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MC 2100 output failure question

Bigears

Bigears

Manager of my own day
Picked up a very clean but inop MC 2100 with a blown main fuse. I replaced the fuse and powered it up on my dim bulb and it was obvious that something was drawing huge current. I suspected a shorted output and after testing all 12, found one Motorola 32-070 shorted emitter to case. I tested all the driver resistors and diodes incircuit and found no failures.

My questions: is the MJ15003G a suitable replacement for the 32-070 and if so, do I go for one, all 3 on the amp board, or all 12? I do not have a way to test leakage and if these are failure prone and the MJ15003G is as good or better, I'll do them all.

Also, i notice the outputs do not have any mica insulators or thermal grease, and two of the 12 have a metal plate behind the output, is this correct?

Finally, the driver boards have some writing on them and I'm not sure what that is all about. This is only the second MAC I've worked on, so I don't know if this is factory or from a previous repair.

Thanks in advance for the expertise.

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Am in the process of doing some cleaning and detailing of an MC2105 that has been recapped (Nichicon Muse) and a new faceplate. Putting in new long-life lamps and new blue for the meters and the Deoxit on controls, etc. The driver boards have writing on them, but rather than what appears to be QC numbers, it is simply a hand-written six-digit number. All 12 output devices in my amp (same as the MC2100 other than meters, I believe) are mounted on what appears to be a steel or iron mount that it is about 1/16" thick, or so. As the amp is working well (make that "sounds wonderful), am not pulling the devices to add any thermal grease, etc. Not sure what I would do about the output devices, as I am fortunate in having a complete working set. Am guessing I would change the six on that side to remain as 'balanced' as possible. But, I would look at the curves for the originals (if available) and compare to the new device to see how close they are before deciding.
 
Wow. I am working on a similar 2100 issue for a friend. As soon as I started to power it up on the variac it was drawing huge current. I was told it had a bad rectifier bridge, but a new one did not fix the problem. On this unit all transistors were replaced so I am thinking there is a problem, there. I have all the old ones, and I think they do have grease on them. Also my driver boards have writing on them. I will have to compare to what I have. John
 
MX117.MC7150 said:
snip-- All 12 output devices in my amp (same as the MC2100 other than meters, I believe) are mounted on what appears to be a steel or iron mount that it is about 1/16" thick, or so. As the amp is working well (make that "sounds wonderful), am not pulling the devices to add any thermal grease, etc. Not sure what I would do about the output devices, as I am fortunate in having a complete working set. Am guessing I would change the six on that side to remain as 'balanced' as possible. But, I would look at the curves for the originals (if available) and compare to the new device to see how close they are before deciding.
Click to expand...

Thanks for your input. My main concern is what to do about the metal output spacers, which seem randomly used on my 2100 and only fitted to two outputs. No thermal grease or insulators is a first as well. I want to do a correct repair which to me means new mica insulators with T grease and at least 6 new inputs. FYI, there are no obvious signs of repair on this amp and not even the screw heads look molested. Still, I can't believe this is how it left the factory...perhaps the experts will chime in and enlighten us...
 
Many Mac heatsinks are 'hot', meaning they are electrically live. Note that your heatsinks are electrically insulated from the chassis. If there's a driver transistor on the same heatsink, its collector may be at a different potential, and thus it is necessary to insulate that one device from the others. When reassembling, the insulator MUST go back to the same transistor location that it came from.

All the output devices should match. The death of one means the replacement of them all in order for current sharing to work properly. But...I dug through my good 'pulls', and found a lone Motorola 32-070 which I'd be happy to send to you for a small donation to AK. PM me if you are interested.
 
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PM sent....

Just to be clear the insulator plate is matched to that specific output device and among that particular amp board? I did keep these matched and I have them arranged to go back on the original amp board but I'm not certain of the order on each amp board, but I believe is unimportant.

Thanks for the help.
 
