MC-2105 refurb

T

teehas53

New Member
I’m the original owner of an MC-2105 (a later production unit, S/N - AHxxxx.) It’s in outstanding condition, always well cared for, and still alive and kicking after more than 50 years. It sounds pretty good although not quite as pristine as I remember. I know the electrolytics (at least) are way past their best-by date. The only service it’s ever had (or needed) was a trip back to the factory for testing and minor repairs in 2002. At that time it had a couple of caps and resistors replaced and tested within spec.

The plan is to do a basic refurb this fall and I am in the info gathering stage. I will replace all the electrolytics, any other caps that are known potential issues and check/replace a few key resistors as needed (driver board bias for sure.) I don’t want to do a full on resto-mod just because I’m in there - my goal is to keep things fairly original but refreshed for reliability. I also want to make sure I cover all known issues so I’m not back inside the unit any time soon.

I’ve read Refugee’s post here (and others,) super helpful! I have the tools, and some experience building amps (mostly tube units for guitar.) I have the service manual from McIntosh. I’ve seen mentioned here that there’s also a service bulletin for this amp (I believe relating to the driver boards.) The contact person at the factory said there were none for my amp, I’m wondering if possibly that’s because it’s a later unit? Any comments/opinions on that are appreciated.

What I’m hoping for here is advice and feedback from those who know this amp better than I. I want to cover the important stuff but not fix things that ain’t broke. Any and all comments are much appreciated!
 
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teehas53 said:
I’m the original owner of an MC-2105 (a later production unit, S/N - AHxxxx.) It’s in outstanding condition, always well cared for, and still alive and kicking after more than 50 years. It sounds pretty good although not quite as pristine as I remember. I know the electrolytics (at least) are way past their best-by date. The only service it’s ever had (or needed) was a trip back to the factory for testing and minor repairs in 2002. At that time it had a couple of caps and resistors replaced and tested within spec.

The plan is to do a basic refurb this fall and I am in the info gathering stage. I will replace all the electrolytics, any other caps that are known potential issues and check/replace a few key resistors as needed (driver board bias for sure.) I don’t want to do a full on resto-mod just because I’m in there - my goal is to keep things fairly original but refreshed for reliability. I also want to make sure I cover all known issues so I’m not back inside the unit any time soon.

I’ve read Refugee’s post here (and others,) super helpful! I have the tools, and some experience building amps (mostly tube units for guitar.) I have the service manual from McIntosh. I’ve seen mentioned here that there’s also a service bulletin for this amp (I believe relating to the driver boards.) The contact person at the factory said there were none for my amp, I’m wondering if possibly that’s because it’s a later unit? Any comments/opinions on that are appreciated.

What I’m hoping for here is advice and feedback from those who know this amp better than I. I want to cover the important stuff but not fix things that ain’t broke. Any and all comments are much appreciated!
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Take some photographs of the circuit boards if you are able to do so. I have one of these amplifiers where a few years back I went through it like you are preparing to do. I have all new output transistors but found the drivers and other transistors to perform good when tested on the curve tracer. I removed any carbon resistor in the audio circuit and caps were selected as to the circuit function. I do not use audio grade electrolytics unless they are in the circuit path and look to see if a film type like Polypropylene, Polyester, PIO or even Silver Mica is suitable and will fit comfortably.

Even after 50+ years, the two main power supply capacitors were checked and found to retain their capacity and low ESR. That says something about the quality of some of the components used. The original output transistors functioned but were all over the place on gain and not very linear. I was tempted to replace the driver transistors, but they looked good. Maybe later...? I do believe I have changed a few on the transistors in the front end, with newer and quieter devices because I recall building a circuit to match pairs when conducting 20 or so milliamps in open air. That was the differential amp I believe.

Take your time and you will have fun. The end result was clean, quick sounding amplifier that you would not even know it was a McIntosh, let alone having the autoformers. It can be that good.

Enjoy.
 
As Jose said, pictures of both sides of the driver boards will help, there were at least two board versions and several other changes.
 
Thanks for the replies gents. I hadn’t planned to disassemble the amp until closer to doing the work, but maybe I should in order to better see what I’m dealing with.

Jose: I haven’t seen much discussion about problems with output transistors. We’re you having audible symptoms that led you to test yours?
 
teehas53 said:
Thanks for the replies gents. I hadn’t planned to disassemble the amp until closer to doing the work, but maybe I should in order to better see what I’m dealing with.

Jose: I haven’t seen much discussion about problems with output transistors. We’re you having audible symptoms that led you to test yours?
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Jumping in although you asked Jose:

My 2105 had transistors with significantly varying gain / curves, which not necessarily an indicator of forthcoming doom and failure, can (will) increase the distortion and as the transistors are load-sharing and quasi-complimentary it can affect not only the linearity and symmetry of the amplification but also the higher gain transistors will be doing more of the work, running hotter junction temps, and will be more stressed.

I replaced all output transistors and bias resistors with new curve-matched devices, not that expensive.
 
Thanks Jeff, I really appreciate the info. I’m learning as I go. Any thoughts on where to source curve-matched transistors?
 
teehas53 said:
Thanks for the replies gents. I hadn’t planned to disassemble the amp until closer to doing the work, but maybe I should in order to better see what I’m dealing with.

Jose: I haven’t seen much discussion about problems with output transistors. We’re you having audible symptoms that led you to test yours?
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The amplifier worked but it was 30 years since I opened her up to check inside. As far as the output transistors, I recall they we no match for the newer ONSemi products. The amplifier received MJ15024 outputs. I like those for the initial transistor amplifier releases and when there are three output devices per side (minus the driver) I use the MJ21193/94 devices, like the MC2200/05 and later big guys. The MJ150254/25 are great for Marantz amplifiers also.

