Restoring an MC2105

DOB had us tuck the diode up against the brim out in the field, the few factory shipped units did have them stuffed under the device however. The key is it has to track the heated semiconductor.

The "War of the Worlds Mars invader" look alikes are best replaced at this time also. The diff pair get noisy and and for a few dollars would be silly to leave them in there.
 
Yes, its clearer now after I can see where it's at in the schematic. I understand it now, why it needs to touch the transitor.

Would you happen to have the non McIntosh replacement equivalents from say Mouser it digikey, for the ufo transistors? I think the pairs above c101, are ksa992. I'm not sure of the two above the back of resistor replacements are.

Thanks again for your help.
 
You are asking me to remember.......ha but I will try. Mpsa06 and 56 for the sentry monitor was engineerings choice, Toshiba BL grade devices were Mac's choice for 3 or 4 dozen other devices back in the 80s.

The last batch of sa970 and sc2240s I got barely met the BL spec and so while the NEC/ Fairchild devices were not on Mac's list they have been regularly available and do meet the Toshiba spec. We will have to see if this continues under omsemi or hopefully gets better as there used to be a higher HFE 1845 and 992 available
 
Tonight I started into the amp, and did the easiest thing. I replaced the two main caps and found that both leaked, one was really bad. The amp actually ran really well, I now question the validity of the previous owner saying that it was tested by the prior to me buying it. Or at least the shop's reputation that he took it to. So for your viewing pleasure.
 

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Ok, thanks for everyone’s help. She’s back and running, from what’ve read, if she not burning up, she’s good. After a couple of hours run time she’s just warm and feels even on both sides. Im adding a little bling to her, making a custom chrome screen.

I replaced all the electrolytic capacitors and a few Mylar ones as well. Updated the driver cards, per the lastest service bulletin. Added some bypass capacitor, per suggestion I read here. Replaced all output transistors, as I did find two that were out. Mica and thermal greased all output transistors. Replaced all .33 and .56ohms resistors. Both output transistors and resistors were from McIntosh. Transistors were almost 50% higher cost then mouser, btw.

I do have some final questions though.

- Is it normal for the 5w resistors on the driver board to be burning hot all the time?
- the .33 ohm resistors from McIntosh, still measured pretty high, .6- .8 ohm. Would it make a different If one were to replace them with similar 5w wire wound resistors that has 1% vs 5%?
 

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Yes the 5 watters run hot. That seems to be the norm. Is you meter measuring 0 when you short it. mine is .3 ohms off because of the replacement leads.
 
did all the crud come off the chrome where the really bad main cap leaked?
 
Out of curiosity, where did you get the main filter caps? McIntosh or were you were able to find adequate subs at Mouser?

-Matt
 
Yes the 5 watters run hot. That seems to be the norm. Is you meter measuring 0 when you short it. mine is .3 ohms off because of the replacement leads.

Yup, great call on the meter, rookie mistake, didnt even think about that. Good to know about the normalcy, of the resistor running hot.


did all the crud come off the chrome where the really bad main cap leaked?

Yes, I used ACF-50, to help clean the rust down to bare metal, then cleaned it and treated it with anti-rust primer and painted over the primer. It was all under the cap, so not visible after you install the replacement cap.


Out of curiosity, where did you get the main filter caps? McIntosh or were you were able to find adequate subs at Mouser?

-Matt

Yes, I found here on the board a reference to a drop in replacement, per Jlovda
. https://www.mouser.com/ProductDetail/661-36DY393F075DL2B

-Rich
 
Excellent - thank you for the link Rich! I have been thinking about replacing the filters in my MC2100. They aren't as expensive as I thought they were going to be.

-Matt
 
If you are refering to the 2 round heatsinks, they are manufactured by Avid and the part # is 2227B. They are pricey, but I did not want to take the chance of them being too small. I think I bought them from Newark, or maybe Mouser.
 

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Any help,I would really appreciate? I replaced the 132-153 and 132-154 with the replacement that are mentioned. 2n5320and 2n5322. The amp runs a lot warmer. Heat sinks are hot. I reinstalled the 132-153 and 154. The heat sinks are now cool to the touch.
 
Is the diode up against the bottom of the new transistor??? Did you install the jumper to take the place of the electrical connection of the old heatsink??? Did you use these instructions to do the update 044-980.
 

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Is the diode up against the bottom of the new transistor??? Did you install the jumper to take the place of the electrical connection of the old heatsink??? Did you use these instructions to do the update 044-980.

I have one of these with that same board revision that just came in, questioning whether I should do the update or not. Everything tested great on the bench. But I was torn between doing the bulletin or just recapping it and calling it a day.

What did you use (or what is recommended) for the bias diode? is there any reason the original won't do?
 
When I did mine, Mac did not have the diodes, I was informed I could use the original one. Just make sure it is placed as shown in my pictures posted above. I believe Mac has them in stock now, I would contact them and see if they do. I believe the current part # is 070-046.
 
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