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Old 25th November 2010, 04:36 AM   #1
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Default Battery Question vs VTVM

Refurbishing a Heathkit IM-13 VTVM. Originally had zinc/carbon battery which I understand had a voltage of 1.55 and to calibrate the instrument, the meter has a red dot at the 1.55 spot and you would have tweaked a pot to align the needle with the dot.

What is available is an alkaline battery which has a voltage of 1.60; I am thinking that I need to calibrate the instrument by tweaking the pot to line up with 1.60 volts (not the red dot 1.55 volts).

Your thoughts and/or comments must welcome. I can attach a schematic if that helps.
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Old 25th November 2010, 04:57 AM   #2
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I would get a cheapie DVM from Harbor Freight and use it as a standard. Far more accurate than the Heath and only a couple of bucks brand new.
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Old 25th November 2010, 05:55 AM   #3
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Thanks for the reply. Its not an issue of not having a DMM as I am well covered in that area (Fluke). I am interested in recalibrating the VTVM with the batteries that are currently available today.
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Old 25th November 2010, 08:35 AM   #4
Enzo is offline Enzo  United States
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Can't answer directly, but with 1% resistors cheap and common these days, try it. Then see if your resistors measure up correctly.

There is calibration and then there is just daily use. Each time you start a measuring session, don;t you generally have to short the leads and turn the set-zero control anyway? They were not expecting a fresh battery in those meters every day. That zero control was there to compensate for battery variations.

As an aside, we mostly prefer alkaline batteries for performance reasons, but aren;t plain old common batteries still on the market? Usually called "heavy duty" or some such.
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Old 25th November 2010, 04:25 PM   #5
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Yes the zero is set each time the meter is used but that isn't involved in the battery circuit as during calibration etc it is done w/o the battery installed as it is getting the power from the AC line. My big box store didn't have those older type batteries; will look someplace else.
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Old 27th November 2010, 04:32 AM   #6
Enzo is offline Enzo  United States
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Oh wait. This is not about calibrating the ohms scales? They are using the 1.55v as a voltage standard to calibrate the DC volts scale against? For that all you need is a DC voltage you know to be a certain value then set this meter to it. it may not have the convenience of aligning to a dot, but should work just as well. If your flikes agree on some voltage, be it a battery or something else, then use that as a standard and set the Heath to it.
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