Switchable VTVM Probes

Many popular VTVMs used a switchable probe: a probe that contains a one megohm resistor for DC measurements, and a switch to bypass the resistor for AC and Ohms measurements.

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For example, here is the classic Heathkit probe from my IM-11 VTVM.

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The button at the front rotates 180° to select either DC or AC/Ω.

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Here is an RCA WG-299E probe from an RCA WV-98C VTVM.

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A slide switch selects either DC or AC/OHMS. My post on repairing a WG-299D has some photos and a description of the internal mechanism of this switch.

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This is a Simpson Model 0150 probe from a Simpson 312 VTVM.

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The Simpson probe uses the hard-to-find mini version of the Amphenol 75-MC1F microphone connector.

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It also has a slide switch to select between DC and AC/OHMS measurements.

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This is a B&K/Dynascan Model PR-43 probe for their Model 177 VTVM. It uses a slide switch as well.

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A B&K/Dynascan Model PR-21 probe for their Model 277 FET Multimeter. Again a slide switch, but in this case the resistor is only 100 KΩ, not 1 MΩ. It’s labeled “Direct” and “100K” rather than DC and AC/Ω.

© Steve Byan 2011-2019