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 Post subject: making old VTVM's and VOM's more accurate
PostPosted: Jan Sat 16, 2016 3:33 pm 
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This is an old technique that I've been using for 40+ years.
The idea is trimming resistors to get exact values. Also useful
for replacing fried wire wound resistors.

Use a carbon resistor that is slightly *lower* in resistance than the
original. Carbon resistor resistance can be trimmed by cutting into
the case and into the carbon. I use a small triangular file but a small
hack saw or dremel tool works fine too. And the trimming can be done with
the circuit powered up which is handy for meter circuits.

Once you get the resistance value you want, clean off all dust and
seal the area with nail polish. One important consideration is you'll
be reducing the power handling capability a bit depending on how much
trimming you do.
Steve

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Jan Sat 16, 2016 4:46 pm 
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Sure is an old technique. Found it in a Triplett dual meter VOM from the early 30's.

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Jan Sat 16, 2016 6:11 pm 
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That's the way we used to trim thick film resistors back in the '70s.

Image

The resistors were "printed" using a thick paste, then fired to cure the material. Afterward, a laser or sandblaster made cuts to achieve the "exact" value needed. This was necessary because the resistive paste could not be screened with absolute precision.

Rich


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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Tue 02, 2016 4:01 pm 
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Location: Central Pennsylvania
The cut to adjust method works on ceramic capacitors as well. I knew a small company that made3 crystal filters that ground the corners off of ceramic caps to get the precise, low values caps needed to set a crystal filter on spec


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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Wed 03, 2016 12:33 am 
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I once visited the Zenith factory in Chicago back in 1980, where I saw how they produce thick film circuits in high volume production. On the fly, they laser trimmed resistors by cutting snippets in an alternating pattern, leaving a longer conductive path, until a certain value was achieved. This was done in 1 or 2 seconds per circuit.


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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Thu 18, 2016 1:01 pm 
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Filter companies used to tune LC and Crystal filters by cutting the ceramic caps with sharp dykes and it works well but the caps will be susceptible to moisture unless they are sealed. For filters it was common to solder a can over the assembly but a cap left open will become leaky as sulfides from the air condense on it and moisture creates a conductive film.

Carbon comp resistors left open to the air will also have problems as carbon will slowly oxidize and the values will change over time unless you reseal the resistor.

thick film resistors are usually not a problem as they consist of a resistive material in a binder that seals it.


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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Thu 18, 2016 5:19 pm 
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If I was going to "trim" a carbon resistor to replace a precision part I might be worried about long term stability issues.

What I would do, and I caught heck for even suggesting this in the past, is to use the highest value precision resistor possible that is lower in value, and then trim as needed using a cheaper resistor in series... that could be the carbon resistor being filed to value. That will assure that the major percentage of the resistance value would be provided by a stable and accurate part, and reduce long term drift.

But, I will point out that the accuracy for most VTVM and VOM designs are limited by other factors. Even short term drift will affect the readings.

Pete

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Fri 19, 2016 4:09 pm 
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Peter Bertini wrote:
... the accuracy for most VTVM and VOM designs are limited by other factors. Even short term drift will affect the readings.

I will second Pete. The way to get an accurate VTVM is to buy an accurate VTVM, and if it is old (say, an HP 400H or 412A), to repair and recalibrate it.

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Fri 19, 2016 11:08 pm 
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There's more to precision resistors than accurate value.

In fact that's one of the less-important characteristics in most applications.
Values can be adjusted by using trimmers or other techniques.

The main advantage of precision resistors is stability, over temperature and over time.
They stay at whatever value they have despite environmental assaults and affronts.

The Leeds & Northrup "rabbit ear" standard lab resistors hold their value to 6 digits over many decades.
The several sets we have at the cal lab were made in the 1950s, and haven't drifted measurably since.

- Leigh

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Fri 19, 2016 11:14 pm 
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In my early ham days I learned how to remove a crystal from its housing and polish it w/toothpaste to wear away some material in order to raise the freq a bit.

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Fri 19, 2016 11:41 pm 
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Pbpix wrote:
In my early ham days I learned how to remove a crystal from its housing and polish it w/toothpaste to wear away some material in order to raise the freq a bit.
That was common practice back in the day when MIL-surplus crystals were plentiful.

It was also possible to lower the freq slightly by rubbing a soft pencil over the crystal face.
I don't know how long such a mod would last, or how stable it was.

- Leigh

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 Post subject: Re: making old VTVM's and VOM's more accurate
PostPosted: Feb Fri 19, 2016 11:47 pm 
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Leigh wrote:
Pbpix wrote:
In my early ham days I learned how to remove a crystal from its housing and polish it w/toothpaste to wear away some material in order to raise the freq a bit.
That was common practice back in the day when MIL-surplus crystals were plentiful.

It was also possible to lower the freq slightly by rubbing a soft pencil over the crystal face.
I don't know how long such a mod would last, or how stable it was.

- Leigh

Yeahh... now I remember the pencil lead trick too. Forgot that.

I've still got some of those military type crystals around here... lol

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