IM-25 modification

[Rinkie Pollack]

Someone beat me to the punch by referencing the Electric Radio article.
 
By background is in the Hearing Aid biz, and we used to use mercury batteries.  They have been replaced by zinc/air batteries, which I believe are similar in that they maintain full strength until the end.  The current ones are marked 1.4 volts, and are very inexpensive.  The 675 size is the largest (all sizes same price, typically), and if it will fit physically, may be worth a try. 
 
I'm not enough of a technical type to know if that's close enough, but I would think it's worth a try.  I don't think they'll last too long, as they are activated when the tab is pulled off and air gets in, but at places like Costco they're only about 30 cents each on a card.
 
Ron K2RP
 
The ones we have now are marked

--- On Fri, 9/18/09, Ron <ronami at YAHOO.COM> wrote:


From: Ron <ronami at YAHOO.COM>
Subject: IM-25 modification
To: HEATH at LISTS.TEMPE.GOV
Date: Friday, September 18, 2009, 6:14 AM


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. 

I wonder if anyone here owns and/or uses an IM-25 and, 
if so, how did they solve the problem.

Personally, after much head scratching, I've come up with 
a simple and cheap mod for the unit that removes the 
need for the Mercurt battery. An addition of just three 
components allowed me to power the Ohms circuit by the 
existing alkaline dual C battery. This shouldn't affect the 
battery life. Accuracy is not quite as good as a Mercury 
cell would have provided but certainly, more than enough 
for the purpose. 

The three components are a diode, resistor and trim pot. 
Totsal cost is about 5 bucks. The trim pot could be done 
without, if one is willing to go through the hassle of 
selecting the resistor to match the diode and an internal 
transistor (it's a set and forget situation). 

-- Ron




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