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McIntosh MA6100 repairs

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Posted on April 8, 2017 at 14:40:21
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 I've just completed several repairs on a McIntosh MA6100, the first one I've worked on, including replacing output transistors and bias control transistors in one channel with replacements sourced (by request) from McIntosh. This has been one of those whack-a-mole repairs where as soon as you think you're done, something else pops up.

I've been playing it for a while to verify that all is well, and I've noticed that the left channel heatsink with original output transistors gets warmer than the right channel heatsink with new output transistors. The right channel, the one I just repaired, stays fairly cool. I also swapped driver PCBs just to see what would happen, and the left channel heatsink remains warmer. Both channels have been bench-tested at full power into dummy loads.

Should I be concerned about original output transistors in one channel running warmer than new replacements in the other?

 

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RE: McIntosh MA6100 repairs, posted on April 9, 2017 at 15:25:30
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 And the answer is that I was right to be concerned. The second output channel died while the amp was on the bench.

I wonder if this amp got hit by a power surge that caused one channel to fail and turned the other into a time bomb.

 

RE: McIntosh MA6100 repairs, posted on April 10, 2017 at 13:41:42
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Therm
On the 6100 I use the MJ15003 outputs but it is important that you check the 22 ohm resistors on the driver board.That sets your bias but also see if there is any DC offset from the differential pair by checking DC at the output.Those 22ohm resistors just be pretty close to 22 ohms.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: McIntosh MA6100 repairs, posted on April 10, 2017 at 15:49:48
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 Thanks, Michael. Interesting that the schematic doesn't point to those as close tolerance parts.

In this case, there was no DC offset problem with either channel. I thought the repair was finished. Then, I noticed that the heatsink on the non-repaired channel ran hotter than the channel I'd repaired. I was getting ready to run some further tests suggested by another McIntosh technician to check for problems when that channel suddenly blew both 5A fuses at the same time, just sitting there on the bench.

 

RE: McIntosh MA6100 repairs, posted on April 12, 2017 at 17:39:20
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Therm
How did you make out?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: McIntosh MA6100 repairs, posted on April 13, 2017 at 15:19:18
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 I got the new transistors in yesterday evening, including the pre-output drivers and outputs. I also replaced the 22R resistors and a few others on the board that were out of tolerance. One 22R resistor was up to 38 Ohms.

I did a safe power up. No fuses blew, nothing is getting hot, and DC offset is under 9mV. I didn't have time to do further testing due to field service calls today.

 

RE: McIntosh MA6100 repairs, posted on April 13, 2017 at 20:17:32
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 OK, on more testing, there may still be a problem.

In one channel, when on, the voltage drop across the emitter resistors at idle is about 2mV, and it remains stable.

On the other channel, the one I just fixed, the emitter resistor voltage drop starts at around 3mV, but slowly creeps up as it stays on, just sitting there, up to 17mV when I turned it off.

I have not replaced the differential pairs in either channel or the next transistor Q405. All other transistors on the driver board have been replaced, plus the output pairs.

I think you have to order Q405/Q406 from McIntosh. Then there is the temperature compensating diode on the heatsink.

Where would you suggest I direct my attention next?

 

RE: McIntosh MA6100 repairs, posted on April 15, 2017 at 08:56:14
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 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.

 

RE: McIntosh MA6100 repairs, posted on April 15, 2017 at 18:17:54
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 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

 

RE: McIntosh MA6100 repairs, posted on April 16, 2017 at 08:58:44
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 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.

 

RE: McIntosh MA6100 repairs, posted on April 16, 2017 at 09:32:40
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 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

 

RE: McIntosh MA6100 repairs, posted on April 16, 2017 at 15:52:28
Thermionic27609
Audiophile

Posts: 411
Joined: March 29, 2009
 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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