Paul:
Thanks for the info, it certainly helped!
Last evening, I had a 3 to 30 pf, 15 KV cap that had quite high leakage at
3.5 KV.
This evening, after cleaning, I can go to 10 KV (DC) with .25 microamp
leakage.
Thanks again,
Barrie, W7ALW
----- Original Message -----
From: "Paul Marbourg" <zborg@comcast.net>
To: <barrie@centric.net>
Cc: <amps@contesting.com>
Sent: Saturday, August 20, 2005 12:23 PM
Subject: Hi Pot Testing of Vacuum Components
> 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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