Hi Barry.
I built a homebrew hi pot tester using an Emco DC-DC converter that runs 2.5
KV up to about 25 KV. I have tested a lot of vacuum components used in
amplifiers and tuners.
1.) It is ESSENTIAL that the dielectric be ABSOLUTELY free of grease,
oil, fingerprints, dirt and other contaminates. I have spoken directly with
a line supervisor at Jennings about this. Her recommendation (Jennings'
in-house procedure) is to de-grease the device prior to testing with a
commercial spray bottle product like Formula ---. If you do not do this,
your leakage current measurements will be worthless. This applies to glass
and especially to ceramic dielectric material.
2.) There may be big differences between testing the component with AC
and DC high voltage. Jennings rates their vacuum caps voltages with 60
cycle AC. They state in their technical bulletins to de-rate the vacuum cap
test voltage to 60% of AC voltage when using DC for leakage testing. This
does not apply to vacuum relays.
3.) Many older vac caps develop copper fingers on the edge of the
annular rings which can lower the breakdown voltage of the device under
test. A common practice is to gradually approach the rated peak voltage of
the vac cap and allow the cap to flash over briefly. This is repeated up to
2 times, if necessary, but no more than this. This can often restore the
cap to factory specs.
4.) If you are within the stated peak test voltage (adjusted for the
type of current AC or DC), and you see large current leakage values, the
device is bad, period. Anything over a single microampere is bogus (some
would argue that a microampere is too much leakage). If you want to use a
vacuum cap at a de-rated working voltage, I would say this is a bad
decision. If the cap does not test to factory specs, it is an unknown
quantity just waiting for Mr. Murphy to come and visit. This is harsh,
since the vac cap may look okay, but hi pot testers don't lie.
73
Paul WN7T
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