I finally thought to look and that "odd" resistor is a 1090M-ohm that's meant for an RCA WG-289 probe. I haven't done anything to check the resistance as nothing I have can do that. I might connect one of these 45M-ohm resistors in series with it and measure the voltage across the 45M-ohm just to see if the 1090M-ohm is still functional.
I finally found a reasonably-priced HK40-6 which should be her midweek and I can finally measure the cathode voltages of my Tek scopes safely and somewhat accurately.
Thanks,
Barry - N4BUQ
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If I'm not mistaken, there's an "odd" resistor with something like 90.9 labeled
on it. I'll have to get that back out and see exactly what value it is. I got
these resistors from an estate sale and it looks like they've never been used
for anything.
Yes, it's just to get the HV set on the cathode of the CRT. Technically, it's
supposed to be within 1%. I have a Triplett 630-NA that has a 6000VDC range
that I've used as a rough check but I want to get this much closer than that.
Thanks for the good info.
Barry - N4BUQ
----- Original Message -----
From: "edbreya=yahoo com" <edbreya@...>
To: "tekscopes" <[email protected]>
Sent: Thursday, February 8, 2024 1:52:17 PM
Subject: Re: [TekScopes] Method to measure HV
Barry wrote "I presume that because the probe uses a 75M and a 75k in series and
the 10M voltmeter is across the 75k, the 10M has minimal effect on the
divider's ratio. Is that correct?"
It depends on what you mean by "minimal effect." In reality, for a commercial
1,000X probe at 75 megs input, the "75 k" termination resistor is likely 75.643
k, which is about right to attain nearly the ideal value of 75.075...k when the
10 megs DMM input R is paralleled. That's why Jean-Paul mentioned being sure of
that 10 meg value.
Your 10X probe deal should work just fine, but be aware of the dependence on the
DMM's input R, and whether it's connected. If you get into a situation where
the 90 megs of R are left dangling out there, it's a current source backed by 3
kV, so can run up to 33 uA into whatever it could contact. It can't hurt you,
but could damage certain parts if contacted. If the DMM is always securely
connected, it's still a current source, but limited to 300 V. The main thing is
to keep it wrapped up to avoid any mishaps. The exact same thing can happen
with any HV probe if it's ground connection is forgotten or inadvertently lost,
so always be sure it's secure.
You can also opt for a higher divide ratio like 100 or 1,000X, which is what
most HV probes do, to get more comfortable measuring voltage, It's quite easy
to adapt any high-meg resistance into a nice HV divider. The higher the divide
ratio, the more flexibility you have in hitting the values right and handling
multiple load values. For instance, you could go with 100X instead of 10X, and
be able to drive a 10 meg or 1 meg input. With the same 90 megs of inlet R,
you'd want the bottom R to be 90 M/99, so 909.09... k. This can be conveniently
made with a 1 M termination paralleled with the DMM's 10 M input R, or, a 10 M
termination with a DMM (or scope) input R of 1 meg. These are all idealized
values, of course. The actual ones have to be worked out. It's most important
to know the tempco, voltco and actual value of the inlet R - everything else
can be worked out to accommodate it.
Anyway, since your application seems simple enough here - just checking a scope
one time (right?), and not needing extreme precision, your 10X thing is already
the way to go. It's good to consider options too, if more of such needs are
expected.
Ed
