Mc Intosh amp, unfriendly problem

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McIntosh is having it's main ground = 12VDC ground, like Rockford or newer Alpine amps. Unlike many other amps which has transformer's secondry floating towards 12VDC ground. It means the input is differential (between RCA center and shield) with opamps, and RCA shield is connected by 100ohm//100nF to main ground. Many ground points in the PCB is taken from screw hole (which is bolted to the heatsink),so if the screw is open, no ground for that point. I have no schematic for McIntosh car amps, maybe you can contact McIntosh directly?
 
Will the amp work properly (no protection mode, insignificant DC offset measured directly across the speaker terminals for each channel, produces clean audio) if you directly connect the secondary center tap to chassis ground?

I've seen a few instances where protection circuit components were leaking and pulling the secondary center tap away from ground.
 
It appears that it's going into protection because there is offset but the offset doesn't appear to be due to a fault in the audio circuit. Something is pulling the secondary ground up. I've never seen one of these so I can't be much more help. You'll have to try to find whatever is pulling it up.

Is it possible that there is a diode on the RCA shield or secondary ground that's supposed to limit the DC on the shield (similar to Rockford or MTX)?
 
Yeah, I've been looking for leaky, shorted or open components that could be pulling up sec. ground .

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Is it possible that there is a diode on the RCA shield or secondary ground that's supposed to limit the DC on the shield (similar to Rockford or MTX)?

The only diode so far that I found, that goes between DC ground and the output rca shields (and heatsink of ch 3/4 ) was the shorted 1N5401 that I replaced.
I'm not sure how Rockford or MTX does it.

I did manage to order the service manual from Mac, I don't think I can go any further until I have the schematic. So I will have the MC431 info available to those who need it.

Let you know what happens
 
I just double checked and the diode is good and installed correctly.

But I misspoke when I said the diode goes between ch 3/4 rca shields and DC ground.

The diode actually goes between the ceter tap of ch 1/2 power supply and DC ground.

The rca shields of both ch 1/2 and 3/4 each, are routed through a 100 ohm resistor to their respective power supply sec. center taps.
 
After getting the schematic on this amp, I quickly realized the solution for the MC431 still going into protection
even with the new parts installed was right under my nose.

It wasn't until I actually laid eyes on the print that I saw that both secondary center taps
of each power supply should be at "Chassis Ground". This in not done with pc board traces, but only gets accomplished when you bolt the pc board back into the case and both power supply/output heatsinks are shunted together. Now, the amp powers up normally.

Actually, I did think of this as a possibility before, and I "clip leaded" the 2 heatsinks together. Where I went wrong was the amp was already in protect mode. This little stunt does not take the amp out of protect mode once initiated. If I had only tried it before a power up I would have save myself $30 and a lot of time.

Oh well.
 
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