VTVM probes

[Fred Olsen]

Richard Post wrote:
>
> Why not just put a small slide switch in the probe ...

It should be pointed out that Heath was not alone in using a switched
isolation resistor in their combination probes; in fact, they were
rather late coming to that method.  Most brands of service-grade VTVMs
(and some FETVOMs) used combo probes.  However, nearly all others did
indeed use a slide switch rather than Heathkit's proprietary rotary
style, with its myriad of custom, now-unreplaceable parts.

The main advantage to the Heath design was its compactness.  Since the
probe tip was mounted on the switch assembly, rather than solidly in the
probe body, ruggedness was not a particular virtue.  I would also
suggest that the choice of modified phone plugs and jacks was less than
optimum, and was based on continuity of design from the shielded cable
and isolated DV probe of the V-7A, et al.  While Switchcraft did what
they could, replacing the 'ring' contact of a stereo plug with an
insulator sleeve, and using different insulation washers and spacing in
the jack, 1500V was still pushing the limits of the design.  That was
also right at the voltage limit of the cable they used.

Don't shoot the messenger.  This is just a design perspective.

Best to all,
Fred (still a dyed-in-the-wool Heathen)

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