"Damian Menscher" wrote in message
...
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.)


They can flash over, that's your main worry. I've seen people make HV probes
with simple resistors though, the best way seems to be to chain them
together and then place them in a piece of plastic conduit then fill that
with mineral oil. You can make a reasonably professional HV probe that way,
and you're only measuring a few Kv so you should be fine. That said, I got a
real Fluke 40 Kv HV probe on ebay for under $30 a couple years ago, I've
seen the 6 Kv ones go for under $20 but with a 40 Kv you can measure CRT
anodes.