Need help with my McIntosh MC250 please - diyAudio
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Old 27th January 2013, 11:56 PM   #1
tofu is offline tofu  United States
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Default Need help with my McIntosh MC250 please

Hi,

I opened up my MC250 today for the first time to check for leaking caps and such, and it looks like someone has already been in here to update a few of them.

I noticed that the left and right channel seems to have different components. I looked at the manual and it seems like they should both be the same, but i'd like to consult you experts first before i take a soldering iron to my amp.

I'd also appreciate if you have any other recommendations as to what i should replace/upgrade.


In summary,
Left channel has a .56w 5w resistor (correct according to manual)
Right channel has a .33 resistor

Right channel also has what looks to be a coil "wire round" resistor on the other side of the pcb, while my left channel has a ceramic resistor in its place (can't read the value)

I'm not very good with schematics of this nature, so i've just been using the manual. I appreciate all the help I can get.

(manual and schematic located here: http://www.berners.ch/McIntosh/Downloads/MC250_ser.zip)
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Old 28th January 2013, 01:00 AM   #2
tofu is offline tofu  United States
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i am also unable to locate the 0.47uf eros caps suggested here: 6th Street Bridge: McIntosh MC-250 update

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Old 28th January 2013, 06:13 AM   #3
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Originally Posted by tofu View Post
i am also unable to locate the 0.47uf eros caps suggested here: 6th Street Bridge: McIntosh MC-250 update


i just went with silmic II caps and vishay for the two large caps on the input.

whoever messed with this amp really screwed everything up. they have the wrong value caps everywhere. i'm replacing the four suspect resistors as well.

Last edited by tofu; 28th January 2013 at 06:24 AM.
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Old 28th January 2013, 12:37 PM   #4
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Thanks for the schematic diagram on this historic amp.
I wouldn't worry if C11 was bigger than specified, they appear to be couplers instead of timing components. C51 & C52 should be correct, I think. It is odd they put a curved plate symbol on caps otherwise specified in pf; caps that small are not electrolytic.
The 0.56 ohm emitter resistor appears to be there for a reason. Don't understand why, but the cost of a new one is more shipping than the resistor itself.
Interesting schematic. I always wondered why nobody made transformer coupled transistor amplifiers. Other than cost. Now I see McIntosh did. I wonder if the transformer protects the output transistors against speaker wire hot shorted to case. It might not. I also wonder if a shorted output transistor permanently magnefies the core of the output transformer. There appears to be no other protection against transistor burn ups. The output transformer will certainly protect the speaker against output transistor burnups.
As far as one 0.47 cap brand versus another, it might be more important that the dielectric be polyprophylene, which people on here say is extra linear, and the right voltage and tolerance, than some particular brand. It might also be important that the cap come through a QA system ending up at a major distributor, instead of coming through an auction site which might be a reject or a remarked part. Home quality tests on capacitors are expensive. Silk screen to custom re-mark a batch of parts is cheap.
Protecting my $300 each used speakers against my $50-$100 transistor amplifier is one of my interests. My $60 ST70 tube amp protects speakers with output transformers. My $50 ST120 transistor amp (used, broken, repaired) has a speaker capacitor to protect against DC. My $100 CS800s (used, tired) has 2 entire systems to protect speakers, which Salas walked me through, before I would dare connect it to the speakers. My $55 PV1.3k (used, burned up, repaired) has 1 speaker protection circuit that burns the PWB land off instead of protecting the speaker, so I've been spending a month designing a protection circuit for it that maybe actually works. McIntosh output transformers are great speaker protection, but probably a $100 per channel solution limited to 50 watts.
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Last edited by indianajo; 28th January 2013 at 12:50 PM.
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Old 28th January 2013, 07:00 PM   #5
tofu is offline tofu  United States
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Indianajo,

Thanks for the input. I ordered the resistors with my caps, so the shipping wasn't too bad. Came out to $20 and change to replace c47 c48 c53 c54 c55 c56 c11 c12 c51 c52, as well as the resistors with improper values (deviating from the manual). Ordered everything from Mouser

I hope the Elna Silmic II's are ok. They seem to be liked on this forum, and were pretty cheap. I'm sure anything will be an upgrade over the old caps and the improper value caps that were used
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Old 31st January 2013, 10:52 PM   #6
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I changed out all the caps and resistors i mentioned in the previous post.

My gain knobs are equal now. I used to have to turn the right channel up a bit to get the same output as left.

Amp sounds more cold and sterile now, though hopefully I get used to it
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Old 31st January 2013, 11:38 PM   #7
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You have the specified 15 mv at R57 and -25 mv at R59? This is your bias current that reduces crossover distortion. Similar values on the other channel. If you don't have enough I'm not sure what you would do about it, but having the output transistors in class AB is important to sound, especially at soft volumes. They have to be turned on.
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Old 31st January 2013, 11:42 PM   #8
tofu is offline tofu  United States
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Quote:
Originally Posted by indianajo View Post
You have the specified 15 mv at R57 and -25 mv at R59? This is your bias current that reduces crossover distortion. Similar values on the other channel. If you don't have enough I'm not sure what you would do about it, but having the output transistors in class AB is important to sound, especially at soft volumes. They have to be turned on.

turns out i still had the EQ on that i was using for my temporary headphone setup while my amp was disassembled.

Amateur mistake
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