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Moving Coil Meter

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Ash Small
Fri May 13 2011, 07:40PM Print View
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Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 3363
After the recent discussion in this thread:



I've started running some preliminary tests, as I've been challenged to build a moving coil meter (or moving magnet meter) and I've decided to take up the gauntlet

I could only find one supermagnet, a broken one I keep in one of my toolboxes, so I placed it in one side of a TV flyback core to create a horseshoe magnet of sorts. I'll sort out something better when I can but this is satisfactory for now.

I then wound a coil around a cigarette filter using some enamelled wire from an inductor from a TV, I think. not sure what the wire guage is, but it's a good starting point for initial tests.

I left the ends long and suspended it above the makeshift magnet as per the photo below, and connected it up in series with a potentiometer (22 k), a DMM and a 12 V supply.

I started turning the potentiometer down from 22k and the coil started to swing.....I turned it up and down a few times and the coil was swinging through 90 degrees.....then the potentiometer burned out......

Anyway, it's proved the concept. I'll refine the setup a bit and run some more tests. I didn't have a chance to ascertain how much current gave full scale deflection, but I do have a starting point now.

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Patrick
Fri May 13 2011, 08:38PM
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Location: Chico, CA. USA
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Oh, now we all did it, we really put Ash's spine up now.
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magnet18
Fri May 13 2011, 11:42PM
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I just built one, stationary coil moving the meter.
It works, assuming I want the entire 0-5A span on a 1cm scale.
and theres no breeze
and noone bumps it
I think it might work better if I use a magnetized iron needle instead of one made of emi shielding...
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Ash Small
Sat May 14 2011, 12:25AM
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magnet18 wrote ...

I just built one, stationary coil moving the meter.
It works, assuming I want the entire 0-5A span on a 1cm scale.
and theres no breeze
and noone bumps it
I think it might work better if I use a magnetized iron needle instead of one made of emi shielding...


You can either adjust the number of turns or add some resistors. It's all elementary stuff. It's a trade off between power consumed and accuracy.

I'll develop mine a bit further and post results.
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magnet18
Sat May 14 2011, 01:40PM
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chapter 8 might be of some amount of use.
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Proud Mary
Sat May 14 2011, 01:54PM
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20 kΩ per volt - i.e. 50 μA FSD - is the established standard for moving coil movements.
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Ash Small
Sat May 14 2011, 05:42PM
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Posts: 3363
Proud Mary wrote ...

20 kΩ per volt - i.e. 50 μA FSD - is the established standard for moving coil movements.


I'll need some better magnets than the ones I'm using. I have a load of supermagnets somewhere, not Neodymium, but a higher temperature spec. which aren't as poweful. It may be a couple of months before I can get them though.

I may get some neodymium ones in the meantime.

I'll need to use thinner guage wire than I'm using at present. I forsee this presenting most of the problems, firstly, due to the difficulty of winding thin wire without ot breaking, and secondly, too thin or too many turns and it will overheat. I'm not yet sure how much of a problem this will be at 50 uA, but any change in resistance due to temperature change will probably affect accuracy. This is where I expect the most problems.

Balancing the movement will also present some problems but I should be able to overcome this. Static balancing is simple enough in principle.

Obviously, the stronger that magnetic field, the easier it will be. The makeshift magnet I used above is no-where near as strong as neodymium ones

Obviously, in a lot of amatear applications, 50 uA FSD isn't required, but that's the challenge I've accepted, so I'll see what I can do.

@ Magnet18, Do you have any photo's of your setup?

EDIT: I'm assuming a larger diameter coil will have advantages, assuming the same number of turns.
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magnet18
Sat May 14 2011, 09:26PM
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15 turns, 20ga, I need more turns. much more turns.
I'll see what happens when I make some changes, I have some nice thin wire and better material for a needle.
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Patrick
Sat May 14 2011, 09:30PM
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Location: Chico, CA. USA
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magnet18 wrote ...






15 turns, 20ga, I need more turns. much more turns.
I'll see what happens when I make some changes, I have some nice thin wire and better material for a needle.

remember the d'Araonsval meter has that arm stretching in a curve across from pol to pole, to linearize the meters scale, otherwise what your showing above is exponetial or logarithmic, i forget which.

im trying to find pics and my book at the moment... to show what i mean.
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Sulaiman
Sat May 14 2011, 09:40PM
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Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2313
For a moving-coil meter, winding the coil will be tedious,
making the pivot/movement will require machining
making and mounting the springs at each end of the coil for centering/restoring-force/lead-out I want to see !
Seriously, don't start this project!

buy/get a moving coil meter movement
and try to completely dis-assemble then re-assemble the movement
now imagine having to make the parts as well

Make a Tangent galvanometer using a magnetic compass and helmholtz coils
this can be calibrated knowing the earth's magnetic field and the number of turns and dimensions of the coils.

