Mc Intosh amp, unfriendly problem - diyAudio
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Old 28th May 2009, 01:24 PM   #1
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Default Mc Intosh amp, unfriendly problem

Anyone worked on Mc Intosh?

I've got a mac MC431 here that goes into protect.

No Power supply or output mosfets shorted.

Rectifiers test ok.

Replaced a 3 amp diode that runs from 12 volt ground to heatsink case that was dead shorted, along with a burned trace on it's cathode. This has a dual power supply (one per channel)

I can get the power supplies to come up if I isolate the rectifier junctions from the board.

I was suspicious of the STK paks in the driver circuit (STK 350-000), so I reconnected the rectifiers and isolated the STK's power pins but it still goes into protect.

Mac hasn't gotten back to me yet about a scheme. not that I expect help from them.
I didn't see much on mac on this forum.

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Old 28th May 2009, 03:16 PM   #2
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I have a few here, I would be checking there Power guard circuitry as well as it likes to cause this sort of trouble. It's supposed to roll back the amp for a soft clip. I have heard nothing but wonderful things about this function...LMK if you are able to actually get a print out of them, I would like a look see if possible...THX...
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Old 28th May 2009, 03:47 PM   #3
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Hmmm, power guard circuit ?!

I'm unaware of that.

What does it do, sense high current from the outputs and reduce the duty cycle of the power supply?

I'd like to know more about it.

Haven't heard a thing from Mac. but I will share info with you.
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Old 28th May 2009, 03:51 PM   #4
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Usually only the caps cause the problem. Check for liquid around any caps, usually the power input 12VDC (tall/brown) caps and the short/fat 10uF caps are already gone bad.

Replace all small/fat 10uF caps, some are around power supply section, some are behind the STK, some are on the corner of the input/loudspeaker terminal PCB.
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Old 28th May 2009, 04:21 PM   #5
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Is any channel producing DC on the speaker terminals when it attempts to power up?
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Old 28th May 2009, 06:10 PM   #6
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Yes, I just measured aprox. 7.2vdc out of speaker ch. 1 and 2

Ch. 3 and 4 had about 1.2v
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Old 28th May 2009, 06:17 PM   #7
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Do you have symmetrical power supply voltages for the regulated and rail voltages (and whatever supply the voltage amplifier IC is using)?
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Old 28th May 2009, 07:03 PM   #8
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No, power supply won't even come up with the rectifiers connected to rest of the circuit.

I connected the rec's to make the speaker voltage tests.

I should mention that the 7.2 volts from spkr 1 & 2 outs were momentary, the 1.2v out of 3 & 4 was constant.

I did measure the rails when the rec's were out;
one PS had + 51.8 / -51.8

and the other had +52.6 / -49.4 if that gives any clues
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Old 28th May 2009, 07:55 PM   #9
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Did you measure directly across each set of terminals for each channel? I appears that you may have used chassis ground and the 1.2v offset is from the difference of ground potential.

The voltage would only be momentary because the supply would shut down when the protection circuit saw the DC offset (assuming that it has DC offset protection).

I'd suspect the voltage amp. I used to see a lot of them fail in various amps back when they were commonly used. Pull the one from the 7.2v channels and see if the amp will power up.
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Old 29th May 2009, 02:01 PM   #10
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I ESR'd the brown caps (2200/50v) at the DC input those appear good, I see lots of Glue, but no appearant sign of leakage. The 10uf caps look fine as well.

I reconnected everything and this time removed power resistors from the STK voltage amp ic from the channel that had the high DC on spk terms. .....It powered up and the other channels had audio..... Looks like a bad STK 350-000

I tried this before with both STK's, one at a time and it still went into protect. Maybe I missed a power pin the first time (thanks Perry)

The low voltage supply is running +15 / -13 volts.
Is that acceptable ? Audio on the good channels seem fine.
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