Mc2100 question

quartersaw

quartersaw

AK Subscriber
Subscriber
My Mc2100 suddenly developed a loud hum in one channel, and then suddenly, the fuse blew. Using my Variac, (with 35 volts output from the variac) the amp still overheated the bridge rectifier with the two output cards removed. (Those big diodes were HOT!) I replaced the four large, press fit Motorola diodes, with NOS parts. The other two diodes, are OK.
I tested all of the power transistors, and they all appear to be OK. (Input board transistors, as well) The only obvious thing that I see, is one of the heat sink mounts is broken. In looking at the broken mount, it appears to be insulated from the chassis.(?) Might this broken mount, be part of the issue? Are the heat sinks actually insulated, from the rest of the amp?
I recapped the amp a few years ago, and I replaced the 39000uF caps, last year, after the onset of this problem, thinking that might be the culprit. No change.
I'm a bit perplexed, at the defective parts that I am NOT finding. I'm taking my time with this going forward, but if anyone could chime in as to where I should go with this, it would be much appreciated! :-)
 
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The heatsinks are live, so if the insulators fail after many years then yes you can have one of the rails dead short to ground...
 
kevzep said:
The heatsinks are live, so if the insulators fail after many years then yes you can have one of the rails dead short to ground...
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Thanks, Kev!
I just called Mcintosh, and they inform me that the heat sink brackets are no longer available. They did have the Bias Diodes, which I haven't been able to find, anywhere else. I'll have to improvise some sort of plastic mount, for the heat sink.
 
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quartersaw said:
Thanks, Kev!
I just called Mcintosh, and they inform me that the heat sink brackets are no longer available. They did have the Bias Diodes, which I haven't been able to find, anywhere else. I'll have to improvise some sort of plastic mount, for the heat sink.
Click to expand...
Let me check my stash. I had a couple of brackets break and NJ AK'er sent me some spares. No promises but let me see if I can locate to send you -- found these 4. Seems like the lines are more cosmetic that structural.


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quartersaw said:
Thanks, Kev!
I just called Mcintosh, and they inform me that the heat sink brackets are no longer available. They did have the Bias Diodes, which I haven't been able to find, anywhere else. I'll have to improvise some sort of plastic mount, for the heat sink.
Click to expand...
I can't remember exactly what I did, but I had to improvise to insulate the heatsinks on the MC250 I was working on....
Looks like you have a solution though!!
 
I measured the individual components, in each of the heat sink assemblies yesterday. I found one open .33 ohm resistor. The others are measured by subtracting the reading on the display of my Multi tester, when I short out the two probes. All of the other resistors are within spec, as far as I can tell. It seems a bit odd, that the one component is dead, but everything else seems to be OK.
 
These brackets are perfect candidates for 3d printing. Something I have not delved into but have a friend of a friend that could fabricate similar parts.
 
I was not observant, as to where the two Mica insulators are placed in the power transistor banks. It appears that the insulators should be under the transistors closest to the two thermal cutouts, on the two inner heat sinks. Is my thinking correct, about this?
 
quartersaw said:
I was not observant, as to where the two Mica insulators are placed in the power transistor banks. It appears that the insulators should be under the transistors closest to the two thermal cutouts, on the two inner heat sinks. Is my thinking correct, about this?
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Nevermind. I found the info in another Mc2100 thread.
Got the amp running today, and if I close my eyes, it sounds like my departed Mc2300's!
 
I have one weird issue with the amp. The speakers are hooked up to the 16 ohm terminals. I couldn't remember any reason, why I did that. I hooked the speakers up to the 8 ohm terminals, and the right channel sounds distorted, and the power output is low. I thought 'oh, ****'.
I hooked the speakers back up to the 16 ohm taps, and all is well. I looked at the underside of the amp with a flashlight, and everything looked OK. Nothing shorted out.
Driving the amp HARD, I get temps on the transistors of 125ºF.-130ºF. via my cheapo Harbor Freight Laser Thermometer. The transformers are very cool to the touch.
 
Pretty sure you have to strap the 16 ohm taps and hook your speakers to the 16 ohm taps when switched to mono for 8 ohm speakers. I couldn't down load the manual off Berner's site but if my memory serves me.....
 
tuffstuff1 said:
Pretty sure you have to strap the 16 ohm taps and hook your speakers to the 16 ohm taps when switched to mono for 8 ohm speakers. I couldn't down load the manual off Berner's site but if my memory serves me.....
Click to expand...
I'm running the amp in Stereo. I rewired my stereo/mono switch. The wiring looked pretty frayed underneath...
 
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