You could also make a taut band mirror galvanometer...very sensitive.

For a compact/rugged unit I would go for a moving magnet design.
e.g. mount a diametrically magnetised disc magnet (I can post a couple if you like)
on a pivoted shaft with the pointer needle.
Wind many turns of wire on a stationary core with the pivoted magnet in the airgap
use a second magnet above or below the pivot points to zero the pointer and act as the restoring force.

An easy to make (large) coil and no spiral coils.
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Patrick
Sat May 14 2011, 09:55PM
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Location: Chico, CA. USA
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Sulaiman wrote ...

Seriously, don't start this project!
I think theve got their heart set on this matter, many of us think this is an unessacary waste of time and effort which they could spend on their other stuff.

But its not like I havent felt similarly with my HV probe obsession, though if I could by them for $6 instead of $2400 I would just buy the dam things.
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magnet18
Sun May 15 2011, 04:10AM
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I'm honestly probably going to end up just shelling out the cash for a bunch of these things, but it's still fun to play around with building one.
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Patrick
Sun May 15 2011, 04:33AM
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Posts: 4445
magnet18 wrote ...

I'm honestly probably going to end up just shelling out the cash for a bunch of these things, but it's still fun to play around with building one.

Just buy the analog meters from ebay, radio shack or whatever source, then use a color printer to print out a scale you like with the graphics and scale you want. you can make them look cool.
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Ash Small
Sun May 15 2011, 07:13AM
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That's very good advice, Sulaiman. When I first suggested Magnet18 should consider making some, I had in mind a fairly crude moving magnet design, which was no where near 50 uA FSD.

This was after he'd said that ~$9 each was way too much for him.

I'm viewing this as 'just a bit of fun'. I don't even need one! .....And I still have my other projects.

However, I've accepted the challenge to try and construct a 50 uA FSD meter, and I'm assuming that a moving coil meter is probably the best way to accomplish this.

I did strip, repair and rebuild a cheap meter when I was a kid, as I mentioned in the other thread. I also suggested that this would be a good starting point for Magnet18.

I don't expect to make much progress until I obtain some decent magnets. I have some somewhere, but I think they are 200 miles away and they are not neodymium, but another rare earth type which are not as strong but will withstand higher temperatures. They are also 'rejects' that I obtained as samples.

Working with the very fine guage wire is a bit of a challenge, as is winding the spring and making the movement, which I plan to do only using basic hand tools.

Even a crude moving magnet design would be quite useful for amateur HV projects where 50 uA FSD isn't required and where one wouldn't wish to risk destroying a more expensive meter. At the end of the day if you build it you can repair it!

Anyway, I'll run some more tests when I locate some better magnets and post results. As Magnet18 said, 'It's fun to play around with building one'.

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Sulaiman
Sun May 15 2011, 08:45AM
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Location: United Kingdom
Posts: 2313
I have some of these

Glue a thumb-tack to each face for a quick bearing
us a second one for zero/restoring force ... almost done!
Get the core & winding from a small ac shaded-pole ac motor (microwave oven fan etc.)
and there's not much left to do!

I still think that a tangent galvanometer is a better hobby project
with a magnetic compass and a Helmholtz coil you can
- measure the Earth's magnetic field at your location (requires a milli-amp meter)
- easily calibrate your meter
- have a calibrated magnetic field generator
- replicate the earliest type of laboratory-grade current meter.
.........

EDIT: if you construct the two coils for the Helmholtz coil well you will also have;
- a reference inductor
- a reference variable inductor
- two coils suitable for resonance and coupling experiments
- a 'wireless-power' experimentation setup
- two really 'neat' bracelets (uber-geek-cool energy-harvesting?)
- a loop-antenna for radio reception
.........................


With a small mirror on the movement and a laser pointer you could have a 'cute' meter
.
.
.
What more could one desire?
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Proud Mary
Sun May 15 2011, 09:07AM
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Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4811
Sooner or later, most amateur constructors come to realize that it is usually cheaper to buy something ready made than to make it yourself. We don't enjoy the economies of scale of the mass manufacturer, and can not spread the cost of highly specialised tools across thousands of production items.

