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McIntosh MA6100 final stage dead.

sfCamil

sfCamil

New Member
Hello,
Sorry for my english pls !

I ask you to help me for a McIntosh MA6100. Recovered from a friend in a bad state.

There was a huge noise and he go in a safe mode after a few minutes. The final transistors were very hot. I just take some measurement in the supply voltages and compared with the schema, nothing was good. I don’t remember the values directly but I decided to directly change the large filter capacitors (10000 µ), the diodes and, the capacitors in parallel on each diode, in 42 V section (sorry I do not see the notations in the PDF that I have on my possession). Measured, voltages are ok.
Here comes the disaster. I just get out the final stages from the socket (dock?) for visual inspection and I put it back. I turn the amp and ….. boom. 4 fuses burn out. One false movement from my part? I don’t know. I paid attention but ....
I checked for short circuit, output to speaker…. nothing abnormal, the protection thyristors ok. But the result is quite heavy.

- Final transistors Q415-Q418 (open circuit). A substitute MJ21196G (better than MJ15003G? from what I have read on other forums)
- Pre-end transistors Q411 - Q414 dead (open circuit). Replace with 2N4923 / 2N4920 pairs
- The diodes D415 - D416 in the emitter of this transistors, dead’s. Replace with 1N4148 ?
- The resistor 10 ohm in the emiter have a value of 18 ohms. Bad?
- Transistors Q407 - Q410 dead (open circuit). Replace with MPSA06 / MPSA56 peers
- One of the two diodes fixed on the final transistors radiators D403 shorted. To be replaced by ...?
2 x 1N4148 in series (I've read on other forums)
- Resistors 0.33 ohms to the final transistors collectors in open circuit. Value: 0
- The diodes D413 - D413 simulated by GE PNP transistors (132-098) died (open circuit). Any substitute pls?

Open circuit = no conductivity, nothing measured, 0

What is happening I have no idea. Or the fact to restore the correct alimentations made it all started ....
If you could advise me for the replacements and any idea from you is welcome….. !!!!!!!!!!

Thank you very much you all.


In attachment a picture with the final stage of MA6100

A9R6331-final.jpg
 
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4 fuses burned mean big short circuit (output transistors fails )
MA6100 don't like mid repair . Full repair would be preferable . If you don't have the knowledge to do it , send it to a good tec man . Don't trust the "info" coming from other forum . They don't have enough knowledge about this item . Trust the guy who service them since more than 30 years . There is many tek able on AK . They will guide you well .
Where are you ?
Don't think that you will be able to refix it in a snap
 
Were all the fuses blown before you worked on the power supply? Are the mica isolators between the outputes and the heatsinks or did someone forget them? The correct output replacement is the MJ15003g from onsemi.

I would pull the driver boards and all the outputs and bring the unit up on a variac so I could monitor the current draw and voltages.

Catastrophic failures like you describe are rare with Mac units but with extreme abuse anything can happen. You might need to take a stack of cash money to your local tech to get him to work on it now that you have been inside it.
 
After some discussion with someone from McIntosh and read all this forums I do all this changes:

- Final transistors Q415-Q418: MJ15003G
- Pre-end transistors Q411 - Q414: 2N4923 / 2N4920 pairs
- Diodes D415 - D416 in the emitter of this transistors: 1N4148
- Resistors R431 – R432 in emitter: 10 ohm (b ad presentation)
- Transistors Q407 - Q410: MPSA06 / MPSA56 pairs
- Resistors R433 to R436 0.33 ohm 5W
- Diodes D413 - D413 simulated by GE PNP transistors (132-098): diode MBR120P
- one of the two diodes fixed on the final transistors radiators D403: 2 x 1N4148 in series attached to radiator
- Removed the two triacs from protection circuit (this was the big problem I need to replace them ... )

I started the ampli with an incandescent lamp on series.all ok ....

Now the ampli it’s working (!!!) … but I have a problem with the right channel. The final transistors (MJ15003G) become hot even without any signal on input. It’s on this channel where the D403 was changed but this problem was present before the crash. Switched the two pc boards between them and its always the same transistors who become hot. Conclusion: it is not on the board the problem. Also removed the output wire to the speaker…. switched the preampli boards … the same problem… same transistors become hot. The replacement of D403 with 2 x 1N4148 not ok? Any other idea?

Thank you very much!
 
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What are the voltages at the various connection pins of the driver board?

If the output transistors ( MJ15003g) are always getting excessively hot, with either driver board, you either have faulty MJ15003s, a bad mica insulator from the heatsink, or a improper voltage going into the transistors via the board.

Third option is easiest to measure, 1and 2 means pulling and testing the new outputs.
 
I can not read your voltages, red is too small to read.....but it appears you do not have 0 volts where it is supposed to be....at the output.

The circled triac part number TR401/402 is part of the crowbar circuit. It is designed to blow the fuses if DC is present at the output. Since I can not read your voltages, and in fact if the output is not 0 DC, the triac is doing it's job, shunting the signal to ground.
 
While I do not think the triacs are your problem, you never know. Since I am sure you will ask the comercial replacement part number are MCR264-6 or MCR69. The original is a MAC11-2. The semiconductor data sheet calls the replacements SCRs and the legs are labeled as anode cathode and gate L to R, the original a Triac,T1T2 and gate L to R.

Now I finished my sandwich and i get to go wrestle with a Arcam.......
 
I have zero interest in going to your website.

Parts typically have names and numbers in a schematic, test points have numbers and semiconductors not only have names but individual connections that are known as base, emitter, and collector to identify voltage problems.

There is no label for bottom part.
 
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