Answering and closing my old thread...
I was cleaning out some storage at home and found this old broken instrument. I decided to have one last look at it and see if I could fix it before scrapping it.
This time I decided to take out the board from the casing since I was going to throw it away anyway. On the underside I discovered a burnt component, a rectifier for the +/- 20 volt supply rail. After a closer inspection it turned out not to be the rectifier itself that was burnt but the de-coupling capacitor before the rectifier. The capacitor was not only burnt, it was completely missing, as in, it had basically exploded and had left metal residues on the board and pads. I
-measured across where the capacitor had been and it showed ~500
. I cleaned the board with acetone as much as I could and ohm-measured again, this time it read >5k
. I figured that some of the capacitor residues must have left a low- \$\Omega\$path between the pads and this was why the transformer had started smoking, drawing too much current on the +/-20V rail (I also de-soldered the rectifier and measured it, it was OK). After that I re-soldered the rectifier back in place and powered on the unit, ready to pull the plug. To my surprise this time the smoke did not come from the transformer but from the underside of the board (I had to flip it right side up when powering it on). I immediately turned the power off and after that ohm-measured across where the capacitor had been situated. This time it showed almost a dead short ~6
. The PCB between the capacitor pads had charred quite a bit so I started to carve off the sot. When I have carved off and cleaned I ohm-measured again and to my surprise the measurement had climbed to ~100
. I repeated the procedure, carving out more and more of the charred PCB between the pad and every time the resistance kept climbing. I was basically scratching out charcoal until I hit the ground plane ~1mm down in the PCB, after cleaning with acetone this time the measurement showed infinite resistance. I again powered on the board and this time... no smoke! And the instrument seems to work just fine!
Now the question is... Why had the ceramic capacitor exploded? Was it just component failure?