VTVM Mod

[Randy Kaeding]

I recently saw a few requests for parts to make up or repair the probe for
Heathkit VTVMs that contained an AC-Ohms/DC switch right at the probe.

Someone came up with a mod back in the early 70s where the 1 megohm
resistor that is normally inside the probe can be installed inside the VTVM
right on the Function switch. This automatically switches the resistor in
and out as necessary when you turn the Function switch.

The following steps show you how to make this mod in the IM-11, IM-18,
IM-13, and IM28. Note that it is not an "authorized mod", but is easy to do
and easily reversible.

1.      If you have the manual for your VTVM available, refer to Figure 2 (IM-11
and IM-18) or Figure 7 (IM-13 or IM-28). If you don't have a manual, you
can still do the mod if you follow my directions carefully.

2.      Find the wire that connect between the "hot" side of the probe jack and
the Function switch. Note that the hot side of the jack only has one wire
connected to it. Disconnect the end of this wire from the lug of the
Function switch (this is switch lug 5). Leave it disconnected for now.

3.      There is jumper wire connected from Function switch lug 5 (where you
disconnected the wire above) to another lug on the Function switch (this is
lug 3). Remove this wire and discard it.

4.      Connect a 1 megohm, 1/2-watt resistor between Function switch lugs 5 and
3 (where the jumper wire was).

5.      Now connect the free end of the wire coming from the probe jack to
Function switch lug 3 (this is not the same lug from which it was
disconnected). You may have to lengthen this wire to make it reach lug 3.

6.      You can now use any straight-through probe with the VTVM, and do not
have to worry about switching the resistor in and out.

Good luck, and happy measuring.

Randy, K8TMK
"One of the Hams at Heathkit"
(25+ years)

--- --- --- --- --- --- --- --- --- --- --- --- --- --
To subscribe: listserv at listserv.tempe.gov
and in body: subscribe HEATH yourfirstname yourlastname
To unsubscribe:  listserv at listserv.tempe.gov
and in body: signoff HEATH
Archives for HEATH: http://www.tempe.gov/archives
--- --- --- --- --- --- --- --- --- --- --- --- --- --