I had smoke coming out of the right side of a Mcintosh MC2000 amplifier.
No tube flashes and the left channel continue to play with both the blue meters all lighted (but not moving).
After the amp cooled downed, I opened it up and saw that resistor R36 has burnt (visible burnt marks, supposed to be 30.1kohm but it now measures 202kohms). The output stage fuse is ok.
I am wondering if there are tube schematic experts here who can help me figure out what other components that I may need to replace in addition to the burnt resistors? There are 2 npn transistors and I am thinking about the 2 output KT88 tubes.
Many thanks for your help
Cheers,
Marcus
No tube flashes and the left channel continue to play with both the blue meters all lighted (but not moving).
After the amp cooled downed, I opened it up and saw that resistor R36 has burnt (visible burnt marks, supposed to be 30.1kohm but it now measures 202kohms). The output stage fuse is ok.
I am wondering if there are tube schematic experts here who can help me figure out what other components that I may need to replace in addition to the burnt resistors? There are 2 npn transistors and I am thinking about the 2 output KT88 tubes.
Many thanks for your help
Cheers,
Marcus
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Yes, you're on the right track. Check the output tubes and both transistors, and the C12 and C15 capacitors.
You can use the diode test function on a dvm if necessary. Make sure the base-emitter and base collector junctions are good on both.
Hi , I agree and would also check the 320 and 60 volt sources. A power supply fault, particularly in the -60 supply could cause the problems.
Thanks for the advices.
It is out of warranty, the local agent will service it though. But this amplifier is really heavy to transport and I like to work with electronics.
The layout of the circuit board and connectors in this amp make it harder to check out the power supply voltages. And it is really heavy to tilt / flip etc.....
It is out of warranty, the local agent will service it though. But this amplifier is really heavy to transport and I like to work with electronics.
The layout of the circuit board and connectors in this amp make it harder to check out the power supply voltages. And it is really heavy to tilt / flip etc.....
Marcus
Q3 and Q4 are emitter followers used as buffers driving the Kt88s..The 31k resistor that burned and C15 is probably all that has to be replaced if the unit still plays..If the unit is not playing,you need to check for collector to emitter shorts or collector to base shorts on Q3 and Q4.
I have worked on three MC2000s in 11 years so they are good amps..The last one was simply an upgrade.
R34 is burnt also. look closely, it is dark in the middle. you had a tube arc. throw that tube away, check the transistor and replace the resistors and you should be fine.
Thanks again. I will replace R36, R34, C12, C15 and the 2 transistors. I may replace the 2 KT88s too. I didn't see the tube arc and they were fairly new SEDs.
I got to figure out how to ensure that the large capacitors are drained (find proper points to get my multimeter leads to probe)...even though I think McIntosh would have proper bleeder resistors etc. And then how to take out the output amplifier circuit board.
The MC2000 sounded great but haven't been too reliable for me...And it is really difficult to service due to its weight and circuit board layout...
Thanks again for all the help.
I got to figure out how to ensure that the large capacitors are drained (find proper points to get my multimeter leads to probe)...even though I think McIntosh would have proper bleeder resistors etc. And then how to take out the output amplifier circuit board.
The MC2000 sounded great but haven't been too reliable for me...And it is really difficult to service due to its weight and circuit board layout...
Thanks again for all the help.
Replace the tube(s)! In most cases a problem with one of the output tubes will be responsible for the damage you have seen.
Do check the transistors as well.
The reliability issues you are having may have something to do with the output tubes you are using. I used to repair certain models of ARC amp decades ago and they returned broken frequently because the owners refused to listen to me wrt the types and brands of output tubes that would be reliable in those amps. (Incessant tube rollers) I suspect you are having the same tube related problems with the 2000.
If you can get matched, burned in 6550 or KT88 this should reduce the problems significantly. Look for a vendor who burns in for 24hrs and then matches, this will catch most of the bad tubes.
Do check the transistors as well.
The reliability issues you are having may have something to do with the output tubes you are using. I used to repair certain models of ARC amp decades ago and they returned broken frequently because the owners refused to listen to me wrt the types and brands of output tubes that would be reliable in those amps. (Incessant tube rollers) I suspect you are having the same tube related problems with the 2000.
If you can get matched, burned in 6550 or KT88 this should reduce the problems significantly. Look for a vendor who burns in for 24hrs and then matches, this will catch most of the bad tubes.
I just tested the transistors. Q3 is shorted. Q4 measures ok.
R34 is burnt visually but measures ok. To be on the safe side, I would replace all the 2 transistors and the 2 resistors.
It is hard to probe C12 and C15. So I will test them when I figure out how to remove the PCB board.
I didn't see the tubes arc-ed, so I guess I will have to replace both of the KT88...It's a pity since they were fairly new.
Thanks for all the advice!
R34 is burnt visually but measures ok. To be on the safe side, I would replace all the 2 transistors and the 2 resistors.
It is hard to probe C12 and C15. So I will test them when I figure out how to remove the PCB board.
I didn't see the tubes arc-ed, so I guess I will have to replace both of the KT88...It's a pity since they were fairly new.
Thanks for all the advice!
It isn't unusual for new output tubes to arc a few times. Is there any warranty? Do you have another more robust amp you could install them in, to burn them in?
Probably not a bad idea just to purchase them burned in and matched, a number of vendors offer this service. Greatly reduces infant mortality and the resultant collateral damage as well as providing well matched pairs or quads.
You guys are right! There is a bad output tube. The local tube seller I brought the tubes from tested them; one of them is bad. I wasn't able to find a single SED Winged C to replace the bad one though. They are hard to find.
I am having difficulty to buy replacement parts from the local Singapore McIntosh resellers. They only do high-margin repairs and no parts selling. And my unit is purchased previously in Europe. I will probably buy parts from Digikey if I don't hear from McIntosh USA (they are probably my last option).
Sad to find this kind of support for a "limited edition" amplifier.
I am having difficulty to buy replacement parts from the local Singapore McIntosh resellers. They only do high-margin repairs and no parts selling. And my unit is purchased previously in Europe. I will probably buy parts from Digikey if I don't hear from McIntosh USA (they are probably my last option).
Sad to find this kind of support for a "limited edition" amplifier.
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