AVO 8 Mk1. Movement sensitivity problem.

[Down Under]

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=189548

I have a problem with an AVO 8 Mk1. I was recently given this meter to see if it could be made to function correctly as it was reading low on all ranges. I appreciated that it had to be something common to all circuits so I started with the meter movement. Initial tests suggested that all was well however, when I did an initial FSD current test, I appeared to be approximately 236 micro amps when it should have been 37.5 micro amps.

I removed the meter movement to check the movement more thoroughly, in particular the hair springs etc and all appeared to be in good order. I then did a more accurate test of the FSD current using a resister in series and accurately measuring the voltage across the resister and using Ohms law re calculated the current at 238 micro amps. I adjusted the shunt to see what effect it would have which was minimal and then reset it back to its original position. To the extent that I could, I checked the pivots associated with the moving coil etc and all appeared to be operating as expected. Note, all of my tests were conducted with the damper resistance in circuit.

My question is, has anyone experienced a similar problem?

Bruce

[pmmunro]

Bruce,

Some loss of sensitivity in Avometer movements is not unknown but nearly an order of magnitude is very unusual and would tend to suggest that something is seriously wrong.

Possibly the magnet has lost field strength or the moving coil has shorted turns. The series combination of the moving coil and the series swamp resistor should be 3333 ohms.

If someone has attempted to dismantle the movement and has loosened the screws which clamp the magnet assembly, this would have been very likely to cause a loss of magnetism.

The magnet can be remagnetised fairly simply - if the correct equipment is available - but there are not many organisations which still have this equipment.

A number of people have reported being able to compensate for small losses in magnetism using small suplementary button magnets but nothing as large as in your case.

Is the "damper" resistance you mention the series swamp wound on a bobbin fixed to the back of the movement? In fact most of the apparent damping effect comes from the "Universal Shunt" chain which has a total resistance of 10,000 ohms and is designed to shunt 12.5 microamps which, together with the 37.5 microamp fsd figure for the basic movement gives the 50 microamp sensitivity around which the meter is designed.

PMM

[Down Under]

PPM

Firstly, thanks for responding as it is appreciated. I am located in Western Australia and there is little help here.

The meter has had a difficult life in that the crack in the case would suggest that it has been dropped at some time.

It is difficult for me to determine if there is sufficient magnetism however, I can try some button magnets and see if there is any improvement. That could point me in the right direction.

I had thought that a shorted turn on the moving coil could be the problem but again I have no way to test this that I know of. I am use to big motors etc where it isn't a problem.

The series resister I referred to as the swamp resister is in fact wound on a bobbin. I thought by removing the movement, and testing it as an individual unit I could isolate the problem. As another test, I connected a DC supply to another meter with a FSD of 50 micro amps and adjusted the current to 35 microamps then interchanged the movements. The AVO movement was about 25% FSD.

I am in my mid seventies and I am a retired electronics engineer, and this is taking me back to some of my first year lectures.

Bruce

[The Philpott]

I would have thought you'd get a boost of 10 or 12% by placing a neodymium magnet somewhere on the concentrator, but if that succeeds and confirms diagnosis, remagnetising not likely to be economical..and that's if you could find a place that does it.

Thinks: Warbird enthusiasts surely must have to get analogue meters refurbed as and when old stock runs out- but that's a social circle that it's notoriously difficult to get into, in the UK at least.

Dave

[jimscoper]

Firstly are you sure this is a Mk1 meter - with a 50uA range. Not teaching granny etc.

The series swamp resistor makes up the movement coil + swamp to 3333 ohms. So if you measure the swamp on its own you can find what the coil resistance is meant to be. A cautious measurement will tell you if the coil is shorted.

There is also a VDR/Thermistor device across the coil/swamp combination. Might be worth checking that has not gone low. Roughly the coil is about 1.2K ohms and the swamp 2.1K on the ones I have messed with.

I have had trouble with the magnets - but not as much as you seem to have.

[pmmunro]

[QUOTE=jimscoper;1471792]Firstly are you sure this is a Mk1 meter - with a 50uA range.
There is also a VDR/Thermistor device across the coil/swamp combination. Might be worth checking that has not gone low. Roughly the coil is about 1.2K ohms and the swamp 2.1K on the ones I have messed with.

The thermistor is only fitted on Model 8 Mark 3 and 4.

PMM