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Old 21st July 2004, 08:37 PM   #1
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Default mcintosh MC2105 needs some help

I have serious problem with my mcintosh MC2105 power amp. With both left and right volumes at the same level the left output from the power amp has a significantly less amount of gain as the right output. I would say about 20 to 30 dB difference. The decibel meter for the left shows this gain difference between the 2 outputs. I try to crank up the left to compensate but then it distorts (noticeably) without that much gain. BTW the speakers I'm using with this are the ESS amt-1a (electro-static). I've tested the amp on diff speakers with the same result and different inputs. For awhile this setup worked well in the living room but then I put the amp in the basement for about 3 years when I got my surround sound and recently I tried getting this same setup to work again. I would really like to get this to work again.
Any ideas on what the problem is?

Should I try to get it repaired or should I try looking for a diff mcintosh amp (newer)?

I really miss listening to this amp (well more like the ESS speakers). Maybe I should try to look for a slightly used newer model.
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Old 23rd July 2004, 02:44 AM   #2
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Ok, after alot of logical thinking I've concluded that the small preamp circuit board in the MC2105 is broke. Here's my logic behind this deduction. The noticable decrease of gain is shown my the decibel meters and the headphone output and speaker output have little left gain. It has to be this little board because the decibel meter uses the output from this board and the headphones work off this board. So, I first tested the transistors to see if they all had the .67volts across base/emitter/collector and all of them turned out ok. Then I looked at the capacitors and there was a .0047 uf ceramic cap that was dead so I replaced it. After the replacement I tried the amp again and there was no improvement. So, now I'm thinking I shoud replace the blue electrolytic caps as well and go through each resistor. Here are some pics that show my amp
Click the image to open in full size.
Click the image to open in full size.

I also measured the input POTS and both are same resistance values on the meter, can I trust my meter or should I replace them anyway?

Does anyone have some ideas of what else might be wrong?

Does mcintosh sell these small boards seperately or does anyone knwo where I can get a used board that works?
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Old 23rd July 2004, 03:50 AM   #3
zobsky is online now zobsky  India
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another easy thing to do

also try swapping the driver (??) boards on the top of the amp between left and right channels to see if the problem follows the boards.

these slide in and out like PCI cards on a computer. might do well with cleaning the copper contacts too.

my mc2100 had a cap that needed replacing on one of these cards
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Old 24th July 2004, 12:33 AM   #4
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I was incorrect about the decibel meters and headphones working off that little board. I switched the two output boards and guess what... the bad channel switch from left to right. Here are the two boards.

Click the image to open in full size.

These boards have different components on them. I asked my dad if this amp was ever repaired and he said yes. Apparently when this amp was bought he moved into a brand new apartment and the builders mistakenly put 220volts for his appliance lines. This blew up the amp
So, the amp was then repaired but with different components (equivalents really). I checked all the resistors and the caps and the little transistors and all of them seem to be in working order. Then I flipped the board over to look at the solderings. Whoever repaired this board must have been an intern for mcintosh. There's a few coroded connections, one is barely connected to the board and there's a few cold solder spots. I hope I can repaired these connections without making a new PCB board for it. Heres a pic of the solder side...it doesnt do it justice but it's not a good soldering job.

Click the image to open in full size.

I realize that age can have an affect on this but the other board is older than the one thats broke and still works perfectly.
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Old 24th July 2004, 05:21 AM   #5
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try setting a hot solder iron against all the suspicious looking solder joints and letting the solder re-flow

i still suspect you have a bad component somewhere on the driver board

glad i could help
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Old 24th July 2004, 09:09 AM   #6
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http://www.diyaudio.com/forums/showthread/t-1310.html

The drivers, no longer available from McIntosh, are available from Magic Parts for $11 each, or see above post for sub.

also read:

http://www.audioasylum.com/forums/am...ges/49299.html
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Old 24th July 2004, 07:25 PM   #7
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Do you have a product number for magic parts?
Is their website magicparts.com?

I think that is the best course of action for this amp and for myself. I'm probably going to buy 2 brand new boards for this amp and then use the old ones for spare parts. Of course if magic parts doesnt have these boards anymore then I'll have to replace components .

This is so great that this amp is going to work again. I miss it's...perfection
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Old 24th July 2004, 07:51 PM   #8
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From the post you didn't read:

PNP
RCA 40409 .7(Ic)/90(V) TO-39
Driver Transistor w/heat sink
RCA 40410 .7(Ic)/90(V) TO-39
Send us an email to dealers@magicparts.com and include your zip code in your subject line. We will promptly reply with the dealers in your area.

"I'm probably going to buy 2 brand new boards "

Tell me another one.

McIntosh quit making that amplifier over 25 years ago.
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Old 24th July 2004, 08:18 PM   #9
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well I mistakenly thought you were speaking of a complete board that magic parts sells. I did read the posts but like I said I thought you were speaking of a complete board.

Thanx everyone for helping me out. I'll give magic parts an email with that information and get those new transistors.
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Old 25th July 2004, 04:15 AM   #10
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Default MC Boards

Hi,

I believe Audioclassics carries both the input and the driver boards! A bit pricey though! You can check them out at audioclassics.com.

Hope this helps.

ckt
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