MC2100 positive output failure

[Bob M.]

Probably very early MC2100. No fuse, open power cord. Replaced electrolytic caps on preamp board. Others tested fine although the ones on the driver cards look horrible and will be replaced. Powered up on Variac and it drew over 2A at just 20VAC. Pulled each TO-3 transistor one at a time and found Q20 and Q22 shorted in all directions. Installed some used but good MJ15015 transistors. Now powers up fine. CH1 works properly; CH2 has no positive signal when loaded with 8 ohms. Does swing positive with no load. The voltages coming out of the driver card don't match CH1; positive drive is at +2V, negative drive is at -36V (same as supply). I swapped the two driver cards; no change, so I'm thinking it's a problem on the positive CH2 heat sink (furthest from the control panel) since the output goes negative just fine.

I've got six MJ15003Gs plus some 0.33 and 0.56 ohm resistors on order. Obviously some of those resistors are still good, since the two shorted transistors were loading the +/- 35V supplies quite well. I'm assuming the no-load performance is just the drivers feeding the output.

Is my thinking at fault? Would love some thoughts from others who had similar issues.

Bob M.

 

[c_dk]

Welcome to AK...

not enough information to troubleshoot but......there are some basics that have been garnered over the decades

The MJ15003gs have a little bit more gain than the original so you must look for oscillation after the unit is assembled.

There was no modification for the fact that the .33 and .56 resistors are inductive......they must be inductive and as far as I know the only source is the factory.

Have the insulated heatsink brackets survived.....the sinks are "hot" and it is not uncommon for these amps that often saw rough service to have a issue with the sink shorting to the chassis or cage. I have seen the supply wires melt.....

I would rebuild the driver cards with the known update as highlighted in numerous MC2300 threads.

 

[Bob M.]

Thanks. I did order wirewound resistors to replace the (possibly open) originals. The heat sinks seem to be quite insulated at the moment. No continuity from any of them (using the mounting screws in the plastic ell brackets) to ground, +35V, or -35V, which seems odd. Perhaps the insulators are doing their job, or perhaps those 0.33 and 0.56 ohm resistors are more open than I suspected. I was considering inserting silicone pads under each transistor. I also have a feeling that the cage may have contacted one or more heat sinks in the past; it was NOT screwed down when I got it. In fact the bottom cover was missing all its screws and feet.

I was more concerned with the fact that with the shorted Q20 and Q22, this basically put a short (through the 0.33 ohm resistors) across the +35V and -35V rails, hence the very high AC current at low voltages on the Variac. After installing non-shorted transistors, that problem went away, but now I have no positive-going output signal on CH2. CH1 produces a sinusoidal output as expected, in fact it had no problem making 100 watts. CH2 will produce sinusoidal output with no load. That tells me that the 0.33 ohm resistors in series with Q20 and Q22 should be good, so there should be a pull-up source on the output line.

I'm sure there are tons of modifications/revisions that should be done if the amp is to be brought up to the latest factory spec, but at this point the owner just wants it working enough to use. The amp looks like it sat under a bar. It was covered in sticky residue. The chrome plating on the top of the chassis is badly pitted and rusted. This is not a ground-up restoration.

You mentioned "known updates as highlighted in numerous MC2300 threads." This is an MC2100. Are the mods the same?

Bob M.

 

[jlovda]

I tried fixing one for a friend and gave up after destroying too many replacement parts.

 

[c_dk]

Only the one transitor is isolated from the heatsink, negative to put the .56 in the circuit. All the others are in contact with the sink.

The driver boards for the two amps are interchangeable, doing the upgrade to the more stable driver transistors was optional for the MC2100s back in the clinic days.....it was mandatory for the MC2300s.

Do you have a positive supply voltage to the positive heatsink for the problem channel? I have seen these amps pass sinewaves with open wirewounds and open transistors.

Have you ruled out the input board transistors or the switches? You have ruled out the driver boards, found failed outputs but could have overlooked the inputs. If you heat the wire receiving tube you can pull out the tinned wire and transplant it to the other channel.

 

[Bob M.]

Thanks to all who've responded. I've checked the audio signal at the input to both driver boards (pin 1) and both are nice clean sine waves.

I've got some thin silicone pads that I was considering installing under all the TO-3 transistors, to completely insulate them from the heat sinks. I don't like the fact that the heat sink cover contacts the heat sinks. I realize that heat conduction is better without any insulator but the risk of shorting out a supply doesn't leave me with a nice warm fuzzy feeling.

Both supply voltages are making it to the heat sinks since I have a full-height sine wave on CH2 when no load is applied. I'm also thinking that the driver on the positive heat sink is passing the signal, but I measuref them; there IS +35V on the collectors of the positive heat sink xstrs. I know all of the supply voltages are present on the two 15-pin driver board connectors.

Previously I wiggled each heat sink individually out of the chassis. Is it any easier if I pull both out AND dismount the driver card socket from the chassis, just to be able to manipulate both to get to the back (wired) sides of the heat sinks, or isn't that necessary or recommended?

Bob M.

 

[Bob M.]

I got around to working on the amp again this morning. Found it easy to unsolder 5 wires and remove the entire CH2 positive heat sink from the chassis. All three resistors were open. I replaced them all along with all three transistors. Put the heat sink back in and now have to do the same to the negetive heat sink, although I suspect it's in better shape, but since I've got the parts anyway I might as well do it.

Hopefully the amp will work better when I finish.

Bob M.

 

[Bob M.]

I did the negative heat sink this afternoon. It had two open resistors but I replaced them all as well as the three transistors. Surprisingly, the amp powered up (on the Variac) just fine and isn't drawing very much current, maybe 1/4A. The good thing is that now CH2 is making at least 25VAC into an 8 ohm load with minimal distortion. I still need to check both channels for output power, distortion, frequency response, and noise, and hook a scope to it and make sure nothing is oscillating. Then I can put the covers on, including wrapping some electrical tape around the mounting edges of the top cover to prevent it contacting any of the heat sinks. There's very little clearance there; in fact the heat sinks touch the mounting edges when I lower the cover.

Fun yet expensive repair. Thanks all for reading and commenting.

Bob M.

 

[kevzep]

Nice work Bob!! Good to see another one saved from the dumpster....

 

[Bob M.]

Thanks. It's not "McIntosh Quality" but it works. The owner didn't want it restored, he just wanted to use it. I'm sure it will sit on a shelf for another 10 years. The chrome plated chassis is a mess: dirty, rusted, and pitted. Without taking everything off of it, it was not possible to even attempt to clean it except for whatever the rag would wipe off. I don't think this unit was destined for the dumpster but I'm happy with the results.

Bob M.