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MC2105 Driver Board Help

zapdog

zapdog

guitardad
Hi guys, this is my first post, so go easy on me. I have a 2105 with driver boards 049-899 which blew a channel a number of years back. At the time I replaced the large 'lytics, but then shelved the project due to lack of time. recently I have replaced all the output transistors (MJ's), all the lytics, performed the Mcintosh update on the driver board (2n5320-2n5322 transistors, diode under heatsink, mpsa05 and mpsa55 pair, new bias resistors, etc.), and still my 250 watt lamp lights up when plugged in in series. By swapping boards I narrowed the issue to one board. I tried pulling Q106 on that board, and the lamp doesn't light up. So I figured bad transistor. I ordered new ones, but while waiting for them to arrive, I swapped in Q105 from the good board, and the lamp lights up! Dang. So I swapped D102- didn't help. All the resistors associated with that transistor test out ok. Is there something I am missing? I feel that maybe a component is failing under load. Q102 and 104 test OK, but I have seen transistors test out OK, but then fail under load. Any suggestions?
 
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I don't want to pull out a schematic and can not remember all the q designations. I think I have 4 or 5 Mc2105 schematics to choose from.

Scan and post the correct schematic screen shot of the questionable board or at least use verbal descriptions of the transistors function.
 
Before you did wholesale updates did you determine what the cause of the blown channel was?

By pulling the class A driver transistor you have basically cut off all audio getting ino the output stage.....so I guess you have determined nothing is a dead short there.

Still you should review your past work to make ure you do not have a transistor in the wrong place of the leads incorrectly installed.

If there was a major initial failure it would not be uncommon for the sentry monitor transitor for that rail fail.

Improper non inductive WWs resistors installed can lead the amp to go into oscillation and as Terry has reported in some cases in certain amps the MJ15003Gs can have just a little too much gain and cause issues.

That solution is going back to the root of the design which uses the first 003 to be a driver so to speak for the other two in the darlington configuration.

In these amps the predrivers determine if the output circuits are acting as NPN or PNP type circuits.

So back to the beginning, what parts failed in the intial diagnosis?
 
Interesting stuff. I will study up tomorrow morning, but to answer your question: one of the output transistors (on the heat sink), developed a short. It seems the consensus on AK is to change all 12 out.

D103 and D104 are new.
 
Reread your posts.....

So one driver board works in either channel, one board works in neither, at least in regards to presumed current draw.

If a channel misbehaves with either driver board: even the "good" one:

Did you replace all the outputs? How many wirewounds were open? The TO3 transistor isolator (mica) is only on one device. Did you get it on the correct transistor?
 
Yes, one board works in either channel, 1 board misbehaves in either channel. Replaced all 12 transistors, positive I have the mica in the right place.
 
zapdog said:
Yes, one board works in either channel, 1 board misbehaves in either channel. Replaced all 12 transistors, positive I have the mica in the right place.
Click to expand...
If one board works in either channel, then it has to be the other board.
You have a driver board issue.
 
Update! Saturation point suggested putting the original diode’s D103 and 104 back in, and guess what? No more dead short. I will try running some music through it now. Thanks for your help all!
 
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