William Beaty wrote: HIGH VOLTAGE SHORT IN CARY 5000
SPECTROMETER TWICE! Our UV/VIS/NIR spectrometer lost its -1KV high
volt source, and the cause both times turned out to be an internal short
on the PMT-control PCB. Both times this fault was easily found with a
DVM: an unexpected leakage path, well below 50Kohms, seen between the 1KV
source and ground. The shorted HV trace and the groundplane are both
internal layers in the 4-layer circuit board. Verify: when the 4-pin
molex cable connecting the main PCB to the PMT-control PCB was removed,
the -1KV at the main board reappeared again (actually measured as -980VDC
when working correctly.) The PCB with the actual problem is
positioned vertically all the way to the right of the instrument. The
-1KV switching supply is on the main board covering the left rear of the
instrument. The HV cable is the cable with the 4-pin Molex connector,
with one pin having a heavy red HV conductor. The PCB design unwisely
routes a 1000vdc power supply trace across an internal ground plane.

Did they just forget to clear out the ground plane in that section? In
your photos it looks like they randomly took a paper punch to internal
planes by the optoisolator looking devices. weird.