Jim Adney wrote:
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?

Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently
it doesn't work to remember numbers after it's been 2-3 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.

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.

This is the "HV test point" (-2.2kV). The anode of the CRT gets the
higher voltage you're expecting: +11.8kV.

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.

Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea
to use so many of them. I suppose I could add more to the chain,
though. Or I could just get a real HV probe....

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.

Thanks to you and everyone else for the advice. I'll certainly think
more carefully about this (and probably just use a real HV probe).

Damian Menscher
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