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 How to calibrate a VOM??

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Tommy_R



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Join date : 2010-10-17

PostSubject: How to calibrate a VOM??   Sun Oct 17, 2010 9:00 am

I have this VOM by Precision Apparatus model 120.

The ohms scales are accurate but the voltage scales both dc & ac are low by 7%.

Seeing that they are all low could one common resistor changed value? But which one??

I have included a link to my schematic, any and all help would be appreciated.





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Johnnysan



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Join date : 2010-10-12

PostSubject: Re: How to calibrate a VOM??   Sun Oct 17, 2010 2:56 pm

I would look at R17-21 and R30-34. Be careful to replace resistors with what is listed in the parts list. Sometimes PACO would use two resistors to make one value (and increase wattage), but the markings on the resistors did not add up to the correct value.
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Tommy_R



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PostSubject: Re: How to calibrate a VOM??   Mon Oct 18, 2010 10:36 am

Johnnysan wrote:
I would look at R17-21 and R30-34. Be careful to replace resistors with what is listed in the parts list. Sometimes PACO would use two resistors to make one value (and increase wattage), but the markings on the resistors did not add up to the correct value.

ok I will take a look see.

Your help is appreciated.
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easyrider8



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Join date : 2010-10-18

PostSubject: Re: How to calibrate a VOM??   Mon Oct 18, 2010 2:30 pm

If you look at the schematic you will see a few resistors which are common to the AC/DC voltage section. These would be R-4, R-16, R-22 these are the ones that would affect all the AC/DC readings.

Dave
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Mike C.



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Join date : 2010-10-13
Location : Central NH

PostSubject: Re: How to calibrate a VOM??   Mon Oct 18, 2010 2:42 pm

What are you comparing the Precision's readings to?If you are using a DMM,VTVM or an FETVOM,the Precision's readings will be lower in comparison because of its lower imput impedance...I would think.
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smeezekitty

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Location : Washington/USA

PostSubject: Re: How to calibrate a VOM??   Mon Oct 18, 2010 9:05 pm

Mike C. wrote:
What are you comparing the Precision's readings to?If you are using a DMM,VTVM or an FETVOM,the Precision's readings will be lower in comparison because of its lower imput impedance...I would think.
You could put the voltmeters in parell and compare them.
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Mike C.



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PostSubject: Re: How to calibrate a VOM??   Tue Oct 19, 2010 6:41 am

smeezekitty wrote:
Mike C. wrote:
What are you comparing the Precision's readings to?If you are using a DMM,VTVM or an FETVOM,the Precision's readings will be lower in comparison because of its lower imput impedance...I would think.
You could put the voltmeters in parell and compare them.

That is not at all what I am getting at.
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Tommy_R



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PostSubject: Re: How to calibrate a VOM??   Tue Oct 19, 2010 6:55 am

Mike C. wrote:
What are you comparing the Precision's readings to?If you are using a DMM,VTVM or an FETVOM,the Precision's readings will be lower in comparison because of its lower imput impedance...I would think.

I have a stable power supply I set to 10v and read to a calibrated Fluke DVM.
Seeing that the supply won't complain to a load of 20k/v or 10meg/v I would think that would not make a difference.

This meter is very cramped inside. Locating this resistors will be a trick and a half.

all help appreciated.

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majoco



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PostSubject: Re: How to calibrate a VOM??   Tue Oct 19, 2010 1:38 pm

As far as I can see (which isn't very far!) the only common resistor to AC and DC is the meter cal resistor R29.

You said that the Ohms readings were fine, but of course if R29 was out of tolerance, you could be compensating for this with the "Ohms zero" adjustment.

The series droppers are switched by the DC+/DC-/AC switch so I can't see that a change in any one of those will affect both AC and DC exactly the same.

The meter cal resistor R29 is said to be 1440 ohms but it may be an adjustable wirewound set in the factory.

Good luck! - Martin
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Tommy_R



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PostSubject: Re: How to calibrate a VOM??   Tue Oct 19, 2010 2:10 pm

majoco wrote:
As far as I can see (which isn't very far!) the only common resistor to AC and DC is the meter cal resistor R29.

You said that the Ohms readings were fine, but of course if R29 was out of tolerance, you could be compensating for this with the "Ohms zero" adjustment.

The series droppers are switched by the DC+/DC-/AC switch so I can't see that a change in any one of those will affect both AC and DC exactly the same.

The meter cal resistor R29 is said to be 1440 ohms but it may be an adjustable wirewound set in the factory.

Good luck! - Martin

Well, in this meter there are no trimmer resistors but I heard that usually resistors get old they increase in value, R29 is probably very accessible so maybe I can but my resistor decade box across it and parallel some resistance to lower the value.
Maybe.

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majoco



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PostSubject: Re: How to calibrate a VOM??   Tue Oct 19, 2010 3:33 pm

I have seen meter cal (or swamping) resistors made in the factory by winding too much thin resistance (hence high-ish) wire on an insulating former - then in "post production calibration" turns are removed until the correct meter deflection is obtained. Resistance wire is difficult to solder properly and often results in a dry joint which works for years until one day........

That's why it says in the parts list - Meter movement 50uA calibrated to 2560ohms.

I remember having to carry out a normal annual calibration check on a Beckman low resistance decade box - I think the highest resistance was 100ohms and lowest 0.01ohms. For some reason the 0.01 and the 0.1 ranges out of tolerance with high resistance, then found that compared to previous history all the results were high by the same amount. Opened it up and gave it a good looking at but nothing significant - started prodding around and found one particular buss wire joint gave wildly erratic readings - unsoldered the other end and the 8 gauge buss wire just fell off! Looking at the end with a magnifier showed the typical crystallisation structure of a dry joint - and this box had been in service and remained in cal for over twenty years!
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