These pix are from a Martin Logan SW10 amp board which is buzzing when turned on (goes away after awhile) and distorting during operation. Clearly something has gotten very hot. Seems like it is the two Blue resistors. One charred the PCB and the other was touching the Red output and blackened the shroud around the spade connector. I don't think there was electrical connection.
This is probably a generic chinese amp which is branded for ML.
PCB code is PCBPBDR818R 2007.04.17 Rev 1.6
Does anyone happen to know of schematics or can take a guess at what the root cause is? The main capacitors don't seem to be puffed out on top but I will try to remove and test.
This is probably a generic chinese amp which is branded for ML.
PCB code is PCBPBDR818R 2007.04.17 Rev 1.6
Does anyone happen to know of schematics or can take a guess at what the root cause is? The main capacitors don't seem to be puffed out on top but I will try to remove and test.
Attachments
The discoloured PCB (speaking generally) is a common sight in a lot of equipment and is a natural response of the board to long term heat exposure.
I would definitely change those two small electrolytic caps in the middle of all that discoloured PCB.
The two large horizontal resistors should be checked. I can't make out from the picture what they are but the one on the right looks as though it might be the same as the other one but with the colours changed due to heat. 620 ohm perhaps. Check them out of circuit.
Beyond that... circuit and more involved diagnostics I'm afraid.
I would definitely change those two small electrolytic caps in the middle of all that discoloured PCB.
The two large horizontal resistors should be checked. I can't make out from the picture what they are but the one on the right looks as though it might be the same as the other one but with the colours changed due to heat. 620 ohm perhaps. Check them out of circuit.
Beyond that... circuit and more involved diagnostics I'm afraid.
The board is discolored from constant heat.
Almost looks like the heat is coming possibly from the board underneath.
The spade with the red wire, im wondering if its arching.
far as distortion Might also be shunt regulators for the op amp.
Check if the opamp is getting +/- 15 volts on the power pins.
shunt regulator is basically zener diodes with dropping resistors
and the dropping resistors get very hot.
resistors can drift, caps can dry out, and zeners can fail.
amp is still working, zeners probably ok. resistors or caps
maybe drifted or dry
can also view the amp in complete darkness and see if there is
arching on the spade at start up. or arching under heavy load
as mentioned the smaller capacitors in that hot spot have been getting cooked
possibly also part of the shunt regulator for the opamp.
Almost looks like the heat is coming possibly from the board underneath.
The spade with the red wire, im wondering if its arching.
far as distortion Might also be shunt regulators for the op amp.
Check if the opamp is getting +/- 15 volts on the power pins.
shunt regulator is basically zener diodes with dropping resistors
and the dropping resistors get very hot.
resistors can drift, caps can dry out, and zeners can fail.
amp is still working, zeners probably ok. resistors or caps
maybe drifted or dry
can also view the amp in complete darkness and see if there is
arching on the spade at start up. or arching under heavy load
as mentioned the smaller capacitors in that hot spot have been getting cooked
possibly also part of the shunt regulator for the opamp.
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The discoloured spade cover (that squishy plastic shroud over it) is like the PCB just down to prolonged heat.
What I can pretty much guarantee you will find is that soldering under the PCB in those areas will be 'crusty', possibly at the point of there being dries and probably very difficult to rework easily, the solder just won't take to the overheated parts easily.
What I can pretty much guarantee you will find is that soldering under the PCB in those areas will be 'crusty', possibly at the point of there being dries and probably very difficult to rework easily, the solder just won't take to the overheated parts easily.
