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MC 2105 repairs

wanders

wanders

Super Member
I've got a bad transistor; specifically Q2 on the input section circuit board. Now Q2,4,6 are a set for the right channel (Q1,3,5 same for the left). I've been in touch with McIntosh and they do stock authorized replacements. My instinct is to replace all six transistors.

Also, the leads of each of these transistors is mounted in a small tube which is on top of the pcb and connected to the trace side of the circuit board. (I believe this tube mount system is common on many for McIntosh components.) Even though I've got a solder vacuum (Hakko F500), it looks like replacing these is going to be challenging - lots of heat to remove and replace the solder in the tubes, lots of ways to make problems.

I'd appreciate thoughts on both issues: 1) replacing one transistor vs three vs six. 2) thoughts, techniques, experiences on de/re soldering these Mac mounting tubes.
 
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Only replace the fail component.
About the mounting tube, some will remove and direct solder to the PCB.

I will remove the solder as much as possible on both sides (the PCB back side and the tube).
Add some fresh solder on the tube side. (this will aid the heating process and flow of solder)
Heat one leg at at a time and gently lift the leg (not all the way, just a bit) with needle nose pliers.
Move on to the next leg
Repeat this process until 3 legs off the tube

You can only lift one leg a little bit at a time.
 
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@Gregory has an ingenious way to remove the legs from the tubes. He heats it a bit once the part is removed from the board. Then holding with pliers, gently tapping this on the bench and the tube stays in the jaws while the transistor drops. Check out good pics of the work on the thread https://audiokarma.org/forums/index.php?threads/mc2100-to-dos.844276/page-8

you have a great amp. Perhaps do the recommended updates while you are at it. This guy we linked to did a very thorough rehab. https://youtu.be/Q3cHIo43BQ0
 
Use a solder extractor as normal to remove the tubes from the PCB with the transistor, once you have the transistor out, it will still have the tubes on the legs, at this point its very easy to desolder them from the legs.
No need to over think it....
 
wanders said:
I've got a bad transistor; specifically Q2 on the input section circuit board. I've been in touch with McIntosh and they do stock authorized replacements. My instinct is to replace all six transistors.

Also, the leads of each of these transistors is mounted in a small tube which is on top of the pcb and connected to the trace side of the circuit board.
I'd appreciate thoughts on both issues: 1) replacing one transistor vs three vs six.
Click to expand...

Only Replace The Suspected / Known Defect. Component(s). IMO.

Many believe that if you replace one channels Transistor with a substitute manufacturer: part number that you should replace the other Channels original Transistor, as well.
Sounds much better than the Electrical Engineers Designed. If the bias support components are of good Quality and the Design was solid, tight , then their is no need.

cferry1
 
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If your getting “authorized” parts, just replace the bad one. They “authorize” the same crappy old parts that they were originally built with. Why buy lots of over priced antique parts to replace the antique parts in there.

Me? I upgrade to better modern parts, gain match them and replace them all when rebuilding.

I’ve removed the old transistors from the tubes.
Pain in the ass.
I decided McIntosh is full of hot air and pull those stupid f***ing tubes and go straight to board mounting. I have no idea WTF good those tubes were meant for.

I like old McIntosh gear but they did some really messed up stuff.
 
On a single-sided board I see no value in those tubes.

On a double-sided board they would be a good substitute for a plated-through hole to carry the connection to both sides of the board.
 
Believe it or not I'm making progress. Got the replacement transistor from McIntosh, installed it OK. Once that was done I found a shorted cap in the left channel OPT ground lead. Fixed that. Still have a power loss in the left side, but less than before. Now, I notice that the left channel, inside heat sink is getting very hot and cutting off the power supply. (s302/s303). I'm thinking that the likely culprit is one of the power output transistors. I removed both power output circuit boards to get at the transistors; however, I'm not able to get any useful readings in-circuit with a dmm. I'm if wondering if there is any way to get better, in-circuit readings, or if removing a heat sink is feasible. There is a lot of wiring and a several resistors involved. Might be worth investing in a different tester rather than messing with a heat sink.
 
I decided McIntosh is full of hot air and pull those stupid f***ing tubes and go straight to board mounting. I have no idea WTF good those tubes were meant for.

:rflmao:
 
Have you tried unplugging and swapping the driver boards to see if the hot heatsink follows the driver board? One of the easiest troubleshooting steps in the output stages of the 2105. Better still, run with just one driver board installed, use the right channel driver board on the left channel outputs and see if it still gets hot.
 
Oh, run with a scope also, look for HF that shouldn't be there. Did you check for DC offset on the output to the transformers?
 
SaturationPt said:
Have you tried unplugging and swapping the driver boards to see if the hot heatsink follows the driver board? One of the easiest troubleshooting steps in the output stages of the 2105. Better still, run with just one driver board installed, use the right channel driver board on the left channel outputs and see if it still gets hot.
Click to expand...

I've been chasing this for a little while and seem to be closing in. Earlier, I did swap the boards without leaning anything, but that was before the heat issue reared its ugly head. I can give that another shot. If the other side gets hot I don't need to worry about the power transistors.

SaturationPt said:
Oh, run with a scope also, look for HF that shouldn't be there. Did you check for DC offset on the output to the transformers?
Click to expand...

I've not used the scope for any dc analysis; only signal. I'll look into that.

thx.
 
Found a blown resistor: spec 22 ohms; read 5.5k ohms. Replaced it and everything seems ok; at least for now. I'll put it through some tests later.

Thanks,
 
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