Damian Menscher wrote:
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
Your resistors are too small. Too much load on a typical high voltage
circuit. You're putting too much voltage on each resistor. When you
buy resistors from RS, you have no idea what you're getting.
When you use a pen as a housing, you have no idea what impurities are
in there and whether it will arc over or through.
If you have regular need for a HV probe, put a watch on EBAY for one
and snag one that goes at a low price. Whole bunch of 'em have sold
near the $10 mark recently.
IF you only need this one measurment, borrow a probe and be safe.
I wouldn't risk my life on a probe that cost me $8 to build when I could
have bought a safe one at a swapmeet for $10.
mike
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