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Old 8th Aug 2009, 3:09 pm   #1
logan123
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Default op amp for avometer movement

Hello all.
I would be grateful for any information on the following.

I have an avometer 8 mk 5 which reads low in a test set up. It takes about 40 micro amps to drive the movement full scale and it should only take 37.5.

I have the movement out of its case and on the advice of an instrument engineer I have tried to alter the magnetism but to no avail.

What I would like to try now is an amplifier which I can put into the case which will drive the meter movement to the correct position. I have an earlier avo which has a magnetic shunt but this model has none. All suggestions are most welcome.

Thank you,

Logan123

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Old 9th Aug 2009, 12:40 am   #2
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Default Re: op amp for avometer movement

Hi, the movement will have a swamp resistor in parallel with it to reduce the sensitivity to 50ua (20k/volt). I think this is normally 10k for a 3k33 movement. Try increasing this to 13k3 by putting a 3k3 resistor is series with the swamp. This should at least bring the voltage range back into calibration.
Regards, Rob.
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Old 9th Aug 2009, 11:45 am   #3
Leon Crampin
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Default Re: op amp for avometer movement

If you alter the swamp to restore the movement sensitivity, all of the shunts and multipliers will need to be changed to restore the instrument's accuracy.

The Thorn AVOs are dogs. Buy a good 8 =< Mk III in a boot sale.

Leon.
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Old 9th Aug 2009, 12:00 pm   #4
AlanBeckett
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Default Re: op amp for avometer movement

I thought that the idea of the swamp resistor(s) was to make all the (production) movements look the same. Hence, all the rest of the components could remain standard.
Alan
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Old 9th Aug 2009, 2:54 pm   #5
Leon Crampin
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Default Re: op amp for avometer movement

In order to use standardised shunts and multipliers on any meter (necessary to achieve a sensitivity to specification) the movement has to be of a defined sensitivity and defined resistance. The swamp serves the purpose both of reducing the movement's temperature coefficient - the moving coil is wound with copper wire - and as a means of adjusting the resistance of the movement "assembly". Remember that the torque produced by a mc meter is proportional to the ampere turns in the movement - voltage does not come into the equation.

Having set the resistance of the movement, its sensitivity is set by means of an adjustable magnetic shunt (proper AVO) or by magnetising and progressively demagnetising a centre pole magnet (Thorn AVO).

Leon.
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Old 9th Aug 2009, 3:36 pm   #6
Kate G7SPU
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Default Re: op amp for avometer movement

Quote:
Originally Posted by logan123 View Post
Hello all.
I would be grateful for any information on the following.

I have an avometer 8 mk 5 which reads low in a test set up. It takes about 40 micro amps to drive the movement full scale and it should only take 37.5.
The meter resistance when measured without the swamp is unlikely to be exactly 37.5uA because of the variation between meters, thus the need for correction. The swamp resistor is inserted in series with the meter to correct the variation in resistance found in such meters, as well as reduce the temperature coefficient, and reduce the sensitivity to 20k ohms per volt, which effectively makes the meter 50uA fsd..

I take it that the meter had been checked before it was dismantled?

For a model 8 Mk 5, the meter plus swamp should have a total resistance of 2667 ohms; the swamp alone would be "around" 1k6 ohms.
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Old 9th Aug 2009, 3:52 pm   #7
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Default Re: op amp for avometer movement

ARGHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH !

Regards

Tony
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Old 9th Aug 2009, 3:58 pm   #8
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Default Re: op amp for avometer movement

Quote:
Originally Posted by Jerry G7SPU
The meter resistance when measured without the swamp is unlikely to be exactly 37.5uA
Did you mean Full Scale Deflection?
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Old 9th Aug 2009, 4:04 pm   #9
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Default Re: op amp for avometer movement

The swamp resistor is added IN SERIES with the meter movement to build out its resistance to the nominal value with as much thermal compensation as can be included. The result is that the movement-plus-swamp presents a nominal resistance for all the "standard" (voltage) multipliers and (current) shunts. The full-scale current is determined by the magnetic characteristics of the movement which may be adjustable by a magnetic shunt (e.g. AVOs pre Model 8 Mk V); for centre-pole movements (AVO 8 Mk V +) no adjustment is possible:the movement must be re-magnetised and then progressively de-magnetised to the correct sensitivity as described.

Fitting an amplifier to "improve" the senstivity seems a "sledgehammer and nut" approach. It also introduces other potential problems, downstream, which will be more trouble than they're worth. If you're that concerned, you could make a "correction chart" and learn to live with it.

