Tweet
In spite being ever careful working on all this equipment, and ever-so-careful when I go probing with the equipment powered up on the test bench, still sometimes I get greeted with a loud BANG! when touching the DMM probe to a particular circuit.
That just happened yesterday, while checking the results of restoring a Mackie SWA 1801 Power Panel, and checking the output potential of its’ switched HV rails. I never have the HV rails engaged when I first bring up a repaired system. I verified the power amp circuit worked on the low voltage supplies, then checked the output level from the MosFET’s at the empty Fuse holders (which would be about the same as the LV buss caps with no signal). The Power Panel had an exploded 4700uF/100V Buss cap, which was a nightmare to removed, being in the middle of the glued-down cluster of buss caps.. I’d already replaced the BYV79 hi current TO220 diodes, while the IRF 540’s measured ok.
When I touched the FET-side of the empty fuse holder, with the DMM probe (200V range), I was greeted with a loud BANG and a flash of light from the spark, which of course made me jump back, now eyeballing the Power Analyzer, getting ready to shut it down. All appeared normal…no change in mains current. I shut it down anyway, shaken back to reality. I HATE THAT!!
I didn’t’ find anything obvious, the MOSFET measured fine, as did the BYV79 diode, and powered the system back up, but now, with the fuses in place to check the switching HV rails, the negative rail wasn’t switching. Powered down, discharged the caps and took the clamp off the MosFETs’ and power diodes to look further. The upper half of the MosFET on the neg side came right off, exposing a blackened pile of soot inside the MosFET, so THAT part exploded, resulting in that loud BANG I heard. Replaced it and the controller IC, all back together and working fine.
That just happened yesterday, while checking the results of restoring a Mackie SWA 1801 Power Panel, and checking the output potential of its’ switched HV rails. I never have the HV rails engaged when I first bring up a repaired system. I verified the power amp circuit worked on the low voltage supplies, then checked the output level from the MosFET’s at the empty Fuse holders (which would be about the same as the LV buss caps with no signal). The Power Panel had an exploded 4700uF/100V Buss cap, which was a nightmare to removed, being in the middle of the glued-down cluster of buss caps.. I’d already replaced the BYV79 hi current TO220 diodes, while the IRF 540’s measured ok.
When I touched the FET-side of the empty fuse holder, with the DMM probe (200V range), I was greeted with a loud BANG and a flash of light from the spark, which of course made me jump back, now eyeballing the Power Analyzer, getting ready to shut it down. All appeared normal…no change in mains current. I shut it down anyway, shaken back to reality. I HATE THAT!!
I didn’t’ find anything obvious, the MOSFET measured fine, as did the BYV79 diode, and powered the system back up, but now, with the fuses in place to check the switching HV rails, the negative rail wasn’t switching. Powered down, discharged the caps and took the clamp off the MosFETs’ and power diodes to look further. The upper half of the MosFET on the neg side came right off, exposing a blackened pile of soot inside the MosFET, so THAT part exploded, resulting in that loud BANG I heard. Replaced it and the controller IC, all back together and working fine.
), I was working on a big CRT set. My screwdriver slipped removing the anode connector and I got a 25KV zap to the hand. As anyone's first reaction would be, I jumped back away from the set- hitting both elbows on a file cabinet. I was not only shaking from the electrical shock, but both funny bones were buzzing at the same time. Lesson learned.
Comment