McIntosh C28 and MC2105

[dukep]

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

[dukep]

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?

[djk]

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 (0.47µF).

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

C309, 310 are the output coupling caps for the pre-driver section, 10µF 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 0.1µF film bypass.

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

C1, 2 are the feedback loop caps, 330µF at 3V. The signal goes through these so replacing these with new ones and adding a 0.1µF 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, 150µF at 50V. Replace with same type and add a 0.1µF film bypass.

C201, 202 are the main filter caps, 39,000µF 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 47µF 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 80/80/150/50µF 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.

[a.wayne]

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 ...

[djk]

"a bit soft and sluggish sounding "

Won't sound anything like you described after you do what I suggested.

[dukep]

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 (0.47µF).
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, 100µF 15V. Replace with same type >and add a 0.1µF film bypass.
Yep, replaced, I'll add the 0.1's

>C309, 310 are the output coupling caps for the pre-driver section, 10µF >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 0.1µF 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, 470µF at 25V. >Replace with same type and add a 0.1µF film bypass.
Those were obviously leaky, white fuzzy stuff leaking. I up'd them to 1000uf 35v.

>C1, 2 are the feedback loop caps, 330µF at 3V. The signal goes through >these so replacing these with new ones and adding a 0.1µF 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, 150µF at 50V. >Replace with same type and add a 0.1µF film bypass.
These are in good shape.

>C201, 202 are the main filter caps, 39,000µF 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 >47µF 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 80/80/150/50µF >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.
_________________


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.

[a.wayne]

Originally Posted by djk

"a bit soft and sluggish sounding "

Won't sound anything like you described after you do what I suggested.

Yes , you might be right ! .....

[djk]

"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 22µF~100µF.

[dukep]

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.