[Boatanchors] VTVM QUESTION

[Garey Barrell]

Dan -

I'm not familiar with the WV77E specifically, but I suspect that all 
that is in the probe is a 1 megohm resistor.  Just about every VTVM of 
that era had a 1M resistor at the probe that was put in series with the 
probe lead for  _DC_  measurements.  The resistor was there to 
"decouple" the probe and lead from the high impedance measurements that 
the VTVM was designed to make, such as the grids of tube stages.  The 
resistor was shorted by a switch on the probe for AC and Ohms measurements.

Some VTVMs had separate probes for AC / Ohms and DC for the same reason, 
with the resistor in the DC probe. 

There are "modifications" around to move the 1M resistor inside the 
cabinet to allow use of "plain" probes.  Unfortunately, while this 
sounds like a good idea, it circumvents the reason the resistor was 
there in the first place!!  Just a classic example of "improvements" to 
circuits made by people who don't understand the design, thinking 
they're smarter than the engineer who designed it!

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
<http://www.k4oah.com>



k0dan wrote:

>This might be a tad off topic, and if so I apologize in advance.
>
>I picked up an RCA Volt Ohmyst (WV77E) VTVM. No probes. Apparently the
>original AC probe has a diode built in but it's not spec'd in the schematic
>or manual I got off BAMA.
>
>Is there any reason I shouldn't build myself a probe with a junkbox bullet
>diode (1N5401, 1N4816, etc.)? If this requires a more spec-focused approach,
>what needs to be inside the probe?
>
>Thanks and 73
>
>Dan
>K0DAN
>
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