How to a adjust a meter movement?

[Antonio Vernucci]

I have recently bought a hi-quality multimeter (AVO eight, mark VI) built in
1984.

The multimeter gives low readings (-15%) on all AC and DC ranges. After many
checks, I have fully isolated the movement and I have determined that it goes to
full scale with a current of 44 uA instead of the nominal 37.5 uA. I have
carefully checked that there is no shunt in parallel.

I presume there must be some way to adjust the meter sensitivity, but I am not
sure how to do it One way could be to reduce the tension of both helical
springs, but I am afraid I would not be able to recover a 15% loss that way.

On the other hand I do not think the problem is with the magnet aging, as I have
an older AVO multimeter (model eight, mark 2) which was built a few decades
before and still works perfectly.

Morever, the question arises why the meter sensitivity has degraded.

Any help?

Tony I0JX

[COLIN LAMB]

Hi Tony:

I do not have a good answer for you, but do have a couple of suggestions.

1. Do not assume that because a magnet used by the same company decades
earlier is good, that the magnet made later is also good. Often, in
production, later manufacturing methods are developed to save money. I have
also seen where production methods are changed and for a few years, the
material has limited lifespan, then the company converts back to the
original method.

2. Check to determine if the same lack of sensitivity occurs at the low end
of the meter. If not, that might indicate a mechanical drag.

3. I salute your willingness to explore new areas of restoration, but make
sure, before you operate, that you have the right organ.

73, Colin K7FM

[Tio Pedro]

"Antonio Vernucci" > wrote in message
. ..
>I have recently bought a hi-quality multimeter (AVO eight, mark VI) built
>in 1984.
>
>

If the bare meter movement is showing a loss of sensitivity, my
first guess would be a weakening field magnet. It would be a
bit unusual for a modern instrument, however....

Pete

[Antonio Vernucci]

> 2. Check to determine if the same lack of sensitivity occurs at the low end
> of the meter. If not, that might indicate a mechanical drag.

> 73, Colin K7FM
Thank you for your suggestion.

After completely disassembling the multimeter, I got access to the front spring
of the movement and noted that first turn had just overlapped the spring binding
post, possibly due to a shock. Just touching the spring, it went back to normal
with no deformation at all. Now the full scale current is exactly what it should
be.

What amazes me is that I could not find any visible adjustment point (e.g. a
variable magnetic shunt). Evidently they were able to produce very precise
meters requiring no adjustment.

73

Tony I0JX

[Antonio Vernucci]

> If the bare meter movement is showing a loss of sensitivity, my
> first guess would be a weakening field magnet. It would be a
> bit unusual for a modern instrument, however....
>
> Pete


Thank you for your suggestion.

After completely disassembling the multimeter, I got access to the front spring
of the movement and noted that first turn had just overlapped the spring binding
post, possibly due to a shock. Just touching the spring, it went back to normal
with no deformation at all. Now the full scale current is exactly what it should
be.

What amazes me is that I could not find any visible adjustment point (e.g. a
variable magnetic shunt). Evidently they were able to produce very precise
meters requiring no adjustment.

73

Tony I0JX

[Scott Dorsey]

Antonio Vernucci > wrote:
>
>What amazes me is that I could not find any visible adjustment point (e.g. a
>variable magnetic shunt). Evidently they were able to produce very precise
>meters requiring no adjustment.

Yes, the whole key to the thing is that the number of windings is the only
thing that actually controls the sensitivity. D'Arsonval figured this out
a century and a half ago and we still use his mechanism for most meters.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[bw]

"Antonio Vernucci" > wrote in message
. ..
>
> What amazes me is that I could not find any visible adjustment point (e.g.
> a variable magnetic shunt). Evidently they were able to produce very
> precise meters requiring no adjustment.
>

Good find. Better quality meters were rated to 1 or 2 percent at FULL SCALE.
Also, many are rated for mounting on a steel panel. Many are not so rated.
The presence of external magnetic fields WILL affect the reading.

Sometimes you just have to go with the real meter indication and re-mark the
scale to the correct numbers.
I've done that a number of times, it shows the non-linearity of lesser
quality meters.

[Paul P[_2_]]

> The presence of external magnetic fields WILL affect the reading.
>
And don't forget the static charge on the plastic or glass face after you
spotlessly clean and polish it. A little spit may help.

Paul P

[Edward Knobloch]

bw wrote:
> "Antonio Vernucci" > wrote
>> What amazes me is that I could not find any visible adjustment point (e.g.
>> a variable magnetic shunt). Evidently they were able to produce very
>> precise meters requiring no adjustment.
>>
> "bn" wrote
> Good find. Better quality meters were rated to 1 or 2 percent at FULL SCALE.
> Also, many are rated for mounting on a steel panel. Many are not so rated.
> The presence of external magnetic fields WILL affect the reading. <snip>
>

Hi,
I picked up a Weston type 301 5mA d.c. meter marked for use
with a 1/8" steel panel.
I find it takes about 6.1 mA to deflect to full scale
without a panel. That sounds as if the presence
of a surrounding steel panel would increase the meter sensitivity
by about 20 percent.
Could someone explain this, please?

Thanks,
Ed Knobloch

[Antonio Vernucci]

> Yes, the whole key to the thing is that the number of windings is the only
> thing that actually controls the sensitivity. D'Arsonval figured this out
> a century and a half ago and we still use his mechanism for most meters.

I trust what you write, but it is difficult for me to understand why the meter
sensitivity is not affected by the counter springs tension.

73

Tony I0JX