McIntosh MAC 4100 weak left channel

sgarwood

sgarwood

New Member
Hello!

I have a McIntosh MAC 4100 that has noticeably weaker left channel audio in Stereo mode. Both left and right seem about the same in mono.

AM and FM both work the same as described. I'll start working the connectors with DeOxit for starters.

Anyone have any ideas, or seen this before?

Thanks

Steve
 
After more testing, the aux 1 and 2 inputs are unaffected.

AM, FM, Phono 1 and 2 are affected.

I'll look at the board connections on the pre-amp audio switching board, but I'm leaning torwards one of the amp IC's or a switching IC.

It's about time to start tracing the audio path with the scope and see where things are going bad......

Steve
 
Simple is always better. Ran through the audio path with the oscilloscope. Compared left to right, had similar input levels for FM source into the preamp- audio switching board. Left channel output at the board was half of what it was for the right channel.

Traced all the way through the board - until I came to C732, near the output. End result, changed both C732 and C733. Original was a 10uv/40vdc cap, all I had was some 22uv/50vdc caps. I will probably go back and replace with the original value later, but the unit is working much better now.

Only thing left is a mechanically noisy tuner. Shot it with DeOxit.... Anyone know of a better fix for this?

Steve
 
Last edited:
McIntosh MAC4100 weak left channel

Hello STEVE

What area will you fix with your deoxit ?
Don't put deoxit everywhere ..MAC4100 FM tuner is really reliable and don't bring any troubles usually ...
 
Those brown ROE caps are failing everywhere.....replace with Nichicon MUSE ES series and be set for another 40 years.
 
DeOxit on the tuner....

I put a coating of DeOxit on a new nylon toothbrush and scrubbed the contact points between the shaft and the frames. It was noticeably showing signs of oxidation. Nothing on the blades other than compressed air from a can. My rule ( and others ) towards DeOxit is that if you can see it, you've used too much.

Tuner operation noticebly quieter. Have to see how it acts the next couple of days before putting the covers back in.....

Steve
 
Brown caps....

The only two brown caps on the preamp board were the two that I changed.

I don't know if there had been others....

The schematic showed that they were polarized, but the two that I pulled appeared to be non-polarized.

I have some higher quality caps on order......

Steve
 
Luckily, the only two brown caps I've found so far were the ones on the preamp-audio switching board. I had not heard of the problems with the failure rate.... Have to do some more research - I'm curious now!
 
Here's a pic of the preamp-switching board, before the two brown caos were removed....
 

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There are two more on the EQ board....

We actually did a comparison of a recapped and original MAC4100. The recapped sounded so much better I now feel it is doing the owner an injustice to not do a complete recap. It will not get any better or cheaper to wait or do a partial job.
 
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