On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher
wrote:
A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope.
674? What's that? Is there a typo there?
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.
That's a pretty low voltage for a Tek scope, so my guess is that this
has already been divided down. This makes it more important that you
make a pretty high impedance divider.
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.
Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter
as the last resistor in the chain? That would reduce the load on your
test point by a factor of 10, which would be a good thing.
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.)
That's just the voltage where they are likely to flash over. Since
you're proposing to work with just 200 V on each one you should be
fine. It's probably still a good idea to solder these together first
and then wipe off the resistor bodys with alcohol to remove finger
oils which can make surface leakage or flashover problems.
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?
Don't touch any of this while you're doing the measurement. .7mA is
probably too much for this circuit; .07mA is likely to be fine.
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.
It's safe only until they flash over. Then it will destroy your meter
and would probably not kill you but really make you wish you'd used a
real HV probe.
Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign
There are probably 50 HV probes within a quarter mile of where you
work. Ask around.
-
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Jim Adney
Madison, WI 53711 USA
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