"William Beaty"

HIGH VOLTAGE SHORT IN CARY 5000 SPECTROMETER

( snip)

The PCB design unwisely routes a 1000vdc power supply trace across an
internal ground plane.


** That is quite an understatement.


Given enough years, the HV slowly eats through the paper-thin epoxy-glass
layer and arcs to ground. Perhaps the same fault will occur with many
other units?

** Guaranteed to.

The cure is to route the small 1KV supply trace across the PCB using a
separate wire.

** Yep.

Found myself doing the same once with a number of high power audio
amplifiers where the PCB had adjacent tracks, separated by only 0.4mm, that
carried +/- 120VDC. The fibreglass PCB was NOT coated with resist and dust,
grime and moisture settled on the PCB in normal use.

End result was "tracking" across the surface between the close spaced
conductors and eventual breakdown - leading to the output stage going into
full cross conduction, blowing fuses AND expensive Hitachi TO3 mosfets to
hell.

Part of the repair was to peel off the offending tracks and replace them
with ( light gauge) PVC coated wire.

If caught early enough, thorough cleaning of the PCB and spray coating with
clear "circuit board lacquer" prevented the disaster.



..... Phil