The Sony CPD-100ES trinitron monitor that I've been using with my main old computer setup has been making a very worrying buzzing sound when it turned off and went into standby. It's been unplugged for a few weeks now but the 19" LCD I've been using in its place just isn't right for aspect ratios etc.
Today I had a go at cleaning it. To do that, I took the monitor apart - just removing the outer casing and taking off the plastic that protects the monitor's main PCB, then carefully brushed dust off and used some compressed air & a blower bulb to dislodge any dust stuck in the transformers. As I was putting it back together, I noticed a black relay shaped object near to the mains plug - blew some air into that.
With the monitor put back together, the monitor doesn't make that worrying noise when going into standby now - which I think was the relay operating too slowly / bridging somehow.
I've also been cleaning up battery damage & trying to figure out this completely unknown 386DX40 motherboard, which appears to be the earlier revision of Keropi's board. Very helpfully, there's a jumper (JP4) to switch the battery from ni-cd (charging) to lithium mode. Pretty odd though, the board actually arrived with the jumper set for lithium mode, so it wouldn't have charged? I've checked with a multimeter, moving the jumper makes the voltage with no battery connected go from 0v to 5v, so it's definitely working as expected and shouldn't result in an explody battery.
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There was some corrosion hiding under the BIOS chip on one of the diodes, but not one trace damaged, on the outer layers.
Also for some reason, the TFX power supply I was using for this machine let smoke out when I turned it on. It's still working, seems to give the correct voltages and I can't find what let the smoke out, could that be a filter cap?