MJATS: Construction of DC Probe for VTVM

[Michael Donnelly]

Mike Donnelly
MJATS at JUNO.COM
--------- Begin forwarded message ----------
From: MJATS
To: Heath
Subject: Construction of DC Probe for VTVM
Date: Fri, 24 Apr 1998 21:05:25 -0400
Message-ID: <19980424.210526.16574.0.MJATS at juno.com>

WOW! I received so many requests for the construction of a DC probe for
VTVM's that I decided I would post it to this list that way it can be
filed away if anyone else comes across a VTVM without a probe.

As I mentioned in a prior post I constructed a DC probe from readily
available parts at Radio Shack. This probe is suitable only  for the
early Heath VTVMs that had SEPARATE jacks for a DC probe and  AC/OHMS
probe. This probe will NOT work with the later model VTVM's that have a
single jack for the probe with the DC to AC/OHMS switch built in. The
parts needed are as follows:

Catalog #278-704 Test Leads.
Catalog #278-1271 Shielded cable.
Catalog #278-1627 Shrink tubing assortment.
Catalog #274-1536 (or similar) .25 inch 2 conductor phone plug
1 Megohm, 5% resistor either .25 watt or .5 watt (preferred). Use a known
good ohmmeter and select a resistor with closest reading to 1 MEG.

Make sure the test leads are the type where the metal prod screws into
the probe body. You will be disassembling the red probe to make the DC
probe. The shielded
cable I used is rather stiff and inflexible, but very strong and well
insulated. If you can find more flexible shielded cable go ahead and use
it. I suppose RG-58 or RG-59 coax would work too. But DON'T go cheap on
the cable. Remember that this cable could pass in excess of 600 volts,
especially if you use the probe for measurements on tube equipment.
Better to be safe than sorry.

Step-by-step Instructions (Sound familiar???)

1. Take the red test lead and unscrew the metal prod from the probe body
until it comes free and exposes the red cable. Cut off the red cable
where it joins the metal prod and put aside. It will not be used again.
Save the red probe body and the metal prod. The black test lead will not
be used either, although now you have a spare AC/OHMS probe!

2. Take one end of the shielded cable and strip the outer jacket back
about an inch, exposing the braid. Peel the braid back and cut it flush
with the outer jacket. Make sure there are no loose braids.  Strip the
center jacket about a half inch to expose the center conductor.

3. Cut the leads for the resistor to about .5 inch or so. Solder one end
of the resistor to the center conductor of the shielded cable. Take a
suitably sized piece of shrink tubing and slide it over the body of the
resistor to cover the solder joint back to the center jacket on the
shielded cable. Leave to other lead on the resistor free and uncovered.
Use a hair dryer or heat gun to shrink up the tubing.

4. Take the end of the shielded cable with the resistor, and starting
from the back end, slowly insert the cable/resistor down the probe body
until the bare resistor lead is exposed out the front end (the end with
the threads for the prod) a few inches. NOTE - depending upon the
shielded cable used you may have to slightly ream out or cut off the
chamfer on the back side of the probe body to get the shielded cable to
go inside. I purposely designed it to be a tight fit so it would not
easily pull out. And in a later step shrink tubing will be added as a
strain relief.

5. Take the metal prod and bare resistor lead and carefully solder them
together. Once cooled, very slowly and carefully engage the metal prod to
the threads on the probe body. Turn the probe body slowly until tight. Do
not allow the shielded cable to turn or twist while turning the probe
body.

6. Find a piece of shrink tubing that will comfortably fit over the
shielded cable and the back half of the probe body. Cut this shrink
tubing to approximately &&&& inches long.
Slide the tubing from the free end of the shielded cable until it goes
down the probe body about &&&& inches. It should also be covering the
shielded cable about &&&& inches. Once properly placed use a hair dryer
or heat gun to shrink the tubing. This tubing will be an effective strain
relief.

7. That is basically it. You just need to strip back and solder the
shielded cable to the .25 inch phone jack (center conductor to tip, braid
to outer shell). And of course....last but no less
important...re-calibrate the VTVM to a known standard. I made up my probe
several years ago and works just fine. If anyone has questions or
comments please let me know. Great to be part of the 'Greenies' group!

Mike Donnelly
MJATS at JUNO.COM
--------- End forwarded message ----------

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