McIntosh ma6100 smoking

Deezel

New Member
Long story short. Guy I was buying from plugged the unit in, hooked up to speakers . I saw small stream of smoke, jumped and turned it off. It was on for about 3 seconds. I cracked the unit open and saw 2 resistors scorched(in pictures) on what I believe is apart of power supply. The resistors were in the same area the stream of smoke came from. Also has a fuse toasted going to left output. I haven't worked on electronics at all in 15 years. So Im asking for help, possible cause of the problem, advice and so on.

View attachment 1261667 View attachment 1261668 View attachment 1261669 View attachment 1261670
20180818_215340.jpg 20180818_194932.jpg 20180818_194943.jpg 20180818_194924.jpg
 

Attachments

  • 20180818_194924.jpg
    20180818_194924.jpg
    164.3 KB · Views: 108
Those 2 resistors feed 2 different unrelated loads from the same power supply. The only common connection is that both branches use different sections of the 4 section electrolytic capacitor near the center of the unit. If that were to have 2 sections shorted, it would burn both of those resistors. As a quick check, mark and disconnect all the leads to that capacitor, power up again, be ready to power down quickly, and see if it no longer smokes.
 
Those 2 resistors feed 2 different unrelated loads from the same power supply. The only common connection is that both branches use different sections of the 4 section electrolytic capacitor near the center of the unit. If that were to have 2 sections shorted, it would burn both of those resistors. As a quick check, mark and disconnect all the leads to that capacitor, power up again, be ready to power down quickly, and see if it no longer smokes.
I'll do that. But what does that cap feed, left right channels?
 
If the fuse is toasted to the left channel, the problem is most likely in the left channel. In my experience, it's usually a shorted output transistor. By process of elimination, you can use a meter to check the power supply diodes but since it powers up without blowing the main fuse, I would probably check the left channel outputs first then work back from there.
 
There are dozens of posts in the McIntosh forum regarding the repair and restoration of these integrated amps.

The blown fuse is almost always caused by a shorted output, in this case the one handling the negative part of the waveform.

I would guess the smoke you saw came from a resistor on the driver board.
 
There are dozens of posts in the McIntosh forum regarding the repair and restoration of these integrated amps.

The blown fuse is almost always caused by a shorted output, in this case the one handling the negative part of the waveform.

I would guess the smoke you saw came from a resistor on the driver board.

The 5watt 1500 and 600 resisters on power supply are touching the PCB and smoked it. But on that right channel both fuses are thin wire 5 amp fuses, not 250v 5a like on the left. Only one is blown . Would that be the cause of the heat?
 
If any repairs are being done to this - dont just turn it on again afterwards to see if fuses blow again - build a dim bulb tester - it makes life so much easier and saves gear.
 
Have you looked at the schematic? There is one in the AKdatabase.

When you do you will see why those resistors have discolored the power supply circuit board over the last 45 years, and that they have nothing to do with the open fuse.

There are many false leads that many others have explored which is why I suggested you explore the previous Mac forum posts with the AK search function.

A pair of new modern outputs and a careful repair of the driver board should get this unit ready for a complete restoration.
 
Have you looked at the schematic? There is one in the AKdatabase.

When you do you will see why those resistors have discolored the power supply circuit board over the last 45 years, and that they have nothing to do with the open fuse.

There are many false leads that many others have explored which is why I suggested you explore the previous Mac forum posts with the AK search function.

A pair of new modern outputs and a careful repair of the driver board should get this unit ready for a complete restoration.
I'm rebuilding that section, thanks. I have the original schematics, just been a while
 
The correct modern substitute parts have been listed in past threads in the Mac forum.
Well, after recapping everything except for the big 3 caps and replacing q412 ,q408,q410 (right output side) and all output transistors , it powers on beautifully without a hum or smoke. Right channel has no output . When I turn balance knob towards the left, sound goes out (makes no sense to me). Switched preamp boards and output boards, the boards play but a little hum in the board that was on the right. Also the left channel sounds good and clean, but I have to turn volume knob to half way to get it to play at talking volume. Fuses are all good.
 
Could speculate but to keep you from chasing shadows best start from a neutral position.

Balance at center,same with bass and treble, presence/ loudness off, just one speaker pair on.

Pull the preamp/amp interconnecting horseshoes.