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Bigears said:
My main concern is what to do about the metal output spacers, which seem randomly used on my 2100 and only fitted to two outputs. No thermal grease or insulators is a first as well. I want to do a correct repair which to me means new mica insulators with T grease and at least 6 new inputs. FYI, there are no obvious signs of repair on this amp and not even the screw heads look molested. Still, I can't believe this is how it left the factory...perhaps the experts will chime in and enlighten us...
Click to expand...
Looking at the heatsinks in the MC2105, only the top output devices mounted on the right side of the heatsink as you face the front of the amp have the thin metal spacer. The other ten (10) output devices do not.

You are now talking to the right person (EW) about your MC2100. While you've got it apart you might as well recap. Only another $30, or so, for good caps. And, you've just saved that much in output devices.
 
I've zero firsthand experience with the 2100, all I have is the schematic. So, with the understanding that schematics have errors, and manufacturers make production changes, here's a scat of the output and my take on it...

mc2100output.jpg


On the positive side, Q15, Q19, and Q23 all share the same collector potential (positive supply voltage of 35V), so IF the 2100 is using 'hot' heatsinks that are insulated from the chassis, an insulator on the transistors is not necessary.

On the negative side, Q17 and Q21 share the same collector potential (both collectors tied to the output line). Q25 (Q26 for opposite chan), on the other hand, has a .33 ohm 5W resistor in the collector (part of the overcurrent sense circuitry). So if I have this right, the only transistor that needs to have its collector isolated from the heatsink is Q25, hence you should have only one transistor with the insulator on it per channel.

Note that the insulator is not tied to a particular transistor (since the whole output stage uses the same transistors), but is tied to the location of that transistor on the heatsink. Just so there's no confusion here. The service manual identifies the location of each of these output devices on the heatsinks, so finding the proper location for the insulated device should be a simple matter.

Leaving the insulator out would not cause any catastrophic failure, but the bias would not function properly and the current sense circuitry would be crippled.
 
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MX117.MC7150: your description of the location of the insulator is exactly where I found them on my 2100, top location only, one on each channel, inside amp board (not sure on that but I have them positioned by amp board so this is certain for reassembly).

Also, I planned to recap this 37 year old amp before putting it back into service, but wanted to square away my outputs before placing an order. Thanks to Glen, the output transistor issue has been solved.

Echowars: thanks for clarifiying the importance of the exact location per channel of these insulators. I took digital images beforehand but only have one channel, now it is clear where they go. I do have the SM, so things should progress smoothly and accurately prior to dim bulb power up.

I believe the bias is regulated by the resistors on the driver board so there is no setting, correct?

Thanks to everyone for help getting this one up and running again.
 
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There is no bias or even offset adjust on this amp, but measurements of both should be made to see if they are in the ballpark.
 
I guess I can measure DC before the auto formers, and bias at the emitter resistor and ground, but I'm not sure what is acceptable for the bias.
 
You can measure DC at the speaker terminals if you like. There's nothing about an autoformer that will block DC. As far as the bias, the schematic has the voltages right on it (100mV from the emitter of the Q23 to the collector of Q25). ;)
 
I retested all the outputs and found another that isn't right. Once again, I'm testing with the diode function of my Fluke. Anyway, I get a diode drop .525, from E- to B+, but from E+ to B- it shows 1.8 and I get 10.4K ohms, so I think I'm gonna need another Motorola 32-070 output.

Anyone holding?
 
Have you tried McIntosh Parts? They have an 800 number and stock an amazing number and array of vintage gear parts.
 
That blows.

You will NOT get an original Motorola part from Mac either. Sure, you can call, but you won't get anywhere. No one has had these parts for years (and the last time I ordered a transistor from McIntosh, they sent me an NTE).

I'll still send the one transistor that I have (since you donated to AK for it and it can't be refunded), but if you can't find a second one to replace the other bad one you found, I'd have to recommend that you buy a set of MJ15003G's from Mouser or Digikey.
 
Since I'm only one original Motorola output away from a full set, I may wait it out for a bit and see what surfaces. I'll probably order the caps and get that part of the project underway.

Where else have you seen the Motorola 32-070 output used? Anything common?
 
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Spent some time on Digikey getting my 2100 cap order started and discovered that they are out of MJ15003G's until March 22 2010! Mouser is quoting 22 weeks!

Anyone have ideas on another source MJ15003G's or just one original Motorola 32-070? Or should my 2100 go into hibernation for 4 months?
 
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