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Author Topic: Heathkit VTVM probe ?  (Read 17656 times)

Posts: 4546

« Reply #15 on: January 22, 2012, 06:55:03 PM »

OK got the HV probe, now want to do the mod to move the 1megohm resistor from the probe to inside the chassis.

Why would you move the 1 meg resistor inside the chassis? Back in the day it wasn't unusual to work with hundreds of volts and a resistor in the probe offers some protection for both the meter and circuit being tested. One thing I never liked about the classic VTVM probe is how it's switched for Ohms and Volts, but some were shielded and that was a good thing. If you forget to set the probe to match the meter you will get bad readings that may not be obvious and start chasing ghosts in a circuit. I have a hunch that if there wasn't a good reason for the probe resistor it would have been eliminated a long time ago and that I can't be the only one who feels that way.

Let's just say you may want to think it over before modding the meter.

For what it's worth, I have an HP 427A which is the FET equivalent of a VTVM. No resistance in the Volts probe and it does have a separate connector for Ohms, but The HP Way [tm] is to use either a coaxial probe with banana plug or a 'scope probe through a BNC to banana adapter. They did offer 1:1, 10:1 and 1000:1 probes for measurements up to 24 kV plus an RF probe good to 500 MC's. The concept of a shielded probe is to reduce stray coupling as the meter is good down to 100 mV full scale. That's exceptionally sensitive and if someone wants full functionality from the meter they will use a shielded probe, although it is an expensive way to go.

If you decide to convert to a standard probe spend a little extra for the premium Silicone wire type as they're extremely flexible and resist damage when brushed against a soldering iron. Been there, done that............


Posts: 5224

« Reply #16 on: January 22, 2012, 08:02:22 PM »

I am not sure about the HV probe, but I was made aware that the 1 meg resistor at the probe tip, eliminates capacitance of the cable. That is gone if the resistor is put into the switch.

Posts: 180

« Reply #17 on: January 23, 2012, 08:36:54 AM »

I believe the probe from the senior, and possibly the junior RCA VOLTOHMYST have similar probes if my memory serves me correctly...So if by any chance you can find one of those, I think that they are either direct plugs in for the Heath in questions, or will work with slight mods...
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