Mc2 Mc450 help

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I’m lookin for a bit of help with an Mc2 Mc450 amplifier that won’t power up.
It was working fine until I cleaned it inside with a can of compressed air.
Now it won’t power up, no lights or anything. The fuses are ok and power is reaching the circuit board.

I’ve very little electronics knowledge, so would appreciate if anyone can point me in the right direction. I have the schematic, but don’t really know how to test if each component is ok.

Thanks
Matt
 
The amp is digitally controlled, so none of the functions like mute and gain operate unless the amp is on.
The amp was off while cleaning.

I think I’ve found the problem, there’s a 2k7 resistor directly after where the neutral starts on the circuit board. It’s faulty, reading around 0.6ohm.
 
Thanks for the reply.

I brought the amplifier to an electronics repair and they managed to fix the amp, although it took them some time as each time they found a fault and repaired it, they would find another fault.

I don’t actually know what was broken in the end, but I suspect that the new fans I installed caused the problem due to the metal logo sticker in the centre of the fan touching positive and negative sides of the heat sink. I could see marks there on the heat sink where it had melted slightly from the arching.

Cheers
Matt
 
Did you resolve the issue? I've worked on many MC series amps so am quite familiar with them.
#H713
Hi,
I need help with a MC450 with an output of -108 / -165 mV DC offset. For analog input pcb 6023, I found a circuit diagram for 6025 but this is different from 6023.
Any idea where to adjust the DC offset? Or should I not deal with it?
THX! D '
 
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I don't have schematics for the 6023 PCB- just the 6025. Can you post a picture of the board? It may be very similar (or the same) as the 6025. Unfortunately, there were multiple revisions of some of the PCBs in these amps. Not all of them are in the service manual. This is common with amp manufacturers (Crest is AWFUL about this), but it does make service work a lot harder.

As far as I know, there is no adjustment for DC offset on these amplifiers- the op-amp takes care of that for you. 100 mV seems way high for one of these. Does it otherwise pass audio acceptably? If you can measure THD, what does it look like for this amp? You should be getting less than .01% at 1 kHz into a 4 ohm load.

Also, how much of the service manual do you have for this? Their service manual is fairly lousy, but it's better than nothing.


FWIW, the most common failures I've seen on these amps are the 5532s on the input board (usually it's the first one in the signal path), the PM7524 multiplying DAC and the Toshiba or NEC IO expander chips on the processor board.

This does not sound like a fault on the processor board to me.


Before anything else, check the power rails on the op-amps.
 
That looks to be the same revision as what I got from them a few years ago.


Look at the output of the 5532 (IC1A) to see what the DC offset is like there. I've never had to adjust VR5, though it does certainly look like a DC offset control. Interesting that the PCB6023 does not have it.

Essentially what I'm trying to determine here is whether the DC offset is caused by a maladjustment or a failed part.
 
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