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MC2105 Disaster! Help me Audiokarma, you're my only hope . . .

perryinva said:
So 9 months of nothing after all this!!! Anyone ever hear from the OP again?
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Apologies for not closing out this thread!

I wound up replacing most of the resistors and caps and transistors, and the amp is working well - at least to my ears. Looking back, my biggest mistake was not starting at the end of the signal path (i.e. audio out) and working towards the audio input. In my defense, there were a fair bit of physical issues that led me down that path, but I should have followed the advice I've seen elsewhere in these boards and not tried to shortcut the process.
 
I think I am coming to the same conclusion, since mine works fine, best to replace all the easy & available parts now, and prevent a failure and waste of time troubleshooting etc. I mean, it’s gonna need it all sooner or later!!
 
perryinva said:
I think I am coming to the same conclusion, since mine works fine, best to replace all the easy & available parts now, and prevent a failure and waste of time troubleshooting etc. I mean, it’s gonna need it all sooner or later!!
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You're in for a real treat Perry!
 
My 2105 sounds fine, but then I said the same thing about one of my C33s before I capped it and replaced the opamps and it is far better now. My 2105 is identical vintage to the OPs, in every way, except EVERYTHING on mine is original, including original Motorolas (all 74-52 dates) & the filter caps (74-49 dates). I had thought they were replaced by the blue color, but no. The last date I can find in mine is 74-52, so safe to assume an early 1975 build (AH6701). So I need to order new wire wounds and the 044-980 kit from Birmingham, and the rest I can buy from Mouser.
 
Correction. I already am building the order for the kit and other replacements from Mouser. I might even go with the wire wounds from there, since mine have no marking of any inductive and include the same ferrite beads. My emitter resistors look brand spanking new, (as does everything in my amp) no heat or stress cracks ant indicator of approaching doom. But since virtually everyone has stated that they drift, and they are 46 years old, it is probably prudent to replace them to preserve the original Motorola power transistors. Same with all the 1/2W 1% and key resistors and transistors. There are just not that many to debate about.
 
I noticed in a lot of the restorations I’ve seen on this older Mac gear that people are using what looks like metal film 1W resistors. I assumed the older carbon resistors were 1/2W and just larger because that was how they made them back then (plus metal film resistors are smaller anyway). Is there any advantage (or disadvantage besides price) using 1/2, 3/4, or 1 W. 1 W looks more correct, and what I ordered, except for the 1 or 2 where only 3/4W was available in 5%.
 
Carbon resistors and metal film resistors have different thermal characteristics. I don't know, not being there when they designed the amplifier, whether the carbon comp. were selected for a reason. More likely it didn't make a difference and therefore the more available / less expensive was chosen (which is usually what I did) but the caveat is that McIntosh did their testing and validation (including destructive testing) with the carbon comp resistors and didn't have any catastrophic failure modes, the metal film they did not (or they caused failures).

As a "veteran" of electronic circuit design I know well what (good) companies do to ensure that all tolerance stack-up and build variations are tested and stable during worst-case conditions and I try to stick with the original design. I no longer have environmental test chambers, shaker tables, equipment for load-dump, ESD, EMF, surge, etc. etc. validation and dozens of units to run to destruct and analyze failure modes so ...

I'm not saying don't change it, just saying that Mother Nature sides with the hidden flaw, and there's always a hidden flaw. It's at your own risk.
 
Well, metal film resistors did not become common in electronic gear until after the original design of the MC2105. (Just like radial caps were only common on power supplies, where axial in audio were normal). And as mentioned, some, such as those shrouded in case of short circuit destruction, should remain carbon. But the superior thermal, life, and audio characteristics are well known. There is little reason for extremely large carbon ones, not related to audio path, and still measure well within allowable spec to be replaced. But I am not of the “keep original for originals sake” or similar mindset. Smaller wattage carbon resistors are known to drift, and others have found many resistors on stressed circuits on power amps, far out from nominal values. That is my primary concern. Many have already replaced all resistors in this vintage Mac gear with nothing but rave reviews. Heck I even found a few toasty carbon resistors from reversed original capacitors in my C33 that needed replacement. So as few and easy as they are to replace, I’m good with that aspect of it, and not worried about the “weakest link” aspect. Of more concern is the stress to other components caused by higher voltages and current due to the drifted parts.

