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In Reply to: RE: McIntosh MA6100 repairs posted by Thermionic27609 on April 13, 2017 at 15:19:18
As a follow-up. I thought all was well, but I started to see upwards bias creep on the left channel, the one I'd just repaired. It was still running warmer than the other channel the longer it ran.
Ultimately the solution had to do with the PCB sockets on the chassis. I noticed that some of the contacts had been pushed apart, and one thing I was told about this amp was that it was damaged by rough handling in shipping at some point. I got out my tube socket tools, retensioned the contacts, and cleaned them thoroughly. Everything is stable now.
My sense about these is that though they are ruggedly built, they need a good bit of detailed service at this point to return them to reliable operation. Integrated amps and preamps of this era are full of features we don't need anymore and, thus, have lots of switches and controls in the signal path.
Follow Ups:
Well,I knew the 22 ohm resistors were out of balance because everyone I have ever worked on were out of tolerance except one. Which outputs did you use in the unit?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I used MJ15003G, the same shipped by McIntosh.
There is one further thing I've noticed. After playing music for a couple of hours, one channel's heatsink is still warmer than the other. All DC measurements I can take between the two channels have them very close to one another. The only difference I can find is that on Q405 on the warmer channel, I measure about 22Megohms between base and emitter. On Q406 on the other channel, no resistance registers on a Fluke DMM.
At idle, there is no difference.
Unfortunately, no generic transistor cross seems to be known for Q405/406 (132-081), meaning I'd have to order from McIntosh, which takes about 3x as long as ordering from Mouser.
Some of that is due to the different numbered components between the channels.BTW,the MJ15003 and MJ15004 you can buy at Mouser or Newark.I bought 100 of them two years ago and I have 32 of them left. 22meg isn't really a problem.Do you have a way to run a power output test and distortion level?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Yes, I bought the MJ15003G from Mouser, but I also bought two from McIntosh because the owner wanted them sourced from McIntosh -- the first time, before the second channel blew on the bench. All are by ON Semiconductor.
I've done full-power bench tests into a resistive dummy load and a crossover distortion check at low power. All that is fine. Both channels are putting out above rated power, checked individually. Clipping is symmetrical.
I don't have a distortion analyzer. Yet.
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