I have a coil that is humming in the SMPS of an automotive amplifier. This amp sustained some mechanical damage thanks to UPS, but I managed to fix everything (except this hum). I poured some epoxy over it, just enought to cover the top of the windings. That seemed to help for about a week or two. Now the hum is back on really cold mornings. Should I try some RTV between the coil and the PCB? More epoxy?
jwv3 said:
Epoxy has an expansion coeff of over 100 ppm/ degree C.
The mechanical stuff in your amp will run from 35 ppm (aluminum) to 12 (typical ferrites)
In english: the epoxy probably cracked due to the cold.
You need to fill it with something that will remain pliable cold.
John
The power supply of a car amplifier usually works at 30-60KHz so there should be no humming at all
LF noise from the transformer may be a sign of malfunction or unstability in the control circuit of the SMPS [This is the most usual case]
It may be also due to transformer saturation caused by a faulty regulator that allows the alternator to deliver 16V or more to the electrical system of the car [I've seen this problem but it's less usual]
You should check the voltage waveform that powers the amplifier and the switching waveforms on the transformer and switching MOSFET gates
Anyway, epoxy or hot glue are very useful to prevent transformer wires from scratching due to heavy car vibrations and shorting between them in the long term
I repair car amplifiers and sometimes I have to rewind toroids found to have intermitent shorts between turns or windings
LF noise from the transformer may be a sign of malfunction or unstability in the control circuit of the SMPS [This is the most usual case]
It may be also due to transformer saturation caused by a faulty regulator that allows the alternator to deliver 16V or more to the electrical system of the car [I've seen this problem but it's less usual]
You should check the voltage waveform that powers the amplifier and the switching waveforms on the transformer and switching MOSFET gates
Anyway, epoxy or hot glue are very useful to prevent transformer wires from scratching due to heavy car vibrations and shorting between them in the long term
I repair car amplifiers and sometimes I have to rewind toroids found to have intermitent shorts between turns or windings
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