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Puzzling issue with c1963 MC250 distortion

C

CGMillard

New Member
I have a much loved system that I inherited from my father:
Mcintosh MC250 amp
MX 112 preamp/tuner
Bozak B400a loudspeakers
All purchased in 1963

Several months ago, the system developed an issue with the left channel, a muffled, distorted, crackly sound. I swapped speakers L-R, and the distortion stayed with the left channel, so I thought I ruled out a bad speaker. I swapped input cables L-R, issue stayed with the L channel, so I assumed the source signal was OK.

Took the amp to a factory authorized Mac tech, who replaced a couple transistors and small caps, tested system and declared it up to spec. Got it home and the distortion remained! Completely baffled, returned amp to tech, and he could not find anything wrong with test equipment and listening test.

Returned home, issue still present. So I replaced speaker cables, ran audio source direct to input with new cables, issue persists.

I am grasping at straws now. The Bozaks are of an age where it's safe to assume that the crossovers need to be rebuilt, but why would this symptom only be evident with the L channel? Either speaker sound great off the R channel and both sound distorted through the L???

So my question for you tech gurus out there is, is it possible that something about the L channel of the amp is interacting somehow with the aged crossovers to produce this result? BTW, both speakers sound fine when driven by my backup (a fairly modern Sony str-de595 av receiver)

Thanks in advance for any help!
 
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CGMillard said:
I have a much loved system that I inherited from my father:
Mcintosh MC250 amp
MX 112 preamp/tuner
Bozak B400a loudspeakers
All purchased in 1963

Several months ago, the system developed an issue with the left channel, a muffled, distorted, crackly sound. I swapped speakers L-R, and the distortion stayed with the left channel, so I thought I ruled out a bad speaker. I swapped input cables L-R, issue stayed with the L channel, so I assumed the source signal was OK.

Took the amp to a factory authorized Mac tech, who replaced a couple transistors and small caps, tested system and declared it up to spec. Got it home and the distortion remained! Completely baffled, returned amp to tech, and he could not find anything wrong with test equipment and listening test.

Returned home, issue still present. So I replaced speaker cables, ran audio source direct to input with new cables, issue persists.

I am grasping at straws now. The Bozaks are of an age where it's safe to assume that the crossovers need to be rebuilt, but why would this symptom only be evident with the L channel? Either speaker sound great off the R channel and both sound distorted through the L???

So my question for you tech gurus out there is, is it possible that something about the L channel of the amp is interacting somehow with the aged crossovers to produce this result? BTW, both speakers sound fine when driven by my backup (a fairly modern Sony str-de595 av receiver)

Thanks in advance for any help!
Click to expand...
Try 'exercising' your Stereo/Mono switch. Hit it with some Deoxit. I had a similar issue with both of my Mc2100's, once up a time. I actually took the switches apart, and cleaned them..
 
quartersaw said:
Try 'exercising' your Stereo/Mono switch. Hit it with some Deoxit. I had a similar issue with both of my Mc2100's, once up a time. I actually took the switches apart, and cleaned them..
Click to expand...
In taking it apart, the switch actually broke-in another post you mentioned that you had sourced a replacement part-could you help me find one? Thank you!
 
MC 250's and MX112 came along much later, 1968 to 74. The early 60's was all about tubes. Don't you mean a B-4000a Bozak. Or do you really have the lowboy Concert Grands referred to asblue-5.jpg B-400's from the late 50's. They are huge. I wish I could help with the MC 250, but ever one has already said what I would have said. I have B410's by the way and low boy B-4005.
 
CGMillard said:
In taking it apart, the switch actually broke-in another post you mentioned that you had sourced a replacement part-could you help me find one? Thank you!
Click to expand...
Try Audio Classics. Is the switch riveted in? Someone here can tell you the best way to drill it out.
 
CGMillard said:
In taking it apart, the switch actually broke-in another post you mentioned that you had sourced a replacement part-could you help me find one? Thank you!
Click to expand...
I have a few switches that are electrically correct, but they are simple slide switches. You'd have to fabricate a way to mount it, to the chassis. It won't look the same as OEM, which has the rocker, to actuate the stereo/mono mode. I could send you one, easily enough..
 
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