IM-25 modification

[Mark V. Johnson]

I too had an IM-25 and loved it. I was able to find a battery that was close, but it took a lot of looking and was expensive. 

 

One suggestion. I'd never get rid of a VTVM and depend on the IM-25. If you read the fine print on the IM-25 spec's, it says that the meter is not suggested for use in "RF Fields." I found that out the hard way one day when working on a transmitter. RF got into the IM-25 and blew it up!

 

My fault, no question, but VTVM's will easily handle RF and non RF applications. So, be careful with the IM-25. 

 

Mark V Johnson VE3DJU/VE3DDI 
> Date: Fri, 18 Sep 2009 06:14:22 -0700
> From: ronami at YAHOO.COM
> Subject: [HEATH-TEMPE] IM-25 modification
> To: HEATH at LISTS.TEMPE.GOV
> 
> 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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