IM-25 modification

[Ron]

Kenneth G. Gordon wrote:
> 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.
>   
Be careful, though. Alkaline replacements -- while
physically identical to the mercury cells -- output
1.55V. Light meters rely on the 1.35V provided
by mercury cells.

I've seen on the web a few regulator designs to
reduce 1.55V to 1.35V. The problem with these
is their current draw, that reduces life of the cell.
There is no component I know off that will drop
a constant 0.2V with varying current and/or
temperature.
> 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.
>   
Exactly. 

> 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.
>   
Thanks much for your response. I was hoping for
a better solution I might have overlooked, but, I
guess, no such luck...

-- Ron


> Ken Gordon W7EK

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