IM-25 modification

[Kenneth G. Gordon]

On 18 Sep 2009 at 6:14, Ron wrote:

> The IM-25 solid state VOM is a marked improvement over 
> the various VTVM models offered by Heathkit over the years. 
> It can measure current and its voltage Sensitivity is x10 better.
> 
> I recently got an IM-25, intending to replace my trusty IM-13 
> VTVM. Turned out, the IM-25 requires a 1.35V mercury 
> battery for the Ohms function, that is unobtainable 
> because Mercury batteries have been banned in most of
> the world. While it's still possible to find NOS cells, their 
> price exceeds the cost of the IM-25. 

Ron:

First of all, I don't have a schematic for the IM-25, so the 
following information may not be of much help with this 
problem.

Hoiwever, secondly, there is an article in the most recent 
issue of Electric Radio magazine in which the author 
replaced the battery in the "Ohms" circuit of his V-7 VTVM 
with a small circuit board using three diodes (two as voltage 
regulators), one cap, and two resistors, which uses the 
filament voltage as the power source.

Thirdly, I own an older 35 mm SLR camera which uses 
TWO of those mercury "button" cells to power the internal 
light meter. When I needed to replace those, a "local" 
battery house found some alkaline cells for me which were 
exact replacements.

Lastly, the reason for using mercury cells is that they 
maintain their rated voltage within a very few percent until 
immediately before they are completely dead.

In this case, except for those who don't want to build your 
circuit into their meters, I think your solution is the best I 
have heard of yet.

Ken Gordon W7EKB

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