MC2105 blows fuses on powerup

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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?:confused:

Thanks,
philosophy

PS: HELLO, this is my first post!:D
 
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.
 
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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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/
 
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. ;)
 
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.
 
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?
 
PM66 blowing caps

Hi

I have replaced transistors and isolated the fault to left channel. Now amp has stopped blowing fuses. Also found that R760 now overheats at powerup but more importantly, the recommended fuse on PM66KI pcb is 1.6Amp but 2.6Amp slow blow is more suited - thanks to Rowemeister for this :).
 
hi andrew

Hi

when I first opened the amp I did notice a 2.6Amp fuse and removed it thinking the same and with safety concerns. Replaced this with 1.6 and blew a box of 10 replacements until Rowemeister alerted me to upgrades in PM66KI that would make 2.6amp replacement suitable. But i take your point.

Any advice on the following:
1) testing the 3 legged resistors
2) relay not switching on although resistor tested ok
3) R760 smoking at power up?

Thanks for the quick response - I am not formally trained in electronics, all my learning is on this website and started with successfully modifying cd63 67 and CD4000.
 
3 legged resistors are 2 resistors in one package. The centre pin is the common pin. Simply measure between either end, and centre. They are usually very low resistance and if theyve failed, theyll have failed open.

Relay not switching suggests DC at the output, IE the protection circuit is working.
 
Hi Jaycee, thanks for the good news :). I am now looking at transistors on pcb, specifically Q754, Q756, Q758 and Q760. Although close inspection does not indicate any obvious physical signs of stress at these components, maybe a dvm test off the pcb will provide more information. Resistors and caps in this area all seem to be good.
Thanks again for your response.
 
My 2105 had similar problem a while(~10yrs) ago. Problem ended up being output transistors on one of the channel. I had to replace output transistors as well as resistors on that channel. One good design about Mcintosh amps are that each channels were on a separate PCB/card as well as transformer that seems way over designed/built(or should I say built like a tank) that can withstand most problems caused by output transistors.

I would start from isolating the problem to one of the channel, I think I was able to power on the amp with one of the channel output card off.
 
I forgot to add that when I opened my 2105 for trouble shooting, I found a few spots on two different wires that insulation started to fall apart. I'm not sure if that had to do with transistors going bad or not but I would look for those as well and re-insulate the wires when you have the amp open. I did not replace the wires, I simply used heat shrinking tube/tapes over the area insulation start to fall apart and started to expose the metal inside.
 
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