We get the best value for our money when we design and build things not readily available in the marketplace.





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Steve Conner
Sun May 15 2011, 09:41AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6725
Yes, but it's fun and educational!

Another interesting type of meter is the electrostatic voltmeter. This might be easier to make and more useful to a high voltage hobbyist than the electrodynamic kind. It is basically a capacitor with a spring balance to "weigh" the electrostatic force between the plates. It works equally well on AC and DC.
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Proud Mary
Sun May 15 2011, 10:53AM
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Steve McConner wrote ...

Yes, but it's fun and educational!


I had a paragraph on satisfaction which I cut out before posting for fear of over-stimlulating the starved and suggestible.

Steve McConner wrote ...

Another interesting type of meter is the electrostatic voltmeter. This might be easier to make and more useful to a high voltage hobbyist than the electrodynamic kind. It is basically a capacitor with a spring balance to "weigh" the electrostatic force between the plates. It works equally well on AC and DC.


I have a wonderful old 18kV electrostatic voltmeter in a mahogany case, and use it where even a few tens of μA would pull down the voltage to be measured. It's hard to beat for high impedance DC measurements like GM and prop tube supplies, but you must have room for the 10" x 10" x 10" wooden case.

Field mills are sometimes used to estimate HV DC sources such as VdGs and C&W, but don't seem to have been touched upon in 4HV. There's a good DIY field mill design here:



See also:

Tant, P. Bolsens, B. Sels, T. Van Dommelen, D. Driesen, J. Belmans, R. Design and Application of a Field Mill as a High-Voltage DC Meter
Instrumentation and Measurement, IEEE Transactions on Vol 56 Issue: 4 Aug. 2007 pp 1459 - 1464

Abstract

Field mill instruments are often employed for the measurement of electric fields, electric charges, voltage potentials, and atmospheric effects. This paper discusses in more detail the application of a field mill for measuring dc voltages in high-voltage laboratories. Some specific problems, which arise when voltages in the kilovolt or megavolt range have to be measured, are emphasized. Safety, signal transmission, and electrode design are dealt with. Field meter theory is summarized and generalized in the function of this application. The design of a practical field-mill-based electrostatic voltmeter is described along with some useful enhancements. Finally, test results of the voltmeter are presented.

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Ash Small
Tue May 24 2011, 04:13PM
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Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 3363
Well, I've some spare cash now, and before deciding on which magnets to order, thought I'd do a bit of research.

While I've seen the arm that you mention on meters before, I can't find any mention of it in any of the articles I've looked at.

Patrick wrote ...

.
remember the d'Araonsval meter has that arm stretching in a curve across from pol to pole, to linearize the meters scale, otherwise what your showing above is exponetial or logarithmic, i forget which.

im trying to find pics and my book at the moment... to show what i mean.



The Weston revision of the d'Arasonval meter is what I'm basing mine on. (What we generally call a 'moving coil meter')

According to Wikipedea, a typical meter has 100 uA FSD, with a voltage drop of 50 mV at full current.

Others here have stated 50 uA. Would anyone wish to add anything to this, or to elaborate a bit?

I've read that the coils are usually wound on an aluminium former, as this acts as a damper, as it is highly conductive. Is anyone able to add to this? (that is all I found on the subject)

Also, some articles mention an iron core, to concentrate the magnetic field. Others mention an iron tube. Is anyone able to add to that?

I'll be ordering some magnets this week, and running some more tests when they arrive, but If anyone can advise on the above points I'd appreciate it. Thanks.
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Proud Mary
Tue May 24 2011, 06:09PM
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Location: UK
Posts: 4811
Ash Small wrote ...


According to Wikipedea, a typical meter has 100 uA FSD, with a voltage drop of 50 mV at full current.

Others here have stated 50 uA. Would anyone wish to add anything to this, or to elaborate a bit?


A sensitivity of 20 kΩ per volt has been the industrial standard for good quality moving coil meters since the 1940s.

One only has to think of the legendary AVO Model 8, that great workhorse of the Thermionic Age:



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Patrick
Wed May 25 2011, 09:56AM
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Location: Chico, CA. USA
Posts: 4445
Ash Small wrote ...

Well, I've some spare cash now, and before deciding on which magnets to order, thought I'd do a bit of research.

While I've seen the arm that you mention on meters before, I can't find any mention of it in any of the articles I've looked at.

Patrick wrote ...

