Hi,
I am trying to fix my ML383 because it is making a loud pop when you turn it on and sometimes (getting progressivly worse) it makes noise from the right channel.
It sounds like a noisy transistor but the weird thing is the noise is independent of volume or input.
I have been searching the web for 2 days and I'm unable to find any schematics for the amp.
I have opened it up and took out the VG boards and they are quite complex, so
very hard to figure out without a schematic.
Also I found that the left and right boards are basically the same but are a different layout, which I find a bit strange. I would think the left and right channels would be exactly the same but okay because of the construction of the digital control they had to mirror the boards and in doing so they had move things around and also screwwed up on one side because 2 transistors that need to be thermally together they forgot to put next to eachother and botched them on the underside of the board
I also found that the PCB on the front the selector knob is on a seperate PCB which they did not break from the main PCB and because of that is now deformed. Is that normal? does anybody else have that?
So if anybody can help me with schematics or any other input that would be greatly appreciated.
Thank you.
I am trying to fix my ML383 because it is making a loud pop when you turn it on and sometimes (getting progressivly worse) it makes noise from the right channel.
It sounds like a noisy transistor but the weird thing is the noise is independent of volume or input.
I have been searching the web for 2 days and I'm unable to find any schematics for the amp.
I have opened it up and took out the VG boards and they are quite complex, so
very hard to figure out without a schematic.
Also I found that the left and right boards are basically the same but are a different layout, which I find a bit strange. I would think the left and right channels would be exactly the same but okay because of the construction of the digital control they had to mirror the boards and in doing so they had move things around and also screwwed up on one side because 2 transistors that need to be thermally together they forgot to put next to eachother and botched them on the underside of the board
I also found that the PCB on the front the selector knob is on a seperate PCB which they did not break from the main PCB and because of that is now deformed. Is that normal? does anybody else have that?
So if anybody can help me with schematics or any other input that would be greatly appreciated.
Thank you.
Ok I fixed it already, noise is gone and the pop is gone. Still would love a schematic for future reference. I asked the Harman group support of which ML is a part of but, they said they only give schematics to authorised service partners. So I guess the only thing to do next time I have a problem is to reverse engineer it. But anyway I'm happy it works again.
Ok, first the noise problem was on the right vg board. I found that on the left vg board there were 4 transistors replaced by an earlier repair. I saw they where replaced by the original transistors mps8099/mps8599. And also on the right board one of these was replaced by an 2n5401 which is not a good replacement for it. So i thought maybe there are problems with these transistors and i replaced them all for MPSA06/56 which is a direct replacement for the mps8099/8599 and is currently available. I just picked up a bunch at a local electronics store for 25 cents a piece. See picture 1 where i marked the transistors i changed
The second problem was the loud pop when turned on. When I was doing some testing I saw in the corner of my eye a little flash on the input power selection board on the power supply board. When i looked closer and tried a few times I saw a big spark fly across the leads of one of the NTC's which are on this little board. I removed it and saw on the bottom side there was a big burn mark and someone has done a repair there by scratching out the trace that goes over the 20V tap of the transformer which has a standoff which is not used for my situation (Europe 230V) but was directly under a trace on the input selection board. Apparently the back EMF of the transformer sometime creates a big spark which even jumps through the board... So I drilled out the standoff's which aren't used see picture 2.
See picture 3 for the input selection board:
So now it is working perfectly!
The second problem was the loud pop when turned on. When I was doing some testing I saw in the corner of my eye a little flash on the input power selection board on the power supply board. When i looked closer and tried a few times I saw a big spark fly across the leads of one of the NTC's which are on this little board. I removed it and saw on the bottom side there was a big burn mark and someone has done a repair there by scratching out the trace that goes over the 20V tap of the transformer which has a standoff which is not used for my situation (Europe 230V) but was directly under a trace on the input selection board. Apparently the back EMF of the transformer sometime creates a big spark which even jumps through the board... So I drilled out the standoff's which aren't used see picture 2.
See picture 3 for the input selection board:
So now it is working perfectly!
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Thanks, but yes I do really like this amp. It sounds amazing, has lots of power to drive difficult loads.
The only thing I don't like about it is the speaker binding posts, they are terrible. They only take spades but when the spade is too big at an angle it won't fit under the wings of binding posts. Really annoying!!!
But I'm sure I'm nitpicking ;-)
The only thing I don't like about it is the speaker binding posts, they are terrible. They only take spades but when the spade is too big at an angle it won't fit under the wings of binding posts. Really annoying!!!
But I'm sure I'm nitpicking ;-)
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