The Dude Posted July 29, 2014 Share Posted July 29, 2014 All right here's what I am working on.. A couple of weeks ago I purchased a non working McIntosh MC2100. Through some trouble shooting guide I felt I narrowed it down to one of the driver boards. So I did the update per McIntosh's recommendations. Now when I originally figured it was a particular board. The bulb would be brighter when it was in. Now the bulb is the same rather it's in or out. So that would lead me to believe the board is fixed. However when I plugged it directly into the wall socket the power transformer squealed a little then the fuse pop. So I went and got another fuse , now I thought the first one was a slow blow, but it may have not. Now the new one is a slow blow. So I removed the board I thought was bad and plugged it directly into the wall which I still had the squealing so I instantly unplugged it. Put the other board back in. Plugged the light bulb variac back in, and just let it run for a couple of mins. Then I tried it directly into the wall and this time no squealing and the fuse didn't pop. Now I didn't let it run for to long. So my question is would it be all right or recommended that I plug in the variac and let it run for a while. Or is it possible there is something wrong and I just didn't let run long enough to find out. Quote Link to comment Share on other sites More sharing options...
djk Posted July 29, 2014 Share Posted July 29, 2014 Run it for days with a low audio signal and a 60W bulb. 1 Quote Link to comment Share on other sites More sharing options...
The Dude Posted July 29, 2014 Author Share Posted July 29, 2014 I will give that a try, should I have speakers hooked up for a load or would there be to much fear of it causing damage. Quote Link to comment Share on other sites More sharing options...
artto Posted July 29, 2014 Share Posted July 29, 2014 The 2100 is a solid state amp. Normally you would use a variac on a valve amp that hasn't been used for a while. The older the amp & capacitors the more important it is to bring the voltage up slowly so the capacitors can "re-form". Generally solid state amps usually don't require this. Another little safety "trick" is to make yourself a series connected light bulb limiter. Just mount a standard light bulb socket on a piece of wood & wire one side of lamp cord to it (the other side goes straight through). Use a 15 to 25 watt incandescent bulb. As you turn up the voltage with a variac, the bulb should not glow at low voltage. As you turn the voltage up and get to around 80% to 90% of maximum voltage the light bulb will begin to glow dimly. If something is wrong with the amp the light bulb will act as fuse and blow before the amp does. Quote Link to comment Share on other sites More sharing options...
The Dude Posted July 29, 2014 Author Share Posted July 29, 2014 connected light bulb limiter Thats kind of what I am doing but with out the variac I might have misinterpreted my set up. Quote Link to comment Share on other sites More sharing options...
babadono Posted July 29, 2014 Share Posted July 29, 2014 Huh? What is being done here? Would someone explain in a little more detail, please? A schematic would be a definite plus. Quote Link to comment Share on other sites More sharing options...
The Dude Posted July 29, 2014 Author Share Posted July 29, 2014 Huh? What is being done here? Would someone explain in a little more detail, please? A schematic would be a definite plus. I am trying to diagnosis a power transformer hum, along with making sure that nothing else goes up in smoke. I guess I thought the inline light bulb trick would help keep that from happening. MC2100 Service.pdf Quote Link to comment Share on other sites More sharing options...
The Dude Posted July 30, 2014 Author Share Posted July 30, 2014 OK moving in the right direction, I now know how a dim bulb should look just that dim. So here is the new run down, I have narrowed down the hum to the other driver board We'll call it the right side (the opposite to which I originally thought was the problem). But for some reason the bulb was still a little bright with the left side board. So I got to poking around and found out that the wire wound resistors that I replaced had the wrong value. Since I knew the left side board didn't hum I removed the right resistor from the right side board. Installed that resistor into the left side board and whala. Very dim bulb, now what about the right side board, I am not sure yet so I went ahead replaced the transistor and now need to source another good resistor. Quote Link to comment Share on other sites More sharing options...
babadono Posted July 30, 2014 Share Posted July 30, 2014 So the light bulb is in series with the 120VAC going into the amp? Even if the input was a direct short wouldn't the light bulb still work? Please I want to learn. Quote Link to comment Share on other sites More sharing options...
