A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope. (The high voltage circuit was going into
overload protection, I think as a result of leakage from the diodes
(5647 tubes). Replacing them with solid-state diodes seems to have
fixed that issue (I wish I could keep it in its original form, but I
simply gave up after being unsuccessful for a few years.).

Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.

After reading specs on a 6kV Fluke HV probe, I realize they're really
just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm
resistor). So I figure I could save myself $75 by building my own.
Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.

One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)

Any other warnings I should heed before doing this? I realize I'll
lose some accuracy (they're only accurate to 5%). I guess I also
have to worry about affecting the circuit when I draw .7mA out of it?
I think my measurement error should be small, since my multimeter has
a 10MOhm imput impedence. Is that good enough, or should I use a
smaller final resistor in the voltage divider to reduce measurement
error?

Finally, my crazy idea of protecting myself from electrocution is to
encase this thing in the plastic barrel of a Bic pen. They seem
about the right size, plastic doesn't conduct, and as long as I hold
it at the low-voltage end, it seems reasonably safe. Am I asking
for trouble here, or does that seem reasonable for measuring only up
to 7kV? I suppose I could build a safer handle onto it if needed.

I'd appreciate any advice that could improve my life expectancy.

Damian Menscher
--
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