[Heathkit] VTVM Model IM-28

[Garey Barrell]

Alf -

The probe is actually pretty simple.  There is a 1M ohm resistor right at the tip, in series with the Positive lead, 
that is used ONLY when measuring DC.  The resistor is shorted out for AC and Ohmmeter readings.  Depending upon your 
intended use, you can make two positive probes, one to use for DC WITH the resistor, and another with NO resistor.  Or a 
slightly larger diameter probe could be built with a small switch to bypass the resistor.

If you do any tube or other high impedance measurements, please do NOT fall for the internet mod to move the resistor to 
the switch inside the meter.  This is a typical case where some 'genius' thinks he is smarter than the Heath engineers. 
There is a reason Heath engineers went to the effort and expense to develop the probe design.  The resistor is there to 
decouple the test lead from the measurement point, and is the second most valuable feature of a VTVM over a DMM.

73, Garey - K4OAH
Campton Hills, IL

Drake 2-B, 2C/2-NT, A, B,
C-Line, TR-4, TR-4C/CW/wRIT
WWW.K4OAH.COM

Alf Fisher wrote:
> Hi,
> I have just been given a VTVM Model IM-28 minus its Test Probe and leads.
> I've downloaded the assembly manual so have quite a lot of information about it such as setting up etc.
> I would like to know the type number of the optional RF probe or what is inside it, so that if necessary I can make one.
>
> I don't suppose there are any of these probes around but if there are, I'd be interested.
>
> Vy 73,
> Alf
> G3WSD
>