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Mcintosh MC-30, Can't find a schematic that matches this version

M

Mickey

New Member
One of my recently acquired MC-30s appears to be an early model. Of the 4 schematics I've found on the web, none seem close. It is an MC-30 Type A-116-B. Transformers are M-150-B and M-152-B. It has no accessory outlet. Here are some pictures of the circuit board inside.

Can anyone share a schematic?

Warmest regards, Mike
 

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None here. All mine have a 470 pf cap parallel with R13, 2.2k. Your R4 (3.3 meg) is in a different postion than mine also.

The ususal recommendation is to remove that selenium rectifier and replace it with 1N4007 and change R30 (3.3k) to 9.1k. Terry has posted a ton of information re: MC-30 in his "Hints and Kinks" thread, above.

BTW, that's a nice looking unit - good luck with it. :yes:

Murray
 
Understood. But without a schematic which even comes close to this particular amp, I'm going to be stuck with deriving the circuit myself.

Thanks anyhow!
Mike
 
Lol. Looking on the cap side of the board, I think I can see the "470 pf cap" just to the right of the last bumble bee cap that's next to the paper covered cap. On mine, it's on the resistor side of the board. :stupid: Learn something new everyday.

Murray
 
skippy_ps said:
Lol. Looking on the cap side of the board, I think I can see the "470 pf cap" just to the right of the last bumble bee cap that's next to the paper covered cap. On mine, it's on the resistor side of the board. :stupid: Learn something new everyday.

Murray
Click to expand...

Come to think of it, I got caught by that very hidden cap on my last MC30 rebuild. It took a while to find that little guy.

Mickey, that MC30 looks quite clean inside for its age. From the looks of the lack of heating around the large resistors, I'd say that amp hasn't been run much. Very nice.

Cheers,

David
 
Every so often you will find a part in a slightly different location even in the same build run, I think they wanted to keep us guessing even 40 years later.
 
Thanks Terry!!! I think that's it. This is the first schematic I have seen with the 220K across the plate and the grid of the 6L6GC tubes or with the -B transformers like this amp.

The amps I have belonged to a friends father. They probably haven't been powered for 25 years. As near as I can figure, they were acquired years apart. Perhaps he had this amp for a mono system and then went stereo whith the addition of a later mc-30.

I'm wordering how close the amps will sound to each other due to the circuit and component differences.

Warrm Regards, Mike
 
Once you get them refurbished, I doubt you'll hear any difference in them. Just make sure the bias is set correctly and the voltages up and down the audio chain are in spec.

These aren't big changes in circuit topology we're talking about....

Cheers,

David
 
Thanks!

Hi there--

Just registered today and you guys have already saved me hours! I'm somewhat of a Bell Sound Systems (of Columbus, Ohio) historian and have pretty much restricted my restoration activities to the 2200C chassis (thus my username). A friend handed me an MC30 yesterday in quite nice cosmetic shape and said, "It was either the dumpster or you." Yikes! The amp has a wicked hum and has all the original caps. So, I started collecting information and hunting down parts until I realized that the typical schematics didn't match the chassis. Couldn't believe I found this thread! Thanks to all of you, especially Terry. The schematic posted here matches the amp I have. I hope I can return the favor in the future.

My plan is to replace basically every cap and to do Terry's mod of the bias supply. Any advice would be much appreciated.
 
Welcome bell! Yes, replace all the bee and 'lytic caps and dump the selenium. tubesandmore.com has a suitable multisection can cap.

Murray
 
skippy_ps said:
Welcome bell! Yes, replace all the bee and 'lytic caps and dump the selenium. tubesandmore.com has a suitable multisection can cap.

Murray
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Hey thanks! I had actually planned to bypass the can and go with individual electrolytics. Any reason not to go with that approach?
 
bell2200c said:
Hey thanks! I had actually planned to bypass the can and go with individual electrolytics. Any reason not to go with that approach?
Click to expand...
The replacement can makes for a neater looking installation but that's about it. It (the can) is a little shorter than the original, like in the pic.

Murray
 

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CarlV said:
Yes, it costs a lot less. :p:

Welcome to AK!


Carl
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Hey, thanks for the welcome! Gee, three nice 'lytics @ about $6 each vs. $30 for the can. I get to avoid making a physical mess, be a cheapskate, and preserve the original appearance of the amp. :)
 
Yep, MX-115. That pic is a a couple of years old and I think that I had just got the second one (in the back) going and I'm checking it out. Still have them (of course) and one more.

Murray
 
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