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Old 29th January 2010, 06:51 PM   #1
dukep is offline dukep  United States
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Default McIntosh C28 and MC2105

So, how nice are these old school units?

They look amazing and sound good on the bench. Rated 100watt/ch but is consumes 500 watts in the peaks.
Amazing that this amp has an output transformer!
Tube sound from a transistor.

I just fixed the amp that was blowing fuses. It had pops and spikes feeding the amp from the input section.
It required all resoldering of the driver boards and input board due to corrosion. It had bad power filter caps in the input section (small board in the belly of this beast), replaced with 2 each 1000uf, 35volt. Finally 2 new transistors for Q5 and Q6, mpsA92. (actually only one 056 AA036 was bad).

Customer is letting me keep them for a while.... I'll be spoiled
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Last edited by dukep; 29th January 2010 at 06:55 PM.
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Old 30th January 2010, 09:19 PM   #2
dukep is offline dukep  United States
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to add,
the other channel showed fried (noisy) input transistors, same type. Poor amp is showing it's age.

likey due to bad 10uf capacitors between the stages not keeping dc blocked.
so 6 transistors and 8 capacitors later changing these in both channels, it is operational again.

http://www.classicaudio.com/value/mac/MC2105.jpg

Anybody have one of these amps??

Any reason for one pair of output transistors (of 4) to run hotter?
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Old 31st January 2010, 11:40 PM   #3
djk is offline djk
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I own a MC2100 and two MC2120s.

Caps wear out.

Even when not in use.

This list is for the MC2100, electrically it is the same as the MC2105, just without the meters.

Even though your old amp is still working, you have no idea how much of the music you are missing.

The parts are inexpensive.

Digi-Key is good, Mouser has no minimum order.

Posted by djk (M) on July 11, 2004 at 04:29:36
In Reply to: Re: MC2105 @ $ 100 even @ 20 yrs old is a best buy for me. posted by julian4@telkomsa.net on July 06, 2004 at 14:58:04:


The sound would benefit greatly from replacing a few dried out electrolytics, and adding a few film types here and there.
C301, 302 is the main input coupling cap. It is a Mylar type so it is likely to be OK, upgrading it to a Polypropylene type with give a smoother sound to the high end (0.47F).

C307, 308 are emitter bypass caps, 100F 15V. Replace with same type and add a 0.1F film bypass.

C309, 310 are the output coupling caps for the pre-driver section, 10F 25V. While a film type would be better, size is a problem. The DC bias across this cap also helps out with its being an electrolytic. Replace with same type and add a 0.1F film bypass.

C303, 304 are the DC power supply caps for the front end, 470F at 25V. Replace with same type and add a 0.1F film bypass.

C1, 2 are the feedback loop caps, 330F at 3V. The signal goes through these so replacing these with new ones and adding a 0.1F film bypass really opens up the sound. Go with as high a voltage as what space permits.

C11, 12 are the DC supply caps for the voltage gain stage, 150F at 50V. Replace with same type and add a 0.1F film bypass.

C201, 202 are the main filter caps, 39,000F at 40V. These may be quite expensive new, although I have seen them surplus for very low prices. I would use then unless signs of leakage or amplifier hum are there. Add a 47F at 50V cap in parallel with each.

C203 is a multi-section cap that will be very hard to find a fresh date code. New caps are small enough that they may be wired to the terminals of the old one, leaving the old one in place. This cap is important as it provides the current for the diff pairs and the VAS. The sections go 80/80/150/50F with the voltages being 200/200/150/150V. If you measure the voltages 100/95/90/80V are typical, so 200V caps are not really needed, but 100V is not enough.
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Old 1st February 2010, 02:34 AM   #4
a.wayne is offline a.wayne  United States
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I had both models in the past , great looking pieces , a bit soft and sluggish sounding , nothing offensive , just a signature sound of Macs from that era . Never get tired of looking at them ...
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Old 1st February 2010, 11:08 AM   #5
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"a bit soft and sluggish sounding "

Won't sound anything like you described after you do what I suggested.
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Old 1st February 2010, 04:38 PM   #6
dukep is offline dukep  United States
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Default djk thanks

see notes below, This is actually a customers amp.

>C301, 302 is the main input coupling cap. It is a Mylar type so it is likely to >be OK, upgrading it to a Polypropylene type with give a smoother sound to >the high end (0.47F).
These are labeled C3 and C4 on my schematic, yet to be done. Got them here. Could size up to 1.3uf (that's in an axial type)? That would be a lower roll off point.

>C307, 308 are emitter bypass caps, 100F 15V. Replace with same type >and add a 0.1F film bypass.
Yep, replaced, I'll add the 0.1's

>C309, 310 are the output coupling caps for the pre-driver section, 10F >25V. While a film type would be better, size is a problem. The DC bias >across this cap also helps out with its being an electrolytic. Replace with >same type and add a 0.1F film bypass.
I believe these were the main problems, transistor on both sides were bad.

>C303, 304 are the DC power supply caps for the front end, 470F at 25V. >Replace with same type and add a 0.1F film bypass.
Those were obviously leaky, white fuzzy stuff leaking. I up'd them to 1000uf 35v.

>C1, 2 are the feedback loop caps, 330F at 3V. The signal goes through >these so replacing these with new ones and adding a 0.1F film bypass >really opens up the sound. Go with as high a voltage as what space permits.
Yep, got that one also

>C11, 12 are the DC supply caps for the voltage gain stage, 150F at 50V. >Replace with same type and add a 0.1F film bypass.
These are in good shape.

>C201, 202 are the main filter caps, 39,000F at 40V. These may be quite >expensive new, although I have seen them surplus for very low prices. I >would use then unless signs of leakage or amplifier hum are there. Add a >47F at 50V cap in parallel with each.
Luckly these were also good. Hmmm... what does 47uf do? vs say 100uf (got both in the bug boxes) ?


>C203 is a multi-section cap that will be very hard to find a fresh date code. >New caps are small enough that they may be wired to the terminals of the >old one, leaving the old one in place. This cap is important as it provides >the current for the diff pairs and the VAS. The sections go 80/80/150/50F >with the voltages being 200/200/150/150V. If you measure the voltages >100/95/90/80V are typical, so 200V caps are not really needed, but 100V is >not enough.
_

again good for now. I've usually separated into 3 caps on tube amps.
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Any suggestions on the preamp, except replacement. It's pretty, but awe-fully complicated. And most of the pots are getting noisy (and that's alot of them.
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Old 1st February 2010, 06:27 PM   #7
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by djk View Post
"a bit soft and sluggish sounding "

Won't sound anything like you described after you do what I suggested.
Yes , you might be right ! .....
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Old 2nd February 2010, 09:24 AM   #8
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"Luckly these were also good. Hmmm... what does 47uf do? vs say 100uf (got both in the bug boxes) ?"

Makes the bass sound tighter, and stops the 'squeeg' sound on turn-off (caused by drying out caps). Try values in the range of 22F~100F.
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Old 12th February 2010, 07:39 PM   #9
dukep is offline dukep  United States
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Default oh my, that was sweet

with all the mod's complete .. The big McIntosh amp returned home.

The customer tested it into some very interesting Hartley 300's. These make a very good match.

Thanks for the info.
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Old 5th November 2012, 02:41 AM   #10
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forgive a newbie question. I've done a fair bit of soldering & building of components, but it's been mostly a paint-by numbers - not electronic design. When you talk about putting in a bypass capacitor, are you just speaking about putting in the caps in parallel with the existing cap (or the replacement cap)

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