never be pristine but hey, their old and have some character. Electronically, they are fresh and sound awesome, yours will to if you go through them.
Keep us posted on your progress.

BillWojo
 
There is not much inside these MC250s - no wonder they sound so good. Aside from the main filter and the multicaps, I counted only six other electrolytic caps in the entire amp:

* Two axial 100uF / 12V on the input board (C3, C4)
* Two axial 500uF / 16V mounted on tag strips (these are in the power supply, I need to trace it more to figure out which part numbers these are)
* One axial 330uF / 3V cap on each power amp board (C11, C12)

Do people typically replace the multicolored poly caps in these as well? I noticed that a number of the diodes in the power supply have black corrosion on the leads. It is on the surface only and comes right off, so I don't think I need to worry about those at this point.

-Matt
 
The caps that look like big resistors? I think they change all of those type out.
How is the clean up going? My tube of Wenol came in, I'll have to try it out.

BillWojo
 
Hey BillWojo,

Have not had a chance to get back to this project. Hopefully I'll have time in the near future.

Let us know how the Wenol works for you. I (very quickly) used vinegar and tin foil and was able to remove a good amount of surface rust.

-Matt
 
Just FYI, If you are still working on the rust I would recommend Barkeeper's Friend. If you get the powdered version and then add some water to turn it into a thick paste it does a nice job removing rust (sometimes takes a couple applications). It isn't 100% silkscreen friendly but usually that's only because the silkscreen was already compromised. Just make sure for the best impact you cover it with plastic wrap and let it sit for 30-60 minutes minimum easy application.

If you are curious why this works, Barkeeper's Friend contains oxalic acid which breaks down rust. Its also easier to find than straight oxalic acid.
 
Wow - I'm just getting back to this project now. Time flies.

I fired the MC250 up last night for an initial test before the recap. It works perfectly and sounds fantastic. This is an early unit, and I've grabbed the earliest schematic I can find and it looks like there are 10 electrolytics on the input and output boards that need to be replaced. I'll have to open it up again and see why I only counted six.

I'm also planning to clean the output transistors and apply new grease (if needed).

-Matt
 
Last edited:
I just finished restoring my mc250. I found about 1/3 of the resistors and the sand block emitter resistors to be out of spec (there had been a bit of a bbq in it before i got it) and am happy i took care of them--it sounds excellent. I haven't done the multi section cap yet, though i have the caps and a chunk of bread board to wire them to under the chassis. Have fun--I found it quite relaxing to work on.
 
I just finished restoring my mc250. I found about 1/3 of the resistors and the sand block emitter resistors to be out of spec (there had been a bit of a bbq in it before i got it) and am happy i took care of them--it sounds excellent. I haven't done the multi section cap yet, though i have the caps and a chunk of bread board to wire them to under the chassis. Have fun--I found it quite relaxing to work on.

I'm into a 2100 right now. Understand I'm a student here but the lessons are fresh to me so maybe they'll help you.
I understand the emitter resistors are inductive (and need to be). If you need to source them go to mac. That being said (I'm shooting from the hip here assuming same or similar values) IIRC they were .33 and .56 ohms. If you're using a regular meter, at that low range you might (read likely ) be reading 1.2 ohms or so with your leads shorted. Therefore you would be reading an ohm and a half on a .33 ohm resistor and perhaps think it's out of spec when it is not.
The rainbow fish caps on mine were way out of spec. I replaced them with films (I'm not sure if it actually improved anything, it just seemed the thing to do while you're there.
The big cans are easy to source and change. I've been lead to believe the best way to test (the big cans) is with amp under load measuring voltages.
I think your plan is sound. Good luck with your project.
 
How does one know the age of a MC250? I just got mine and was wondering...Good luck with yours eedork. Couple a pics..:)
 

Attachments

  • Mcintosh 2.jpg
    Mcintosh 2.jpg
    67.4 KB · Views: 72
  • Mcintosh 4.jpg
    Mcintosh 4.jpg
    115.8 KB · Views: 77
  • Mcintosh 3.jpg
    Mcintosh 3.jpg
    78.6 KB · Views: 75
  • Mcintosh 9.jpg
    Mcintosh 9.jpg
    65.5 KB · Views: 73
There were 7,883 Mc250 units built between March of 1967 and May of 1979.

The serial number range was from 10L01 to 91L43. As to the age, you'll have to extrapolate from that based on your serial number.

Larry
 
How does one know the age of a MC250? I just got mine and was wondering...Good luck with yours eedork. Couple a pics..:)

Thanks!

There should be a serial number on the back panel. The serial number is fragile though, so yours may have been scrubbed off at some point in the past.

-Matt
 
If it's all original, look for date codes on the caps, probably built just a few months after.

BillWojo
 
So I just double checked, and there are indeed only six electrolytic caps to replace aside from the the two main filters and the two multicaps:

* Two axial 100uF / 12V on the input board (C3, C4)
* Two axial 500uF / 16V mounted on tag strips (these are in the power supply)
* One axial 330uF / 3V cap on each power amp board (C11, C12)

There are also six poly caps (tropical fish variety). What is the consensus on these - should they be replaced as well?

Final question, does anyone know of the R3/R4 potentiometers? I assume these are used to set the DC voltage on the emitters of Q5 and Q6 to 9.1V, respectively. Is this correct?

Thanks!
-Matt
 
Back
Top Bottom