VTVM 1/4" plugs..

[Robert Myers]

The point here is that there are times when you may measure the voltage
across a suspect resistor by putting this probe across it directly, or on
wires leading *to* the resistor in question; all this being done with the
VTVM *already* on the other end of the probe.  Now say you then snip one end
of that that resistor's lead (or unsolder it completely) becuase you want to
replace it.

Upon doing this, if you leave the probe connected to the circuit as it was
(at the resistor mounting holes, say), and now disconnect/unplugged the VTVM
from the *other* end of the probe, you will now have the full source voltage
across that probe -- which is to say, that probe will be at an open circuit
section of a circuit, and actually will have whatever the open circuit
source voltage would be, as if right across the isolated source on its own.
The current is 'zero' (until the voltage causes an arc), and this is why it
will see the huge (it could be anything) source voltage -- even if the
impedance of that source is very high (as it likely would be for extremely
high voltages)

A extremely large resistor inside the probe itself can prevent it from
actually being across a truly open circuit.

Cheers,
-- Rob Myers

-----Original Message-----
From: Heathkit Owners and Collectors List [mailto:HEATH at LISTS.TEMPE.GOV] On
Behalf Of Emil Dular
Sent: April-03-09 1:52 PM
To: HEATH at LISTS.TEMPE.GOV
Subject: Re: [HEATH-TEMPE] VTVM 1/4" plugs..

This exchange got me curious, so I dug out my Heath 336 high voltage probe
instruction sheet, dated 24 Jun 1966.  Just as Paul observed, the old single
pin Amphenol mic conector (Heath P/N 432-1) and common Switchcraft phone
plug (Heath P/N 438-3)  are used, as was common back in the day of tube
TV's.

The sheet clearly states: "Caution never use the probe on DC voltages above
30,000 volts. While the conductors inside the handle and the test lead never
carry more than 300 volts when the probe is properly connected, THESE PARTS
WILL BE EXPOSED TO THE FULL 30,000 VOLTS, IF NOT CONNECTED TO THE VTVM."

Pretty clearly, the standard phone plug is good for ranges most VTVM's of
that era would see, 500 volts was pretty common with a 1000 volts on a nicer
unit.

Emil WD4SCZ

> I just put 1560 VDC on a typical phone plug.  It is even a stereo plug.
> All sections tested.  It didn't show any leakage or signs of arcing over.
>
> I won't say the test is all conclusive.  Just thought I'd give it a test.
> Bob W5UQ
>
>
> Kent Kressenberg wrote:
> > PJ-055B plugs are often still available... keep an eye on ebay.
> > bought a lot of TWELVE (12) of them a few months ago...
> >
> > 73 de WB4SYU,
> >
> > Kent
> >
> >
> >
> >> If you can find some military surplus headsets (oldies) they usually
> >> have a
> >> PJ-055B plug. These have a much wider insulating band between the tip
> >> and the
> >> sleeve. I have a couple here with a 3mm insulation ring, one of the
> >> Heath plugs
> >> has a 2mm insulation. All my "standard" plugs appear to be about 1mm,
> >> depending
> >> on where they were made. I have even thinner insulation rings in my
> >> parts bin for
> >> guitar cables (OK for audio).
> >>
> >> Up to the recent exchanges of notes on these I had never really paid
> >> much
> >> attention to the insulation rings.
> >>
> >> Cheers
> >>
> >>
> >> Keith Harvey VA3KRV


The 1/4" phone plug and the familiar mono screw on mic connector were
> the common choice for probe attachment back in the radio tv repair day.
> My observations, FWIW. Hope it helps.
> Paul K0UYA

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