IM-25 modification

Mark V. Johnson mvjohn at SYMPATICO.CA
Fri Sep 18 13:53:59 EDT 2009


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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Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

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