My question really centered on what is the actual wattage rating of the PCB mounted carbon resistors. It is not listed anywhere in the service manual. On the service bulletin for the upgrade of the driver boards to larger wattage heatsinked transistors, Mac lists 18 ohm 1/2W resistors for the bias resistor replacements, which I had ordered years ago in anticipation of doing this restoration when I finally had the time. (It had been noted that these values were critical to lower distortion. ) No mention of them being carbon units. The 1/2W metal film units I bought are probably less than 1/2 the physical size of those on the boards. And pictures of other units, show people had replaced theirs with similar sized ones. The cost difference is not significant enough to matter to me. And I have no problem installing physically smaller ones at all, as long as they are rated the same.
 
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perryinva said:
My question really centered on what is the actual wattage rating of the PCB mounted carbon resistors. It is not listed anywhere in the service manual.
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Note 1.) in the schematic notes section of my MC2105 service manual says that all resistance values are 1/2 watt, 10% tolerance unless otherwise noted.
 
Thank you! I had been over my copy a bunch of times and never found that! DOH! Did you use 10% 1/2W resistors? Just curious. I ordered 5% 1W for general and 1% where specified, and also for the 18ohm units. The ones in your pictures of the input board where there are both original carbon and new metals are on the same board look almost the same size. On my 1/2W ones in stock, even the leads look flimsier than the originals. (Not a big deal at all, again, just saying!).

Just as a note, so far, except for the 0.47uF tropical fish caps, (all right on the money, but still replaced) none of the capacitors I’ve replaced to date, or resistors I’ve checked have been within spec. The axial electrolytic caps were at least 25% high and as much as 60%, with much higher ESR than all the replacements I installed. Remaining parts come in tomorrow UPS.
 
Just to close this thread, I too ended up replacing all the electrolytic and film caps & resistors on the input & output boards and did the driver update. I replaced the electrolytics on the meter board. All my diodes looked very good. Checked measurements on all of them. All the small signal transistors checked good and I replaced most of them except the matched pairs and Sentry transistors. I used the 41kuF C-D filter caps. I also resoldered every PCB wire & lead connection and all the mains connections. Everything measured near perfect after replacement except my main voltages were all slightly high: 35VDC measured 37, 80 measured 88, 90 measured 100 etc. but the amp runs much cooler now than before and upgrade in SQ is very significant, especially bass, & noise floor. The transformer hum is a fraction of what it was and is definitely non invasive now. Even put in new 1866 bulbs. After not using this amp for the better part of 10 years, it is definitely back in long term rotation again with one of my recently restored C33s. I am extremely pleased with the combination with both a pair of Advent Legacies that have recapped xovers, and a pair of ADS L520s.
 
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Hello , Sorry to hijack this thread but I would like to ask if anyone knows what i can do about replacing my speaker wire connections of the same 2105 amp. The one speaker connection has a broken screw, so I would like to replace the whole system and put a more robust one (if possible),I think I saw another thread of someone doing it here somewhere.Thanks
 
Orion7 said:
Hello , Sorry to hijack this thread but I would like to ask if anyone knows what i can do about replacing my speaker wire connections of the same 2105 amp. The one speaker connection has a broken screw, so I would like to replace the whole system and put a more robust one (if possible),I think I saw another thread of someone doing it here somewhere.Thanks
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I made some notes here: https://audiokarma.org/forums/index...so-terminal-block-notes.961548/#post-14861185
 
Thank you for quick reply , I managed to get the broken screw out ,but the right angle pin broke so thinking of filling hole with solder to give it contact until I get a new replacement ,Thanks so much Johnny
 
Orion7 said:
Thank you for quick reply , I managed to get the broken screw out ,but the right angle pin broke so thinking of filling hole with solder to give it contact until I get a new replacement ,Thanks so much Johnny
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Mouser sells all of those parts ...
 
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