The Dude Posted July 30, 2014 Author Share Posted July 30, 2014 Yes the light bulb is in series with the amp and outlet. When there is a short the light shines bright. When there is no short the bulb is extremely dim almost hard to tell if it's on, but is on. I don't know more then that, maybe others will chime in. I know it acts as a fuse as well. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 7, 2014 Author Share Posted August 7, 2014 (edited) I will say this light bulb idea is a great way to pin point what the problem is. I will say I read a lot about people rebuilding something completely and hope that it works. I have tried this before and I crashed hard with the project. This time with the light bulb and replacing things in steps helped me see what 2 completely different problems look like. Bottom line I am making great progress on this project, and have learned a thing or 2. I am down to a scratchy channel in this amp, and rebuilding the power supply which will be next. Once that's done and everything seems to be working ok, maybe even after I get an oscilloscope to see exactly what might be going on. I will then do some mods like by passing the coupling caps and things like that. I will all ways post my updates here on this project so maybe it will help others learn what I have. Edited August 7, 2014 by duder1982 Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 11, 2014 Author Share Posted August 11, 2014 Ok if anyone knows the schematic well "cough DJK cough" I could use some help. I have a scratchyness going on in the right channel when I first fire it up, but it goes away. Then in the left channel when I try to turn it up it comes in but is real distorted. Now I thought I would try to rebuild the input board as I had the caps, now when I plug in the inline light bulb it doesn't dim down. I replaced the caps and resistors according to the schematic in the audiokarma database, I guess it could be something I did incorrectly or was could it be the changes in the schematic aren't correct. Some help would be greatly appreciated. Thanks Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 11, 2014 Author Share Posted August 11, 2014 I'd start with a complete voltage check against the schematic. Maybe you already did that? That's the definitive starting point. No I haven't, is it good to start at the power supply and work my way through the schematic. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 11, 2014 Author Share Posted August 11, 2014 Another thing that was brought to my attention is that the schematic in the Audiokarma data base may be for newer units. So if I replaced resistors and caps based off of that they could be wrong for my unit. The only caps and resistors that should be replaced are ones specific to a technical service bulletin. Luckily I have the original schematic so I can go back and double check which ones should be which. Then I will start with voltages, and go from there. Quote Link to comment Share on other sites More sharing options...
DrWho Posted August 12, 2014 Share Posted August 12, 2014 A bad cap in the audio path will be scratchy if there is a DC bias across it. It is often much worse while it is charging. Do the resistors really go bad in these old amps? I understand the aging of a cap, but wirewound resistors are just metal....and it's encapsulated. 1 Quote Link to comment Share on other sites More sharing options...
babadono Posted August 12, 2014 Share Posted August 12, 2014 (edited) So the light bulb was dim before the last thing you did, now it is brighter? Double check the last changes you made, we are all human, we all make errors. The light bulb being brighter is indicative of the input drawing too much juice(Thank you Dr. Obvious). You have a short or partial short somewhere. Is it relatively easy to dis connect the power supply? Looks like the left and right "power amp" boards unplug fairly easily. What happens when they're unplugged? Edited August 12, 2014 by babadono Quote Link to comment Share on other sites More sharing options...
babadono Posted August 12, 2014 Share Posted August 12, 2014 Dirty contacts that are capacitvely coupling instead of making good contact MIGHT be contributing to your problems. Dirty volume pots also. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 12, 2014 Author Share Posted August 12, 2014 Do the resistors really go bad in these old amps From what I have been told and little testing to find this true, that carbon comps drift. So some have been replaced. Now you mentioned a bad cap, there are a couple of electrolytic that still need to be replaced on the input board I can try. But could a poor quality cap do the same. As far as my problem goes, here is what I came up with last night. I re installed the original values of caps and resistors on the audio input board, but the light was still shinning bright. So I went back to the output boards, and checked individually, which came back to the last board I updated. I don't know what is wrong as it wasn't a problem before I did the input board. Unless something in the input board caused something in the output board fail. I will have to do some digging on this. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 12, 2014 Author Share Posted August 12, 2014 Is it relatively easy to dis connect the power supply I will have to check. Dirty contacts that are capacitvely coupling instead of making good contact MIGHT be contributing to your problems. Dirty volume pots also I have cleaned them numerous times, though I guess it wouldn't hurt to disconnect them and ohm check them. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 12, 2014 Author Share Posted August 12, 2014 Thanks for the ideas folks. Quote Link to comment Share on other sites More sharing options...
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