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 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Wed 20, 2019 12:36 am 
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bob91343 wrote:
Years ago HP solved the meter linearity issue by automating the scale printing for individual meters. I don't know the dates they did that or for which models.

It's described in the March, 1961 issue of the HP Journal:
https://www.hpl.hp.com/hpjournal/pdfs/I ... 961-03.pdf

It was first applied to the HP 400H, 411A, 412A, and 425A.

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Last edited by stevebyan on Nov Fri 22, 2019 6:01 am, edited 1 time in total.

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 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Wed 20, 2019 12:39 am 
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bart wrote:
Interesting to see Stevebyan, I will start restoring and create a similar document with the results. Really good job there!!

Thanks. I also recorded the pre-replacement values of the resistors, just to get some data on how many drift out of tolerance.

I described the restoration of my IM-28 in this thread:
https://antiqueradios.com/forums/viewtopic.php?t=365180

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 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Wed 20, 2019 12:50 am 
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Barry H Bennett wrote:
So, how many million $ and million days did it take?

I only replaced two precision resistors, though the AC accuracy suffered a bit because I should have replaced two more resistors in the AC divider. So cost was about 40 cents. It took an evening to make the initial resistor measurements, a couple of evenings to actually restore the VTVM, and an evening to calibrate it.

Barry H Bennett wrote:
And how can one do better than the linearity of the meter movement and dial scale itself ? ;-)

I got lucky, the meter linearity and scale are better than spec.

I don't expect it to remain that accurate over time and temperature, but I'm confident that it meets Heathkit's 3% spec on the DC ranges (except the low end of the 1.5 volt range).

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