MC 2105 Output stage

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grant_MH

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Joined 2009
#1
The left channel is drawing way too much current and gets very hot very quickly.
Sound was perfect for about 5 minutes then random noise and booms began after power on.

found that the 1st transistor, Q117, is the one that is hottest. Replaced the resistor on the emitter, R141, 0.56 Ohm 5 watt wirewound with a 0.5 ohm, same power rating. Still draws excessive current. checked all the resistors on the output board and they seem within limits (+/- 10 0/00).


As all the previous stages are dc coupled with isolation caps having a hard time pinning down the fault.

Anyone have any ideas, please?

thanks in advance, Grant
 
#2
Been watching this thread and no takers so I'll chime with my 2 cents worth. Have you checked any of the components on the driver board of the bad channel? Taken any voltage measurements? If all of the components test OK on the heatsink assy., the board is highly suspect. I may regret saying this but if you can't find any bad parts you could, dare I say, swap the driver boards and see if the problem follows. You do realize that if it is the board you could turn your good channel into a bad channel. Check all of the voltages against the schematic, ie the base voltage of Q117 and Q115, should be -34.5 and +.6 respectively. If these are bad the board is suspect. I don't know how old your amp is but it is probably loaded with carbon comp. resistors, check those as they tend drift, usually up.

What were the cross numbers for the xsistors you needed? How many numbers did you bug Mc for? I always try to squeeze in a few when ordering from them, same for Crown too.

Craig
 
#3
Hi Craig,

thanks for the reply!

Hard to measure the voltages on the board as there is little room to manipulate the DVM probes. I did find on the board that the resistances were mostly within 10 percent and I replaced the electrolytic capacitors. found one bad transistor which I replaced - tested the transistors using a Fluke and all but one had reasonable readings, i.e. voltage drops of 0.6 V +/- 0.1 V. I compared the readings with the board on the "good" side and they seemed to be within reason.

Haven't reassembled the amp yet and given the smoke test but will have time today.

As for Mcintosh, I guess it depends on who answers the phone in the service department when you ask for industry part numbers for replacements. The 1st tech told me that power output transistors could be replaced with MJ15003s. The 2nd tech she could only give me the Mcintosh part numbers, not the industry numbers. I have a couple of numbers I'll post on the thread when I can collate them.

These days it seems that vacuum tubes are easier to find than BJ transistors.

I'll let you know how the smoke test goes.

Grant
 
#4
Tubes are much easier to deal with as circuit changes are much easier. If you think BJTs are hard to deal with try and find some old Mosfets in that style of case, very hard to find at a decent price, like under $30. The MC2200 used the MJ15003/4 outputs too, 018/019/020/021 for drivers. Will check out the schematic some and try to think of something.

Craig
 
#5
I think that you're correct, that it may be the board wherein lies the culprit although I can't prove it yet. I have to abandon my efforts for today as we have to leave for a family reunion in Mississippi tomorrow. rather be doing this!

I'll apprise you of the smoke test result when I get back next week. thanks for the posts and helpful suggestions and helping with the project so far

all the best, Grant
 
#12
Bias resistors for the PA transistors I ordered from Mcintosh arrived today and I am really disappointed as they cost $5 each but they are not the same small originals that fit and conform nicely to the tight volumes inside the heat sinks - I can imagine what Mc charges for the drivers

thanks for the help - you've probably saved me some money!
 
#13
B I A S Diode?

after re-examining the 2105 schematic I noticed diode "D103," labeled "Bias Diode." In the schematic notes this is described as heated by the driver transistor Q111. After removing the transistor, which is bonded to the heat sink, I found the diode. The heat transfer grease appeared to be completely dry. The only id on the diode is mc part # 070-046. I have no idea what to replce it with. I'm going to replace the drivers and feel that this diode may be the cause of the original problem : after warming up for about 5 minutes the amp produces loud "booms" and random noise. I think that it's more likely that this diode is failiing to limit the current in the drivers after they heat up than that they are faiiling.

Any ideas on a replacement for this diode?

thanks , Grant
 
#14
Hello Grant,
although I am nowhere near the expertise of the other readers, no one answered you in a few days so I'll go ahead.
Either you get the diodes from McIntosh (I'm sure you've thought of that) or, replace BOTH diodes with a new type.
Are these diodes rectangular-shaped?
I checked my 1996 ECG catalog, and although it does actually contain references to McIntosh parts, it did not have your diode listed.

Seems you're not the first one to have this problem: http://www.diyaudio.com/forums/showthread.php?threadid=123607

Good luck
🙂
 
#15
Anyone have any ideas, please?

thanks in advance, Grant [/B]
Click to expand...


I would go around checking voltages and see if anything looks odd.

If you get to the point where you are still stuck I would take out all the transistors and check them for hfe. While the transistors are out I would check every resistor value. I would also check caps for shorts.

Its rare I get to this stage and I know its a pain to take out all the transistors but it always turns up the problem.

The last time I got to that stage a transistor turned out to have a hfe of 1 !
 
#16
Problem has been resolved

Guys ,

thanks a million for your interest and help, I really appreciate it. There's a valuable repository of information and trouble shooting techniques.

The problem I had was of my own making : I replaced the 0.56 r bias resistor on Q117, and Q117 itself, and must have accidentally shorted the bias resistor/PA transistor on the sister board. Anyway, after checking every resistor, capacitor, diode and transistor on the PCBs, both boards having the same readings I found the burned resistor and open transistor. Now that they are replaced the amp works great! Still plan to implement the suggestions offered for upgrading, in particular the capacitors.

many thanks to all who took the time to read, think and respond!

Grant
 
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