I took a look at the amplifier, and I actually relaced the driver transistors also when doing the output. For some reason I thought I didn't. The outputs were culled for gain, I don't think it makes that much of a difference between Yellow/Green and Yellow/Blue in the two locations.

Some people believe in a certain service bulletin is a mandatory audible update. You must recall how many of those amplifiers lived in a cabinet and into a bookcase or shelve without much air movement. If you had the amplifier out in the open, all would be fine.
 
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The FSB was born in the MC2300, and carried into the MC2100/MC2105 which share their design. I’m told that it was a mandatory service when 2300s came in for service but optional on the 210x back then. My understanding is that it improved (fixed?) a stability issue when run hard/hot.

It has now been half a century since these were produced and everything drifts, nothing gets better or more stable.

One of my 2105s did have a failure that took out the driver board (a late version amp and driver boards) several years back, not running hard but was on for a long time as background music, standing alone on a credenza. My bad anyway as it was original and due for a complete service. It took out several components on the driver board including the NLA stabistor and did damage to the NLA PC board along with other damage and it took a while to source components for the repair. I have seen many similar failures here on AK. Without a thorough post-mortem on the several MC210x failures it is hard to say how many were failed output transistors, drifted bias resistors, or could be attributed to the driver board stability concern.

It is my opinion that you’re in there, why wouldn’t you spend the relatively small amount of money to do the FSB update on a valuable unit that is over half a century old and has some unavailable components when the FSB is designed to improve stability? I see no reason to spend that much time and effort and ignore McIntosh’s recommendation there, JMO.

McIntosh Parts used to sell the kit, not sure that they do anymore, but it is basically a few resistors, transistors, AVID flying-saucer heatsinks, a couple of jumper-wires, you’re probably refreshing that board anyway. The big price items would be all of the new output transistors, the power-supply filter caps, replacing the bias resistors is far more tedious than rebuilding this board, … again just my opinion.
 
Again, thanks so much for the replies!

I completely agree with the idea of doing whatever is necessary to stabilize/improve the amp. I haven’t been able to locate specific info on the SB we’re discussing but will continue my search. Replacing the output transistors (and driver?) are on my list now.

It’s obvious that I need to get inside the amp to see which driver boards I have. That will happen sooner than later now that I know what has been shared here.

The discussion of transistors is a little beyond the realm of my experience, but I want to learn. I understand the concept of matching them, and basically what a curve tracer does. I don’t have access to one so not sure what to do about that. There seem to be a variety of options out there, some for applications that I will never need. I’m not building a professional tech bench, and I only plan to work on analog audio equipment, nothing digital or computer related.

Jeff: you mentioned you bought extra transistors so you could find enough that matched for your purpose. Just curious, how many more than needed did it take to get to the matched groups you eventually used? And I believe there are three on each heat sink, is that correct?
 
If you would like I could explain to you how to build a tester that will actually mean something to you, and you can sort them out yourself. These testers that run 10ma or so are on a 9 Volt battery or USB not for power devices. You would be wasting your time more or less, other than seeing if the device functions and the gain at 10 milliamps.
 
Jose Jimenez said:
If you would like I could explain to you how to build a tester that will actually mean something to you, and you can sort them out yourself. These testers that run 10ma or so are on a 9 Volt battery or USB not for power devices. You would be wasting your time more or less, other than seeing if the device functions and the gain at 10 milliamps.
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That would be much appreciated! I want to do this job once, and do it right…
 
SaturationPt said:
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Thanks for this link. I also really appreciate your comments on the importance of doing the upgrade and replacing transistors. This is exactly the kind of advice I was hoping to find here - I’ve expanded my expectations re: the scope of the project.

I still have studying to do, especially about which transistors are the best replacements for both driver boards and output stage. It’s new territory for me and your comments and suggestions have been very helpful!
 
teehas53 said:
Thanks for this link. I also really appreciate your comments on the importance of doing the upgrade and replacing transistors. This is exactly the kind of advice I was hoping to find here - I’ve expanded my expectations re: the scope of the project.

I still have studying to do, especially about which transistors are the best replacements for both driver boards and output stage. It’s new territory for me and your comments and suggestions have been very helpful!
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Do you have a small 12 Volt power supply, any digital Volt meters, a 500 or 1k Ohm potentiometer, wire, odd and ends resistors, preferably some less than 20 Ohm?
 
Jose Jimenez said:
Do you have a small 12 Volt power supply, any digital Volt meters, a 500 or 1k Ohm potentiometer, wire, odd and ends resistors, preferably some less than 20 Ohm?
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I have a digital multi-meter, a 12v power supply (DC I assume?) and plenty of wire. I don’t have the pot or many resistors floating around but will happily buy what’s needed if you give me values.
 
Jose Jimenez said:
If you would like I could explain to you how to build a tester that will actually mean something to you, and you can sort them out yourself. These testers that run 10ma or so are on a 9 Volt battery or USB not for power devices. You would be wasting your time more or less, other than seeing if the device functions and the gain at 10 milliamps.
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I would be interested in this also. Matching power transistors. It would really make a good thread. Not many of us have curve tracers for this work - or at least - I don't - LOL
Like you say, the small (atlas) testers and the like are really not suited for power devices.
 
Refugee said:
I would be interested in this also. Matching power transistors. It would really make a good thread. Not many of us have curve tracers for this work - or at least - I don't - LOL
Like you say, the small (atlas) testers and the like are really not suited for power devices.
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Yep, I was just thinking about a new thread on this topic this morning. Will do that soon.

For those of us who are not full-time techs, I wonder if there are any resources for just buying matched pairs/sets? There are a few places that do this with tubes, but apparently not transistors?
 
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