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Old 1st April 2015, 01:45 AM   #1
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Default New to me McIntosh MC2205

Hi Everybody,

Just got a McIntosh MC2205, it has some issues and was hoping to get everyone's ideas. The previous owner said that they noticed some blackish liquid under it one day and so they stopped using it. Prior to that he hadn't noticed anything wrong with it and even when he did notice the liquid it was working. So it sat, until I bought it, unused. I know that the black liquid was most likely the potting tar around the power transformer. My question is could the transformer be over heating due to something that's worn out like a bad diode or cap straining it and if so where do I start to find the problem? Here's some pics of the bottom of the transformer. I only noticed very slight traces of grease/black stuff on it.

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Old 1st April 2015, 02:12 AM   #2
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I'd be interested in seeing some pics a little bit farther away.
Not close ups.

Take some before, during, and after pictures.

So that you remember how it was,
what you've done or will do and how
the wiring was, as you go through it.

First thing is get some compressed air
and gently blow all the crap out of it.

That might have started the problem
then not being able to derate led to others.

You're right up there in MC Country,
with parts aplenty and cheap.
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Old 1st April 2015, 06:31 AM   #3
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... and inside pictures are probably a factor 100 more useful....
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Old 1st April 2015, 07:07 AM   #4
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Is it the left, middle, or right transformer?
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Old 1st April 2015, 12:17 PM   #5
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Those are inside pics and two pics of the inside of the bottom cover, it's the middle transformer, the power transformer. It powers up.

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Old 1st April 2015, 09:43 PM   #6
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I was thinking of going through and changing out all the electrolytic caps with Nichicons and I was wondering about changing out the trim pots, I saw one person change them out with higher precision trim pots
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Old 2nd April 2015, 11:07 PM   #7
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Change all the electrolytic capacitors, and possibly all cappacitors, the trim pots look relatively new, I suspect they have already been changed out once, on a previous repair.
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Old 3rd April 2015, 01:19 AM   #8
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Posted by djk (M) on March 4, 2012 at 17:02:45
In Reply to: input tweaks - McIntosh MC2120 posted by raq on March 4, 2012 at 11:48:03:


This is what I do:

"Which caps are they DJK? I'll try them in my 2125."

Add one 47F across each 42V rail at the main filter caps

Add a 0.01F~0.1F film cap in parallel with C239, 240 (little board on the input jacks, very tight for space)

Add a 0.1F film in parallel with C203, 204 (input coupling cap to impedance buffer)

Add a 0.1F film in parallel with C207, 208 (output coupling cap from impedance buffer)

Add a 0.1F film in parallel with C219, 220 (feedback cap)

Add a 0.1F film in parallel with Q215, 216 (bias transistor)

Add a 0.1F film in parallel with C305, 306 (15V regulator outputs)

Add a 0.1F film in parallel with D304, 305 (reference diodes for regulators)

I cut off all the push on connectors in the power supply and speaker relay and soldered them direct. I soldered a pair of 0.1F film across the speaker relay contacts and cleaned the relay contacts with a sheet of paper and applied De-oxit.

I sprayed De-oxit on the RCA jacks, cleaned the input sensitivity selector switch (very important), and the edge connectors for the L and R amp boards.

I removed the 10-32 screws on the main filter caps and used De-oxit before replacing.

Do not move the chassis with the bottom off, the glass could crack. Put it on a small piece of carpet to help turn at different angles.

The 47F are under a lot of stress, I used 100V caps even though they only run at 42V.

Even though this changes no measurable performance specification, it will change the sound more than you can believe.

Jackhammer bass from an amp with a DF of only 14?

************************************************** **********
You should just be able to jump across the input caps and sensitivity switch with no problems there.

If you're trying to use better input jacks the grounding and the 0.01F caps to ground become important. If you try and eliminate the input attenuator you will probably need a cap to ground from the node of R201 and C203 on the driver card, somewhere between 10pF~100pF.

Good luck.
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Old 3rd April 2015, 01:42 AM   #9
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Thinker,

There you go. The good thing is you got it all blown out and cleaned
up so far. That's always a good place to start.

Have you Identified the problems source yet?

or is it still working okay just running hot?
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Old 8th April 2015, 10:22 PM   #10
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Brought it to Audio Classics for a quick look over, not a true bench test. Was told to look into the output section resistors and transistors. The output relay wasn't turning on, and the meters weren't doing anything. Got it home and thought I would first do a thorough cleaning. Took the whole thing apart, deoxit the controls and connections, rust removal and waxed the chrome, checked the resistors and transistors that control the relay didn't find anything, and put it back together. Now the output relay is working. I'm guessing something was dirty? and the meter responded but, the right side power guard light came one and the meter was showing full power. I felt the stacks and the one was very warm, the rest cold. I switched the power boards and it stayed on the same side. I then removed the stack and started testing the transistors with my multimeter, the first three are the same type. The first two tested at .45 - .48 the third read .25 and the third one conducted both ways so I'm thinking it's broken. Thoughts?
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