Vintage Mcintosh MC250 help needed!!

[Mr. p-car]

Hi group,

Have a newbie question that I want to pick the group's brain with.

I recently purchased an used 1960s era solid state Mcintosh MC250 amplifier, the AMP is in almost perfect condition a 9 out of 10. It is absolutely beautiful!!! Which leads me to believe that it has been redone recently, upon opening the bottom cover I can see that all the caps has been replaced and with little dust.

I have an issue with the AMP that I need some help diagnose and hopefully repair.

The AMP plays beautifully until the volume is turned up. The left channel will distort with the volume turned up but only in stereo mode. If I set the AMP to mono both channel sounds good and will not distort even turned 80% up.

So this leads me to believe there is a problem in the input circuit, since it is a vintage era amp the circuit is quit simple but I don't have too much eletronic diagnostic experience to be able to identify the faulty circuit to repair it. So I want to seek some help from the forum to see if someone would be willing to guide me through the repair process and find the fault.

Here is a link to the schematic of the amp
http://www.analogstereo.com/pdf/sm/MC250_ser.zip

I have a voltmeter, scope (don't really know how to use it), soldering iron, etc....

Thanks in advance!

Happy new year!

Robin


BTW, I have tried various inputs and swaped the input around, and I am 100% sure is the left channel on the AMP side. The distortion will follow the AMP's left channel, not speakers.

My current setup consists of:

NAD 5100 CD player
Counterpoint SA 5.1 two channel tube preamp
Mcintosh MC250
B&W 801 Matrix 2

[unclejed613]

the service manual gives you the transistor voltages for the input circuit. check the Lch input transistor voltages, since the input preamp is bypassed during mono operation. minor differences (within 10%) aren't usually a problem, but i'm suspecting you will find one of those transistors with voltages WAY off.

you have 2 preamps, so you can cross check between the channels (which would be useful even without a schematic)

[anatech]

Hi Robin,
Make sure all the caps on the input PCB are good. They tend to open up.

There is no way someone can teach you how to use your scope. What I would do in this case is feed a sinewave in and follow it through the amp, looking at the input board first. Try to have someone local help you out here.

-Chris

[unclejed613]

i also noticed the manual tells you the main signal path, it's the dark line that goes from input to output. feed in a sine wave and follow it through the amp. also look at signals in places like the opposite (inverting) input of the diff amp (that pair of mirror image transistors) in both the preamp section and the ones on the amp board itself, so you can get used to what to expect to find with the oscilloscope.

[Frank J.Y. Lin]

I am also an owner of MC2505,the brother of MC250 with meters.
I can share with my experience of fixing it.
My amp had similar probelm few weeks ago, there was some distortion and noise on the left channel.
I tried to fix it with cleaning the variable volume control resistor first.
After cleaning, it just restored.
So I suggest that you may try the simplest way, clean the volume control resistor up first.
Maybe it will work?!
Gook luck!

best regards,
Frank

[Mr. p-car]

Thank you guys so much for your input

Unceljed, thanks for the heads on the transistor voltage, I didn't even notice they were there... I need new glasses.

Frank, when I turn the gain control there is absolutely no noise or static. Clear as a whistle, so I am guessing the gain control should be working fine.

Here are some updates.

Both channel are displaying the same exact transistor voltage reading, but at some point it is reading different from what the manual states.

Q1 -0.1v (Correct), -0.6v (correct), 20v (Incorrect, should be 13.5v)

Q3 -0.1v (Correct), -0.6v (correct), 22.7v (Incorrect, should be 15v)

Q5 -1v (Correct), 21.1v (Incorrect, should be 14v), 20.4v (Incorrect, should be 13.5v)

Red wire point #9 40v (correct), green wire point #12 -40v (correct)

Again, both channels display the same exact voltage reading.

What now?

[anatech]

Hi Mr. p-car,
The last MC-250 I repaired needed those caps changed, badly. Right now you may have a leaky C56, reducing the tail current and allowing the positive voltage to rise. At any rate, if you have no way to test those capacitors, just replace them with the same capacitance but a slightly higher voltage rating.

You may as well check R111 and R112 to make sure they are the correct values. Without seeing it on my bench it's a little more difficult to guess at your problems.

-Chris

[Mr. p-car]

Anatech,

Thank you for your reply. I believe this AMP was rebuilt not too long ago so all the cap are new on the input board.

I did take a look at the C56 cap as you mentioned, and it is very interesting what I found. The C56 on my input board is a 470uf 25V cap where the manual says it should be a 640uf 25v. So maybe that's where the problem is !! Interesting that both the C55 and C56 have the incorrect caps as what the service manual says them should be.

Also the resistor R111 and R112 is only showing 2.3k instead of 3.3k.

But does this explains why only the left channel distorts?

[djk]

Mono uses right input only.

"470uf 25V cap where the manual says it should be a 640uf 25v. So maybe that's where the problem is !! "

Trivial, a power supply issue.

"Also the resistor R111 and R112 is only showing 2.3k instead of 3.3k."

That's why you get 20V instead of 14V.

Follow the signal through the left side on the input card, only three transistors. Probably clips on negative going waveforms.

Check R109

[Mr. p-car]

Please excuse the stupidity from my part, here are some more questions. I am a gear head (mechanical stuff), only have very basic electronic background and just enough to be dangerous.

I keep on reading "follow the path", from what I understand basically I would need a signal generator on the input side to start?

And then attach the probe from the scope to check out the wave form and identify after a particular circuit the wave form doesn't look right?

Compare the signal with the right channel and see if I see any difference?

The left channel only distorts when turned up, in theory I wouldn't be able to pick up the problem component just by looking at a signal generated when it is not under load? (sorry! car term) How would I duplicate that?

TIA