Hey guys,
My family DVD player (we still have one) has been sitting disconnected for several months after I discovered its case was being held at a non-ground potential. I accidentally noticed that dragging a HDMI connector across the back of the case yielded small sparks and from there I investigated. Whilst my TV is earthed the DVD player is a two-prong AC plug affair, and both function perfectly fine.
Measuring the case with my
non-true RMS meter reveals a case pot of about 177V AC compared to house ground. The ground in the house is functional. Grounding the case over a 10K resistor yields a 400mVAC drop (less than a mA average) and over a few ohms it is not really measurable with my DMM. ( ~0.003 VAC)
Running the DVD player's power supply after removing it yields the same results, so I know the problem is isolated to this one board.
This is the left (primary) side of the PSU board. What look like fine separations in the ground plane around the edge screws are not: they are just black silkscreen I didn't notice to clean up
I can only see a few ways my problems could occur:
C110 (MOV?) lets too much current throughThe small amount of capacitance this has would let
some mains through, but if it were big enough to cause this problem then why would they put this on a non-grounded device? DC resistance checks out to be above my meter's max of 20 meg, so it's not a short.
EDIT: C111 and the optocoupler both measure good for DC resistance tooInsulation on the large transformer has gone... but surely then the device would have its voltage taps skewed unevenly -> things would be out of spec -> the device would not work. When I discovered the problem it was still working.
Fault on the secondary sideUnlikely given the voltage levels & the fact it's AC.
Should I try removing C110? As far as I can see it's for noise suppression, not safety, but I could be wrong. Is this behaviour normal for non-grounded devices that use their case as ground return path?