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Old 8th May 2018, 10:12 pm   #1
DPUGH72
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Default Avometer 9 SX needle not damping

Have a avo model 9 sx and it works and seems reasonably accurate but the needle swings wildly, for example when you zero on ohms range then release leads and touch together needle swings well past the zero before resting. whereas I notice my other Avos the needle always moves smoothly across the dial. Could this be something to do with a certain resistor wired across the movement that can causes this damping? or is something more serious causing this.
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Old 9th May 2018, 7:41 am   #2
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Default Re: Avometer 9 SX needle not damping

I've got a few like that, haven't got to the bottom of it yet.

Out of interest does it dampen ok when + and - are shorted with left switch to 50uA and right to DC?
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Old 9th May 2018, 12:55 pm   #3
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Default Re: Avometer 9 SX needle not damping

After reading this Thread, out of interest I thought I would check my 9sx,
as I can't say I had noticed anything unusual with the Meter Movement.

However, after checking, my 1971 9sx, the Pointer seems quite Damped.
My 1963 9sx however, the Pointer does swing quite a way past the Zero
and seems a lot less Damped.

Since the Electrical Damping (The Shunt Chain) must be pretty much
identical, I can only assume the difference must be down to the Meter
Movement itself.

Thoughts that come to mind are:
a) Pivot adjustment How Tight.
b) Spring Tension It wouldn't take much of a difference on such a sensitive Movement.

Just my thoughts, I'm sure there are a lot more people out there with far more knowledge
on the Meter Movements than me.


Ian
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Old 9th May 2018, 4:48 pm   #4
DPUGH72
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Default Re: Avometer 9 SX needle not damping

Yes I have noticed it does seem to damp when set to dc and 50uaA similar to my other Avos. this model is dated july 1965.That is interesting superscope as you say your 1971 9sx is more damped than 1963 model as my 9sx is from a similar time period,maybe as you say it is the movement itself.
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Old 9th May 2018, 5:30 pm   #5
The Philpott
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Default Re: Avometer 9 SX needle not damping

I haven't encountered this on a 37.5uA movement, but my first thought would be to check if the needle zeroes consistently after use. (plus or minus 1/4 to 1/3 of one per cent of FSD) Next- is the movement balanced properly? (has the tail weight lost some mass?)

It may well be neither of these things but they are the simplest to check.
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Old 9th May 2018, 7:43 pm   #6
Superscope
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Default Re: Avometer 9 SX needle not damping

On a working Meter, set to DC 50uA and the Terminals Shorted, there will be extra
(Electrical) Damping regardless. Hence it would always be more Damped than an un-shorted Meter .

In order to isolate the difference, I won't call this a Fault as I don't believe it is, Like
for Like must be compared.
Problem is, actual Damping is pretty much a Subjective matter without a proper Test
procedure. Not sure one exists.

That is one of the reasons I tested two allegedly identical AVO's.

Assuming the Shunt Chains are of identical Resistance, the only difference must be
in the Meter Movement itself.

I will when I get a spare Hour carry out a few Tests on a Range of AVO's from different
Periods.
Although, I have the feeling, older Meters might be less Damped, but that is only a feeling at this stage.



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Old 9th May 2018, 9:46 pm   #7
The Philpott
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Default Re: Avometer 9 SX needle not damping

Just shorted the leads on my 'second best' Model 8 (a Mk III from 1968) on all of the resistance ranges. It's quite tricky to time how long it takes for the needle to come to a complete stop, but it appears to be 2.9 to 3.2 seconds...A Model 9 of the same generation should be similar, i would say.

A less sensitive meter from an earlier generation (Model D) takes around 1.2 seconds.

Dave
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Old 15th May 2018, 6:50 pm   #8
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Default Re: Avometer 9 SX needle not damping

DPUGH72,

You describe the characteristic of you meter movement by stating that the pointer swings "wildly".

This is usually due to an open circuit in the shunt chain which is connected permanently in parallel with the movement in these meters. The basic movement has a sensitivity of 37.5 microamperes and a resistance, including the swamp, of 3333 ohms. The shunt chain has a total resistance of 10 kohms so that the parallel combination is 2500 ohms. At full scale deflection, 50 microamperes flows in the parallel combination with 12.5 microamperes in the shunt.

It is the shunt which provides most of the damping on the moving coil and pointer.

Could you please check your meter again as you may find it is one third more sensitive than it should be. Also check that as many of the current ranges as you can test are fully functional.

If you need a circuit diagram or other information, please let us know.

PMM
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Old 16th May 2018, 5:47 pm   #9
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Default Re: Avometer 9 SX needle not damping

Following on from Dave's idea of Timing the Meter Movement from FSD
to a complete stop, I tested a number of Meters I had to hand and find the following:

1943 Model 47A 1.8 Secs
1944 Model 48A 1.8 Secs
1956 Model 8MkIII 3.0 Secs
1963 Model 9sx 2.2 Secs
1966 Model 9MkII 2.0 Secs
1970 Model 8MkIV 2.5 Secs
1971 Model 9sx 2.5 secs
1979 Model 8MkV 2.2 Secs

The Meters above are all working and in Spec or close too.

The above are NOT calibrated results, just me and a Stopwatch and a bit
of Judgment as to when the Needle actually stops moving.

To me at least, 2.5 Sec felt very slow and under-damped compared with a
2.2 Sec Movement, but that only amounts to 0.3 of a Second!
I think a lot of it is Subjective.

Ian
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Old 19th May 2018, 2:37 pm   #10
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Default Re: Avometer 9 SX needle not damping

Quote:
Originally Posted by pmmunro View Post
It is the shunt which provides most of the damping on the moving coil and pointer
I had these exact symptoms on an AVO 8 MK-II I bought recently. An o/c fixed shunt explains both the lack of damping and the over-reading. With reference to the circuit diagram, a few checks should locate the o/c resistor. They are close tolerance types, in my case wire wound. I replaced it with a combination of fixed resistors to get the exact value.
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