[Heath] IM-18 Test Probe

Tue May 31 12:12:52 EDT 2016

There have been discussions lately about plugs on Heathkit VTVM probes.
Gary Clark is the Meter Master and his comments about the issue follow.

Vr,

Gary H. Harmon, Jr. - K5JWK - HAM Radio and ATARI Archaeologist

6003 Archwood

San Antonio, TX 78239-1504

(210) 657-1549 <home>

(210) 884-6926 <cell and text>

(210) 657-1549 <FAX with prior notification>

gharmon at idworld.net

"Retirement = Every day is a Saturday except Sunday"

http://www.grissomroadcoc.org

From: Gary Clark [mailto:gc at sdlcforms.com] 
Sent: Thursday, May 26, 2016 9:45 AM
To: Gary H. Harmon, Jr.
Subject: Re: FW: [Heath] IM-18 Test Probe

Gary,

The combination probes that came with the Heathkit IM-11 and later VTVMs as
well as EICO, PACO, RCA, B&K and others are no longer available. Nobody
makes them any more. Furthermore, after some period of use they become
unreliable, particularly causing problems on the lowest ohms range. The way
VTVMs were designed previously, with three separate test leads was a far
better arrangement.

I refurbish and re-sell all different makes of VTVMs and I solve this
problem with all of them the exact same way. I make a new test lead set and,
if the VTVM has a microphone connector instead of a ¼ inch phone jack, I
replace it with a phone jack.

When I make the replacement test lead set I use high quality ¼ inch phone
plugs I buy directly from China on ebay. (All the best stuff comes from
China.) I use a standard mono phone plug and, even though the insulator
spacing is much smaller than on those original Heathkit probes, I have never
had a problem on the high voltage range. BUT, if you are worried about this,
use a ¼ inch stereo phone plug and leave the ring part unconnected. This
doubles the insulated spacing between the sleeve and the tip.

In addition, unless the instrument in question has ac voltage ranges lower
than 1.5 volts full scale there is no reason to use a shielded input cable.
RCA sold a 4.5 inch VTVM for several decades that had three separate input
leads and none of them used a shielded cable. You don’t need a shielded
cable on the dc input anyway. Only the ac input can benefit from one, and
then only on very low ac voltage ranges. 

The way I make the replacement input lead set is shown in the drawing below.

Regards,

Gary Clark

(The VTVM guy)

-----Original Message-----
From: Heath [mailto:heath-bounces at puck.nether.net] On Behalf Of Richard
Sent: Thursday, May 26, 2016 4:41 AM
To: Heath at puck.nether.net
Subject: [Heath] IM-18 Test Probe

The IM-18 test probe has a 1/4 inch phone plug as the interconnect with the
meter and the test probes. This plug has a wider insulator between the
center pin and ground on the plug in order to be able to handle the higher
voltage measurements. Has anyone found a reasonable replacement for this
plug? I have several meters without the test cable and I would like to make
some appropriate cables. The switchable DC / AC-Ohms probe can be replaced
with two separate probes, so that is not a problem.

Richard KC9UB
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