As a committed AVO user, I would suggest getting the meter properly re-magnetised to yield the benefits of a "good" instrument. Alternatively an earlier Model 8 (they're not expensive at auction or in boot sales) will provide all the benefits with a measure of "tweakability" (and with greater reliability than the Mk V + if you're inclined to "go inside").

Remember Occam's Razor (which is engraved on every metrologist's heart): don't add unnecessary complications.
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Old 9th Aug 2009, 5:02 pm   #10
Leon Crampin
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Default Re: op amp for avometer movement

You may have gathered I'm not a fan of Thorn AVOs. Is it really worth having a centre pole movement re-worked, only to fit it into a meter with crummy switches and unstable cermet resistors, with a few shunts thrown in which generally function as fuses? New boots will also be required on the buttons.

So, would you even bother to repair a Mk V when you can still get a good Mk III for relatively little money? The "proper" AVOs are almost everlasting when fairly treated.

Leon.
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Old 9th Aug 2009, 5:16 pm   #11
G Barham
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Default Re: op amp for avometer movement

Quote:
Originally Posted by SPCh View Post
..... Alternatively an earlier Model 8 (they're not expensive at auction or in boot sales) will provide all the benefits with a measure of "tweakability" (and with greater reliability than the Mk V + if you're inclined to "go inside").........
Dear SpCh,

This is something I've been pondering for a while ..I'm fortunate (or not, depending on one’s viewpoint) to own an immaculate Mk7.

I also have a Mk2 which seems to be just as good as the MK7 with regards to accuracy. As I regularly use 2 Avos would you advise me to sell the MK7 (examples of which are selling for silly money on the devils auction site) and buy a nice Mk3 ? I've heard it said that the MK4 was a retrograde step with regards to quality etc.

Also, in your opinion, are the movements of the earlier Mk3s etc. as robust as the later MK 5 + ?

Could it be that after the Mk3 price played a major part and the designs got cheaper and cheaper whilst still hanging onto the name and the basic accuracy but sacrificing longevity ?

Sorry about all the questions but like you, I'm a great lover of the things but, unlike you I've never delved into the history or technicalities of the what was, IMHO, the world’s best analogue meter.

Regards

Tony
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Old 9th Aug 2009, 5:29 pm   #12
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Default Re: op amp for avometer movement

I have a Mk III and a Mk V. They both work splendidly, although I actually prefer the '25' scale on the Mk III, despite claims that it is/was too 'cluttered'. I did blow up the high current (thick film) shunts on the Mk V, but fortunately I had a scrap spare. I also have a couple of 7s. I'm intrigued by your comments because it has longed seemed to me that the same thing happened to AVOMinors. The Mk IV and its immediate predecessor, of which I have several and use them all the time, are beautifully made, whereas the Mk V I regard as cheap tat!
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Old 9th Aug 2009, 6:13 pm   #13
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Default Re: op amp for avometer movement

Leon Crampin/G. Barham

Your questions largely overlap, so I'll give an overlapping reply:

Far be it from me do decry ANY of the AVO models, but it is generally acknowledged that the rot set in after the Model 8 Mark III. It is no coincidence, I'm sure, that this was the last of the designs under the authoritative gaze of Hugh MacAdie, "father" of the Model 7 (and following), and son of the original inventor (Model "1" = 1923) and founder of the company.

The Mark IV was a low cost, but electrically compatible, version of the Mark III and it shows.

The Mark V was an even lower cost, but electrically INCOMPATIBLE version of the Mark IV.
Its advantages are the independence of the ohms setting pots and the increased basic sensitivity (100mV/50uA instead of 125mv/50uA). The original Model 8 accessories don't fit the Mark V +, either electrically or mechanically, so it omitted the Low Resistance and Insulation ranges. The mechanics, as everyone seems to agree, were done on the cheap (on Jules Thorn's personal instructions), and I have to admit that I've seen more instances of failing magnetism on the centre-pole movements than on the earlier "Alnico block" types.

If it were me, I'd go for a Model 8 Mark II or Mark III, or a Nato TS-1.

(Don't sell the Mark 7, Tony; it's will always be a collector's item).

