I was wondering if someone can help troubleshoot a Mcintosh MC431. Channels 1 and 2 work just fine, but 3 and 4 do not. The powerguard light is not on, but will flicker on occasion. The main power supply voltage output will read -1.9v and +1.9v, which should be -37v and +37v. If I disconnect the jumpers from the power supply to the main board (on 3 and 4), the voltage comes back up to normal on the power supply. Also, shorting TP1 and TP2, defeating the powerguard, does nothing. I already changed just about all the electrolytic caps, the ones that I didn't I checked them with an ESR and Cap meter. I also isolated the preamp from the main output, swapped around the voltage amps, checked all of the transistors in the output circuits, checked and swapped around the powerguard ic's. Could this be a powerguard problem, Over voltage protection? I'm running out of ideas.
I don't know the amp and don't work on car amps, but it's telling you that you have a shorted out-put or at least a something is a near short and the power supply has built in current limiting and is folding back to keep from over heating and burning up.
It was not working before I swapped out the caps. This is my friends amp, and he told me that there was a screw through the speaker wire, which was shorting out one of the channels to ground. When I first got it, the protection light was on, I changed out just about all of the electrolytic caps, then channels 1 and 2 started working. I changed out one cap at a time because I did not want to make that mistake of putting one in backwards. I'm pretty sure that they are all in the correct way, but will recheck them again.
Start checking between the channels that work and the dead ones. something is pulling your rail voltages to ground, do you have the schematic for the amp if so send me a copy or post it so we can take a look at it.
Don't know how much help i'm going to be as I got out of electronics in the early 80's but i'll give it a shot.
Don't know how much help i'm going to be as I got out of electronics in the early 80's but i'll give it a shot.
Wait a minute, how much current does the amp draw at idle with nothing connected but power???
I ask because if those rails for channels 3 and 4 drop to 1.9 volts when connected properly, then your 12 volt current draw would be horrendous trying to supply that level of a short. that is what blows the power supply in most amps anyway is severe shorts dragging every last drop of current out of the supply.
Try and supply channels 3 and 4 with 1 and 2's power supply and see what happens < jump connections to swap 1&2 with 3&4 > If they still drop them your amp is the trouble if not then it your power supply that has issues.
You can also just disconnect 3&4 from their respective power supply connections and then jumper 1&2 to 3&4 and see if the amp plays. At low power levels you can run 4 channels off of two channels power supply. Idle current will double but nothing else should happen unless 3&4 still have issues.
I think your power supply might be sick, not your amp. Mac used power limiting in these amps and that may be the main problem area. One thing for sure though if channels 3&4 had a severe short large enough to drop you rails out then your idle current draw would likely be tripping those resettable circuit breakers located out back < by the way I own one also and it also has power issues, would love to have the document if you have one..please >😉
I ask because if those rails for channels 3 and 4 drop to 1.9 volts when connected properly, then your 12 volt current draw would be horrendous trying to supply that level of a short. that is what blows the power supply in most amps anyway is severe shorts dragging every last drop of current out of the supply.
Try and supply channels 3 and 4 with 1 and 2's power supply and see what happens < jump connections to swap 1&2 with 3&4 > If they still drop them your amp is the trouble if not then it your power supply that has issues.
You can also just disconnect 3&4 from their respective power supply connections and then jumper 1&2 to 3&4 and see if the amp plays. At low power levels you can run 4 channels off of two channels power supply. Idle current will double but nothing else should happen unless 3&4 still have issues.
I think your power supply might be sick, not your amp. Mac used power limiting in these amps and that may be the main problem area. One thing for sure though if channels 3&4 had a severe short large enough to drop you rails out then your idle current draw would likely be tripping those resettable circuit breakers located out back < by the way I own one also and it also has power issues, would love to have the document if you have one..please >😉
I have been comparing from the channels that do work. Also swapped components around.
I haven't measured the current, I can check that. That's a really good idea swapping the power supply jumpers. I'm going to work on that now. I do not have the mc431 schematic, but do have the mc440, using that as a reference.
I haven't measured the current, I can check that. That's a really good idea swapping the power supply jumpers. I'm going to work on that now. I do not have the mc431 schematic, but do have the mc440, using that as a reference.
You were right, it's the power supply. Finally a step in the right direction. I swapped the power supply outputs and channels 1-2 are now dead, but 3-4 are working. Now I'll have to track down the bad component(s). Any suggestions on where start? I already tested the large caps; and swapped them, from 1-2 to 3-4 a while back. Is it possible that the mosfets are weak, even though they test out good?
Could you post a pic of the power supply area in question please ? thanks. For it to be dropping out like it it is it has no current handling capacity which could be the transformer or the circuit breaker had bad contacts, or simply bad solder or a cracked trace somewhere.
The possibility of bad IC's should also not be ruled out, but I would look for the bad current handling things first which would be as I listed above. It sounds like something really simple and possibly bordering on stupid like bad solder joint somewhere or bad overload circuit breakers. Seem them engage before and not handle any current loading at all. I also have seen all the legs of the power devices on these amps have bad solder joints on them. I won't open one up without reflowing all the power devices attached to the sink < a very common issue with these in my experiences >
The possibility of bad IC's should also not be ruled out, but I would look for the bad current handling things first which would be as I listed above. It sounds like something really simple and possibly bordering on stupid like bad solder joint somewhere or bad overload circuit breakers. Seem them engage before and not handle any current loading at all. I also have seen all the legs of the power devices on these amps have bad solder joints on them. I won't open one up without reflowing all the power devices attached to the sink < a very common issue with these in my experiences >
I found the problem. It was all 4 of the pwm switches. They were shorted. I checked these before I recapped and they were good before that, something must have happened after. Thanks for your help.
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