Mc Intosh amp, unfriendly problem

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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.

THNx
 
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...:)
 
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
 
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.
 
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.
 
On most amps, the difference won't cause any problems but I don't know about this amp. It's likely only a Zener that's out of tolerance (assuming that you were using the proper reference - chassis/secondary ground - when measuring the voltage).

PPI is one that has to be nearly perfectly symmetrical. If there's too much of a difference, the DC offset can't be zero'd.
 
Scanlines said:

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

Even with zeners this is way out of value. I would start looking for shorted something in the frontend. I have seen this before on my own Mac amp.

It should be a very close match of 14.8 to 15.5 with maybe a few hundred millivolts discrepancy between the two rails. Also check bypass caps on the op-amps. Something is pulling your rail down on the negative side only it appears. But here again I am posting what I found in my own amp long ago....:) I replaced a bad opamp and VIOLA ! it was alive ...and it played for a few years till the power guard came out to haunt me...
 
This mac is still suffering from offset problems.

I installed the new STK 350-000 and replaced all 6 short, fat 10uf caps. .

The amp still goes into protect mode....UNLESS...rca's are connected from ch. 3 and 4 inputs to my bench 12VDC Pioneer deck. Then, it powers up and I have audio.

I can only assume that the rca sheild from the Pioneer deck is drawing the offset down enough to take it out of protect.

I'm looking closely at ch 3 and 4 because In protect mode, I'm measuring 1.66v on each spkr term.
While I get .30v on ch 1 and 2.
( the amp stays in protect if I switch the rca's to the input of ch 1 and 2.)

When it's running, I've got +14.9v / -14.9v on the low voltage.
Maybe I should suspect op amps ?.

Also, I measure 2.66v on rca sheild of 3 and 4 to DC ground.
.8v on 1 and 2.
 
You've found the bad 10uF fat/short caps? Sometime the track and through hole around it are ruined (due to elko's chemical leak), you can check it's connectivity (especially between top layer and bottom layer via through hole).

Also, I measure 2.66v on rca sheild of 3 and 4 to DC ground.

This could be a good clue. It should stay at 0V. Many of the PCB's ground point are taken from the screw to the heatsink, the heatsink is the ground path.
Have you install the screw's which connect the PCB to the heatsink (ground)?
 
Thanks for the replies !


Is the RCA shield for channels 3 and 4 connected directly to the secondary center tap of the power transformer for channels 3 and 4?

Nope they're not directly connected. Lumanauw is right on. There is a 100 ohm resistor (and it checks good) between 3 and 4 input shields and the center tap of the secondary (of that power supply)

Input shields of 1 and 2 route through another 100 ohm resistor (tests good) to the secondary center tap of it's power supply.

Is the secondary center tap of all power transformers at 0v DC (ref to chassis ground)?

No.
Here's how it checked out currently;

Ch 3/4 secondary center tap to DC ground .19v running
2.67v in protection.

Ch 1/2 center tap to DC ground .71v running
.73v in protection.

Have you install the screw's which connect the PCB to the heatsink (ground)? Many of the PCB's ground point are taken from the screw to the heatsink, the heatsink is the ground path.

Yes, the heat sink screw is in.
Also, there is a 3 amp doide between DC ground and the heatsink. So I would expect the heatsink to be .6v above DC ground.
When I got the amp, this diode was dead shorted and the circuit trace obliterated.
Sure wish I had a schematic.

I did double check the thru holes on those 10 uf caps I installed and they are electically continuous to the other side.
 
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