[Heathkit] Solid State VOM model IM-25 Help needed

[Ron via Heathkit]

Dave,
Belated answer, I know   :-)

Zeners are nowhere near the accuracy and stabity of mercury cells. What is needed 
to duplicate the original IM-25 performance is a precision voltage reference. For my 
IM-25 I used a LM185 (plus four 1% resistors). If you still wish to fix your IM-25, do
see these two files:
https://www.dropbox.com/s/ggfncxv6cbrb7nf/Heathkit_IM-25_MercuryCellReplacement.jpg?dl=0https://www.dropbox.com/s/976hoxxpwdelw80/Heathkit_IM-25%20Mod.jpg?dl=0

Regards,
-- Ron 
 
      From: Walt - WB2VSJ <wb2vsj at earthlink.net>
 To: 'David' <maxtaz at shaw.ca>; Heathkit at mailman.qth.net 
 Sent: Wednesday, April 8, 2015 8:35 PM
 Subject: Re: [Heathkit] Solid State VOM model IM-25 Help needed
   
Mercury batteries are no more.  Closest you can get is a hearing aid battery
at 1.45V (Zinc Air) but they don't last too long.  That battery is also used
to bias a transistor in addition to providing a 1.35 V reference  for the DC
calibration. 

I was going to use something like this in a few of my meters that also used
mercury cells:

http://www.ebay.com/itm/5pcs-LM2596s-DC-to-DC-Buck-Converter-Adjustable-Powe
r-Step-Down-Module-/281436507027?pt=LH_DefaultDomain_0&hash=item4186ec4b93

Walt - WB2VSJ


-----Original Message-----
From: Heathkit [mailto:heathkit-bounces at mailman.qth.net] On Behalf Of David
Sent: Wednesday, April 08, 2015 19:51
To: Heathkit at mailman.qth.net
Subject: [Heathkit] Solid State VOM model IM-25 Help needed

Greetings. I was at a junk flee market in Yuma where I found the IM-25.
What I would like to know is what is being used to replace the 1.4 volt
Mercury calibration battery. The one in this unit I got is dead.
Thanks for any help you can give me.

Dave    VA7DG 


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