Mcintosh MC-250 Humming power transformer, am going to recap; your thoughts?

CruzBay

Super Member
Hi folks, I'm not a component level guy yet, but I've been inside these things many times. I've also been in television engineering for some time, so it's unlikely I'll defibrillate myself by accident.

I've got an MC-250 I've had since the late 90's that's recently developed an intermittent humming in the power transformer, and it's heated up some.

I've decided to open the thing up and replace any capacitors in there that need replacing, thinking a recap will fix things.

BTW, the device still plays flawlessly, and has for 18+- years, but the transformer thing has worried me. Serial number 46L44, and I do have a service manual (DOB sent it to me after a clinic)

I'll be going off this parts list: http://www.diyaudio.com/forums/solid-state/250113-mcintosh-mc250-capacitor-replacement.html Credits go to DIYaudio.com, ahankinson


So I'll be replacing 8 caps:

C11, C12
Was: 470µF, 4V
Replacing with: Vishay 470µF 6.3V MAL202133471E3

C51, C52
Was: 100µF, 15V
Replacing with: Nichicon 100µF 25V UKZ1E101MPM

C53, C54
Was: 10µF 25V
Replacing with: Nichicon 10µF 25V UES1E100MDM1TD (Bi-Polar)

C55, C56
Was: 640µF, 25V
Replacing with: Nichicon 1000µF, 25V TVX1E102MCD (Axial)
OR: Nichicon 680µF, 35V UKL1V681MHD1TN


Anyone have another suggestion?

Should I replace the diode bridge while there? The above list seems pretty short.

Ignore this stuff and call someone who can be more comprehensive?

Thanks to everyone in advance.

Mike
 

Attachments

  • MC-250 underhood.jpg
    MC-250 underhood.jpg
    119.6 KB · Views: 30
Check for high current draw at idle, check for warm elyticaps, measure and scope power rails to check bridge and big caps. A shorted power xformer winding is unlikely but possible. This xformer can be rewound.
Other possibilities include removing and reheating power xformer to melt potting compound.
 
Last edited:
Cool, thanks...

Looking for a variac on the website, unless anyone's got suggestions on one.

Your suggestion on repotting the transformer was the first one I got from an amplifier designer I had lunch with today.

I'll be doing things for him in the future, so he's suggested I bring it in at some point.

edit- looks like a 20 amp 130 volt similar to what I used back in the day is under $100...

Would like to have amp meter, all I see have voltage only...I've got an amprobe and plug adapter, I could use, but it would be nice to have a meter on the variac.

Going through the
So You Want To Repair Audio gear, Eh? Here's the tools you need thread now...
 
Last edited:
If you build a dim bulb tester you can always screw a bulb to AC plug adaptor into the socket and then your tester leads can go in the slots so you can use your multimeter to measure the current draw.

I dim bulb is always nice to have to plug any unknown unit in first to make sure nothing is shorted when you want to do a quick check up.
 
There are many different versions of the MC250 - make sure you have the correct schematic for your unit. I'm sure you do since DOB sent it to you.

Last year I acquired a rusty and beat old MC250 that still worked but had some transformer hum like yours. I also noticed that one set of heatsinks was noticeably warmer than the other. This was an early unit that actually had some adjustments - one pot per card; apparently there are also versions with two pots per card and then all later units had no adjustments at all. I dug up the service manual and discovered that the one channel was not where it should be (I can't remember now if it was an offset or bias adjustment). After dialing it back in per the service manual the transformer hum disappeared and the temperature evened out. It's working and sounding great now, although I still plan to recap it at some point just due to its age.

-Matt
 
I would not count on a recap fixing it. It not uncommon for the bias resistor to drift and if the differential pair semiconductors go off all kinds of strange things can happen.

If your lunch companion can do a bench test it would be advisable before any recap.
 
That's what I'm thinking, thanks Chris.

He's suggested the same, and as we'd be working together- and I need to be trained on his test bench anyway- I'll use my 8B with my computer's sound and hold on my MC-250. I do love the sound the '250 provides...

Don't have enough space, or a 2255 would be snuck in!
 
I recently restored my 250. I think about half the resistors were out of spec, including the bias resistors. Do the multi section cap too while in there-- I mounted 4 caps to the inside of the chassis and disconnected the leads from the can.
 
I recently restored my 250. I think about half the resistors were out of spec, including the bias resistors. Do the multi section cap too while in there-- I mounted 4 caps to the inside of the chassis and disconnected the leads from the can.

Did you go with metal films for the replacements resistors?

I'd also love to see some pictures of your multicap replacement if you happen to have any.

-Matt
 
Yes, I went with metal films. I tried to get the 1% tolerance ones, usually vishay (I buy from digikey because they are based here in MN). I don't have any photos unfortunately. I didn't make it pretty. I used Panasonic fc, and hot glued them to the side of the chassis. I rerouted the leads to the caps, and ran a ground wire off the ground point to the caps. I used heat shrink over all leads so if a cap comes loose it won't arc or short.
 
Hello cruzbay

If you don't have all the necessary tools you can do this :
disconnect all the secondaries of your humming power transformer . Keep it this way and connect it to the mains during one day long and you will see if the hum is generated by the transformer itself (alone) or not .
Unpotting the power transformer is easy
 
Back
Top Bottom