.
remember the d'Araonsval meter has that arm stretching in a curve across from pol to pole, to linearize the meters scale, otherwise what your showing above is exponetial or logarithmic, i forget which.

im trying to find pics and my book at the moment... to show what i mean.


i may not have said this all quite right let me get another post to you regarding this matter.
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Ash Small
Thu May 26 2011, 02:54PM
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Posts: 3363
I scored this for a tenner on Ebay last night, and collected it this morning. It was two miles away.


AVO Model 8 Mk III


I'll open it up and have a look at the movement later. I'm particularly interested in looking at the 'concentrator'.

It's giving some strange readings on the Ohm scale, but I'll try some new batteries and see if that sorts it out.
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Proud Mary
Thu May 26 2011, 04:17PM
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Posts: 4811
Ash Small wrote ...

I scored this for a tenner on Ebay last night, and collected it this morning. It was two miles away.


AVO Model 8 Mk III


I'll open it up and have a look at the movement later. I'm particularly interested in looking at the 'concentrator'.

It's giving some strange readings on the Ohm scale, but I'll try some new batteries and see if that sorts it out.


Coo, that's a real beauty, Ash, and well cared-for by the looks of it.

You should put a fresh 15V battery in it, before drawing any conclusions about the resistance ranges.

All being well with the new battery, I would resist the temptation to open it and look inside. It has survived all those years without your intervention!
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Nicko
Thu May 26 2011, 04:45PM
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Location: Nr. London, UK
Posts: 613
Ash Small wrote ...

I scored this for a tenner on Ebay last night, and collected it this morning. It was two miles away.


AVO Model 8 Mk III

I'll open it up and have a look at the movement later. I'm particularly interested in looking at the 'concentrator'.

It's giving some strange readings on the Ohm scale, but I'll try some new batteries and see if that sorts it out.

Congratulations. Seconded to what PM said (don't mess too much with it - they are subtle and quick to anger). I don't know why but I've fallen in love with AVOs recently. Got one at a boot fair in Tunbridge Wells about a month ago for 13 quid complete with pristine leather case (Its a Model 47 or a Model 40, not sure!). Leads a bit crummy, but otherwise OK. Seems mostly ok - movement has been replaced. Now got 4 of them!



The place to go for info on AVOs and how to look after/repair them is the UK Vintage Radio Repair and Restoration Discussion Forum on "Specific Vintage Equipment" .

They've been extremely helpful with my questions and also maintain a register of 1000's of these things' serial numbers. They will also tell you when yours was made.

HTH,
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Ash Small
Thu May 26 2011, 04:50PM
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Posts: 3363
Proud Mary wrote ...


Coo, that's a real beauty, Ash, and well cared-for by the looks of it.

You should put a fresh 15V battery in it, before drawing any conclusions about the resistance ranges.

All being well with the new battery, I would resist the temptation to open it and look inside. It has survived all those years without your intervention!



This one has been converted to take an A23 12V battery in series with an AAA 1.5V battery. This gives 13.5V (according to my limited maths)

Is this sufficient, or should I add another AAA to bring it up to 15V?








Also, the battery cover is missing, but I'll make up an aluminium one, I think. (If it works OK once I've replaced the batteries.


Maybe you are right, opening it up could be considered as 'cheating', but I've not been able to find any detail about the iron magnetic field concentrator online. (Some articles refer to an iron core or a piece of iron tubing inside the coil)

I downloaded the AVO service manual, and that seems to state that the concentrator remains in the scaleplate assembly when the coil is removed.



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Proud Mary
Thu May 26 2011, 05:15PM
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Location: UK
Posts: 4811
Ash Small wrote ...

Proud Mary wrote ...


Coo, that's a real beauty, Ash, and well cared-for by the looks of it.

You should put a fresh 15V battery in it, before drawing any conclusions about the resistance ranges.

All being well with the new battery, I would resist the temptation to open it and look inside. It has survived all those years without your intervention!



This one has been converted to take an A23 12V battery in series with an AAA 1.5V battery. This gives 13.5V (according to my limited maths)

Is this sufficient, or should I add another AAA to bring it up to 15V?








Also, the battery cover is missing, but I'll make up an aluminium one, I think. (If it works OK once I've replaced the batteries.


Maybe you are right, opening it up could be considered as 'cheating', but I've not been able to find any detail about the iron magnetic field concentrator online. (Some articles refer to a piece of iron tubing inside the coil)

I downloaded the AVO service manual, and that seems to state that the concentrator remains in the faceplate assembly when the coil is removed.


Oh dearie me - we can only hope that the person responsible for such an awful bodge job on the batteries hasn't been tinkering elsewhere.

