MC2105 blows fuses on powerup - diyAudio
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Old 28th June 2008, 11:12 PM   #1
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Default MC2105 blows fuses on powerup

I have a McIntosh MC2105, and it blows its main power fuse immediately on powerup. This is a new development. The first time it happened, the amp turned on for a few seconds and then blew the fuse. Now, the front lights flash, there's a tiny buzz from the transformer, and the fuse blows. (this all happens instantaneously)

Any ideas?

Thanks,
philosophy

PS: HELLO, this is my first post!
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Old 29th June 2008, 02:14 PM   #2
djk is offline djk
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You may need a new transformer.

The panel lights on the MC2105 are not fused. If they short out, the lighting winding will overheat, develop a shorted turn, and the transformer will be ruined (been there, done that).

To know for sure, you need to check the supply and outputs for shorts too.
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Old 29th June 2008, 03:40 PM   #3
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Thanks for the advice. Just out of curiosity, if one or both of the BIG caps (between the heat sinks) went bad, would that cause a similar situation? I ask because that was suggested to me in another conversation...
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Old 29th June 2008, 08:35 PM   #4
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I've poked around the circuits with a tester, and no faults are present in the lights. The main problem area appears to be under the big can caps where every component reads as shorted. When placed across the speaker terminals, I also get a short reading. I do not get any shorts from the power transistor boards. Maybe my transformer is OK? Could you tell me which leads would be the telltale leads for checking the transformer for damage?

When testing resistance on the big caps, the rear one reads high resistance and keeps building, the front one reads at zero. Would you know where to get caps like this? Or even a transformer if I indeed need one?

thanks again!
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Old 29th June 2008, 09:10 PM   #5
kevinkr is offline kevinkr  United States
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Usual things to look at first are the rectifiers, electrolytics and output transistors. You need a meter capable of measuring low resistance circuitry, and the charging current drawn by the electrolytics when you attempt to measure whether they are shorted or not can be very misleading as the time required to get a valid reading can be long.

Your symptoms are indicative of a dead short across one of the high current secondaries of the power transformer. Bridge rectifiers in my experience tend to be less reliable than electrolytics - I have had far more rectifier than cap failures.

Sounds like you do not have a lot of experience troubleshooting nor the required tools to do the job. I'd recommend contacting McIntosh up in Binghamton and see what it would cost to get them to service it.

http://www.mcintoshlabs.com/
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Old 30th June 2008, 11:42 AM   #6
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I didn't say that I was going to fix it myself, I wouldn't attempt that.

I would, however, like to know whether it is worth fixing, and have some idea of the problem before I lug this monster to a repair shop for the $75 to $90 diagnosis. (I think I'll skip the shipment to Binghamton, there are plenty of McIntosh capable repair shops in NYC.)
All I'm trying to do is get an idea of just how terminal the situation is. Just the shipping and diagnosis could eat up a large % of the value of the amp. Then the parts, which when added to the repair total could be more than it would cost to get another MC2105.

It is my very lack of experience which brought me to ask you some advice. If already knew what to do, I wouldn't have asked.
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Old 27th September 2009, 09:59 AM   #7
nuryev is offline nuryev  Australia
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Default did you succeed with repair

hi, I am having similar problem with my PM66KI amplifier and was curious as to what was the core fault, bridge rectifier, transformer or capacitors?
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Old 27th September 2009, 11:20 AM   #8
djk is offline djk
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Most probable cause is blown outputs, then the bridge.

Since you have no experience in this area (otherwise you would have not asked this question), I must point out that it takes a lot of time and study to learn to repair amplifiers.
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Old 27th September 2009, 02:26 PM   #9
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Hi Philosophy. I have had lots of experience "under the hood" of a 2100, which is the industrial-looking, non-meter version of the 2105. A few thoughts:
-Is the amp worth you investing time and/or money to fix the problem? Yes, 2105s in excellent condition are currently commanding close to $1000.
- I suspect the main power capacitors - those beer-can size capacitors - may be fried. Very common for old electrolytics. They are screw terminal parts, so installation only requires a slotted screwdriver. I know you don't plan to do it yourself, but the technician time for changing these out is minimal given what is needed.
- The rectifier is mounted to the rails directly under these big caps and that could be burned as well. If you don't have experience testing diodes/rectifiers, then I would suggest leaving that to an experienced tech.
- Finally, double check the MOV's that are mounted between the fuse housing and the input board. MOVs are the brown flat discs that look like ceramic disc capacitors. While burned out MOVs shouldn't cause the fuse to blow, check it anyway.

Good luck.
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Old 28th September 2009, 05:16 AM   #10
nuryev is offline nuryev  Australia
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Default hi

thanks for the reply - I have found this out the diy way when I started modifying cd players so and find the directed research and learning all part of the fun. So any directions would be appreciated.
BTW, i have already started by testing the torroidal transformer out of the amp (fuses are still blowing), so I suspect primary wiring may have a short.
resistance across secondaries =0.5 ohm and across primary=7.8 ohm, is this normal?
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