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Old 5th April 2009, 08:30 PM   #1
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Default MC 2105 Output stage

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
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Old 8th April 2009, 02:25 AM   #2
llwhtt is offline llwhtt  United States
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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
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Old 8th April 2009, 06:10 PM   #3
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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
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Old 9th April 2009, 01:33 AM   #4
llwhtt is offline llwhtt  United States
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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
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Old 9th April 2009, 01:42 AM   #5
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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
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Old 9th April 2009, 02:17 AM   #6
llwhtt is offline llwhtt  United States
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You mentioned electrolytics, the big one has + gnded, is yours? Did you change them before the problem or during this repair?

Craig
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Old 9th April 2009, 02:43 AM   #7
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both electrolytics have same polarity as originals - big one, C121, 100uF 63 VDC, has positive to ground

replaced during current repair

Grant
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Old 9th April 2009, 08:32 AM   #8
djk is offline djk
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Drivers
RCA 40409 700mA 90V 3W 100Mhz ~2N5681+heatsink
RCA 40410 -700mA -90V 3W 100Mhz ~2N5679+heatsink
Outputs
MJ15003

Real 40409/40410 are still available for obscene prices.
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Old 15th April 2009, 09:44 PM   #9
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did find the 40409/-10 at

http://www.vibroworld.com/parts/tech16.html


now awaiting the parts arrival, then the smoke test
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Old 15th April 2009, 09:46 PM   #10
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meant to say hope that these are the "real 40409/40410s"
the prices is $15.50 with the heat sink
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