Using a patch cord swap the R preout to the l amp in, l pre to R amp in.

Does the dead channel stay or swap sides......again balance at neutral.
 
Last edited:
Could speculate but to keep you from chasing shadows best start from a neutral position.

Balance at center,same with bass and treble, presence/ loudness off, just one speaker pair on.

Pull the preamp/amp interconnecting horseshoes.

Using a patch cord swap the R preout to the l amp in, l pre to R amp in.

Does the dead channel stay or swap sides......again balance at neutral.
Yes, it swapped sides
 
Ok so it is not now showing a power amp problem but a preamp issue.....

You must clean all the switches and controls with Deoxit d5 (red can) and the potentiometers with Faderlube (green can). This includes the push buttons other than speakers selectors.

These units have a issue with ground drains, we would routinely set these units aside during amp clinics I hosted back in the day to spend 2-3 hours soldering them. It is now a given, along with a recap, during any restoration. Again this point has been made often in the Mac forum but first a contact clean is mandatory.
 
Ok so it is not now showing a power amp problem but a preamp issue.....

You must clean all the switches and controls with Deoxit d5 (red can) and the potentiometers with Faderlube (green can). This includes the push buttons other than speakers selectors.

These units have a issue with ground drains, we would routinely set these units aside during amp clinics I hosted back in the day to spend 2-3 hours soldering them. It is now a given, along with a recap, during any restoration. Again this point has been made often in the Mac forum but first a contact clean is mandatory.
Well ,I already cleaned the hell out of them all with the red can, before I turned it on. Ill clean them again. But didn't use the green can at all. I'll get one. And yes, I am planning on correcting grounding issues.
 
Ok so it is not now showing a power amp problem but a preamp issue.....

You must clean all the switches and controls with Deoxit d5 (red can) and the potentiometers with Faderlube (green can). This includes the push buttons other than speakers selectors.

These units have a issue with ground drains, we would routinely set these units aside during amp clinics I hosted back in the day to spend 2-3 hours soldering them. It is now a given, along with a recap, during any restoration. Again this point has been made often in the Mac forum but first a contact clean is mandatory.
Got it working now, thanks. Very quiet. But I'm not familiar with this era McIntosh sound. Its playing more bass heavy, with highs pushed back. Sound about right?
 
Sounds like you have the loudness on. The loudness curve, as well as the presence curve is in the owners manual, I believe a copy of which is in the AKdatabase.

That said, did you replace the electrolytics? Did you solder the ground drains? I always run a spectral analysis to see what these 40 year old units response curves look like at multiple output levels. The tone circuits will go out of alighnment as the electrolytics fail.

No one has ever described a Mac amp from being weak on bass so if a listner is used to a anemic amp they might guess a linear output amp has a lopsided character......a proper bench test, even a truncated one like we did at amp clinics, should give you insight as to what response your unit is delivering.

What did you do to get the channel back?

Sounds like your unit had accumulated a bunch of issues, judging it before you are finished restoring it is really quite unfair other than indicating more work to be done.
 
Sounds like you have the loudness on. The loudness curve, as well as the presence curve is in the owners manual, I believe a copy of which is in the AKdatabase.

That said, did you replace the electrolytics? Did you solder the ground drains? I always run a spectral analysis to see what these 40 year old units response curves look like at multiple output levels. The tone circuits will go out of alighnment as the electrolytics fail.

No one has ever described a Mac amp from being weak on bass so if a listner is used to a anemic amp they might guess a linear output amp has a lopsided character......a proper bench test, even a truncated one like we did at amp clinics, should give you insight as to what response your unit is delivering.

What did you do to get the channel back?

Sounds like your unit had accumulated a bunch of issues, judging it before you are finished restoring it is really quite unfair other than indicating more work to be done.

I keep loudness off and try to always get a flat response. Everything sounds good and even, imaging is nice and its extremely quiet. But it seems that the balance definitely goes towards the bass.
To fix the problem I remove right preamp board and removed a dust ball that some how got in and blocked connections.
No ground drain work has been done yet, but I will get on it. I also have yet to replace the 3 large caps. Might explain why it seems like i have to turn volume knob up too much (10 or 11 o'clock) before it is up to a good loudness.
Do I need to replace the plastic looking caps on the preamp as well? Or just the electrolytics?
 
Back
Top Bottom