In the normal course of events, one can compensate for declining battery voltage by means of set zero, but I would advise you to use the full 15 V for re-commissioning so you can test the set zero across its whole range, and perhaps identify any tinkering or other short-coming.

RESISTANCE
Before testing, the pointer should be adjusted to zero in the following sequence :
(1) Set left-hand switch at “ RESISTANCE.”
(2) Join the leads together.
(3) On the “Ω” range, adjust to zero by means of the knob marked “ ZERO Ω.”
(4) On the “Ω ÷ 100” range, adjust to zero by means of the knob marked “ ZERO Ω ÷ 100.”
(5) On the “Ω x 100” range, adjust to zero by means of the knob marked “ ZERO Ω x 100”
To test a resistance, set the right-hand switch at the range required, the leads being connected across the unknown component. Resistance is read directly on the “Ω” range, but indications should be divided or multiplied by 100 on the other two ranges.


It is often possible to identify past bodging and tinkering by examining the fastenings with a jeweller's loup, as the bodger will commonly use screwdrivers or sockets of the wrong size, which leave their own characteristic markings behind them.
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Sulaiman
Thu May 26 2011, 05:42PM
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Location: United Kingdom
Posts: 2313
As Small'
It's amazing how many seemingly unimportant details turn out to be very clever bits of engineering
- often too late to be fully appreciated. (oops !)

If in good condition it's a useful meter

if minor repairs (resistors etc.) can get it to a useful state then it would be a nice little project.

If there is bad corrosion or the meter movement (magnet, pivots, coil, springs, pointer et.) needs repair then it can be a significant project, hours of restoration calibrated vs. a $5 dmm.
(that I've previously tested the calibration of common ranges to better than 1%)
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Ash Small
Thu May 26 2011, 07:13PM
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Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 3363
Proud Mary wrote ...

.
In the normal course of events, one can compensate for declining battery voltage by means of set zero, but I would advise you to use the full 15 V for re-commissioning so you can test the set zero across its whole range, and perhaps identify any tinkering or other short-coming.

RESISTANCE
Before testing, the pointer should be adjusted to zero in the following sequence :
(1) Set left-hand switch at “ RESISTANCE.”
(2) Join the leads together.
(3) On the “Ω” range, adjust to zero by means of the knob marked “ ZERO Ω.”
(4) On the “Ω ÷ 100” range, adjust to zero by means of the knob marked “ ZERO Ω ÷ 100.”
(5) On the “Ω x 100” range, adjust to zero by means of the knob marked “ ZERO Ω x 100”
To test a resistance, set the right-hand switch at the range required, the leads being connected across the unknown component. Resistance is read directly on the “Ω” range, but indications should be divided or multiplied by 100 on the other two ranges.


It is often possible to identify past bodging and tinkering by examining the fastenings with a jeweller's loup, as the bodger will commonly use screwdrivers or sockets of the wrong size, which leave their own characteristic markings behind them.


Well, I've taken a couple of A23's apart and made one 10 cell battery (reads 15.6V on my DMM). I can't get it to zero on the Ohms/100 range(lowest it will go is 50 (equal to 0.5V), so I assume it was adjusted inside to allow zeroing on the 13.5V it was operating on before.

I found the instructions to adjust this earlier, but can't seem to find them now.(I'm sure I will if I keep looking) but this means I'll have to open it up anyway. (The internal adjustment should be set so that the external adjusters are midway when it is zeroed, to allow for adjustment in either direction)

I was on the UK Vintage Radio Repair and Restoration Discussion Forum earlier, but I'm not sure if it was there or somewhere else.

EDIT: The 15V battery is only used by the Ohms X 100 range (according to the circuit diagram), so the above fault isn't due to the 15V battery

EDIT EDIT: It's a resistance problem. I shorted the terminals using some silver plated wire and it zeros. I'll try cleaning the terminals, that should improve things further. Time for some new leads I think.
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Proud Mary
Thu May 26 2011, 08:55PM
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4811
Does it display believable readings in voltmeter mode?
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Ash Small
Thu May 26 2011, 09:29PM
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 3363
Proud Mary wrote ...

Does it display believable readings in voltmeter mode?


When connected to a 12V battery charger with a 21 watt bulb as ballast, it reads 8.5V, my DMM reads 9.17 (there could be a voltage drop due to high resistance in the terminals)

The same circuit gives 1.2 amps on both the AVO and DMM.

I'll set up a better test with a car battery in the morning.

(It's only a cheap 4 amp battery charger.)
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