As an extra, a maxim of the late Prof. F.C. ("Freddie") Williams,
more memorably expressed than Lord Kelvin's rather prissy original version:

"To measure is to know - so long as you know just WHAT you are measuring,
and HOW you are measuring it"
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Old 9th Aug 2009, 10:27 pm   #14
Kate G7SPU
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Default Re: op amp for avometer movement

Quote:
Originally Posted by Station X View Post
Quote:
Originally Posted by Jerry G7SPU
The meter resistance when measured without the swamp is unlikely to be exactly 37.5uA
Did you mean Full Scale Deflection?
Quite so! I meant 37.5 fsd.
I'd be interested to know how far off calibration the readings are, or were before dismantling.
The thing I love about AVOs is that they are not as entirely straightforward as the first appear.
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Old 10th Aug 2009, 12:27 am   #15
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Default Re: op amp for avometer movement

Quote:
Originally Posted by glowinganode View Post
Hi, the movement will have a swamp resistor in parallel with it to reduce the sensitivity to 50ua (20k/volt).
Sorry guys, I meant SHUNT not SWAMP.
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Old 10th Aug 2009, 11:08 am   #16
SPCh
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Default Re: op amp for avometer movement

AVO (or any other) meter calibration is not simple (or even possible) with typical workshop equipment.
What are you going to calibrate it against ?
How sure are you that your "reference" is accurate ?

All calibration requires a massive act of faith in something (or somebody) else !

For a Model 8 Mark V the nominal movement fsd is 37.5 uA. The accuracy specs. (on DC) allow +/- 1% of this fsd value (i.e. 0.375 uA) between 10% and 100% full scale.
The factory calibration is generally better than this - typically 0.10 to 0.15 uA.

How will you measure 37.5 uA to 0.1uA accuracy ?

A DVM won't do, since it can have a READING error of at least +/- 1 x l.s.d.
on top of any MEASUREMENT error - rarely claimed as less than 0.3% of reading.

On the minimum 4mA range (e.g Fluke 76) the l.s.d. = 1 uA, and Fluke claim only 5 x l.s.d., so even if the measurement is perfect, the reading will be uncertain by at least +/- 5uA !
Adding extra digits wouldn't help, either: the measurement error (0.5% on the Fluke) would then become significant.

If you use a high serial feed resistor and measure the voltage across it with a DVM, the feed voltage can be made high enough to reduce the effect of the reading errors to something below what you can accept, but then you have to consider the accuracy of the feed resistor. "Classical" decade boxes sometimes achieve 0.1% (though rarely claim better than 0.25%).

The best you can do, in practice, is to take many simultaneous measurements with other instruments which you (hope you can) trust, compare the Ohm's Law results for each measurement, and make a statistical inference as to their, and hence the target meter's, performance. The best resistors for these tests are - wait for it ... - the shunts and multipliers in well-treated AVO multimeters !

Metrology is a science in its own right, and rarely taught, I suspect, on modern engineering courses. To get the answer nearly right (i.e with a quantified error) you have to rely on the likes of an AVO (that's what made them so expensive) or a Fluke (with its "bulk" indication uncertainties).

Sorry to be so cheerful, but that's life.
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Old 10th Aug 2009, 12:08 pm   #17
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Default Re: op amp for avometer movement

SPCh,
What a splendid Post!
I suspect you are correct about modern Engineering courses, hence the not unusual phrase one hears 'it must be right, it's digital'.
Alan
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Old 10th Aug 2009, 5:13 pm   #18
Brian R Pateman
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Default Re: op amp for avometer movement

Very good critique of the calibration process. As an Instrument Engineer with over 35 years experience I am always surprised at the tendency to rely on a digital readout with loads of numbers after the decimal point as gospel.

For the sort of work most of us do on our vintage wirelesses, televisions, audio equipment etc, the standard industrial +/- 2% is more than adequate.

Those of us who restore vintage test equipment may require something better for calibrating back to "As Built" standards. It's a case of horses for courses.
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Old 10th Aug 2009, 5:47 pm   #19
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Default Re: op amp for avometer movement

Dare I add:

Yes horses for courses (but not rabbits, greyhounds or donkeys !)

I think this thread (which has long been OT) has run its full course

(with - no offence to anybody - all the aforementioned livestock !)
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Old 10th Aug 2009, 9:20 pm   #20
logan123
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Default Re: op amp for avometer movement

Hello, I write to thank you all for the advice, I have learnt a lot from it.

I would have liked to have had some details of a suitable amp, so that I could have tried it.

Thank you all once again,

logan123.

Last edited by Darren-UK; 10th Aug 2009 at 9:28 pm. Reason: Format corrected.
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