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 Analogue VOM movement under damping
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 Return to top of page · Post #: 1 · Written at 11:27:50 AM on 7 October 2014.
Kxdniu's avatar
 Location: Melbourne, VIC
 Member since 19 May 2014
 Member #: 1577
 Postcount: 92

Hi,

I have a Taylor 88B analogy multimeter and it has a problem.

The movement is not properly damped and the pointer takes a long time to return to zero.

My question is what control the damping in this kind of movements and is this fixable?

If it can be fixed, how?

Thanks

Kevin


 
 Return to top of page · Post #: 2 · Written at 11:47:43 AM on 7 October 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 4398

Is it a moving coil or moving iron type? Moving iron is very slow and would mimic a damping effect.

A nice sharp close-up photo of the guts of the movement would help with diagnosis.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Brad.

A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 3 · Written at 12:14:13 PM on 7 October 2014.
Kxdniu's avatar
 Location: Melbourne, VIC
 Member since 19 May 2014
 Member #: 1577
 Postcount: 92

Hi Brad,

It is a moving coil movement.

I will get some photo later when I get home.

The pointer moves around when the meter is bumped slightly.

Regards

Kevin


 
 Return to top of page · Post #: 4 · Written at 12:28:41 PM on 7 October 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 4398

The pointer moves around when the meter is bumped slightly.

This is fairly normal, especially on older models, though depending on the model there are varying degrees as to how much movement takes place after the bump. All the ancient University gear I used when I was at tech doing my trade course behaved this way. Meters that measured amps and watts were more prone than those only measuring volts.

Also, many meters are designed only to be used either in an upright position or lying flat. Where the feet are on your instrument will give that indication and the meter may provide an inaccurate reading when positioned another way.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Brad.

A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 5 · Written at 1:30:17 PM on 7 October 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 4013

many meters are designed only to be used either in an upright position or lying flat.

The Taylors I used back in the day were designed, like the more expensive Avo, to be used lying flat.


 
 Return to top of page · Post #: 6 · Written at 11:56:55 PM on 7 October 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

I don't know if he's still operating, but I had two meters rebuilt some time ago by I believe Hanson in Moorabbin. Lots of Aeroplanes with instruments in that area.

Some meters had an "OFF" position, but you can short neg to positive & put it on amps. when traveling. That dampens the meter, as it and the OFF, causes the movement to become a generator, and generating power takes energy and that dampens the movement.

Marc


 
 Return to top of page · Post #: 7 · Written at 4:13:45 PM on 9 October 2014.
Kxdniu's avatar
 Location: Melbourne, VIC
 Member since 19 May 2014
 Member #: 1577
 Postcount: 92

Sorry for the delay.

Open the meter and get the movement out.

This one is under damping and I don't know why.

All my other meters don't have this problem.

The pointer takes long time to settle at the zero position.

Any ideas are welcome.

The following are two photos of the meter.

Taylor Moving Coil Multimeter
Taylor Moving Coil Multimeter


 
 Return to top of page · Post #: 8 · Written at 10:29:41 PM on 9 October 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

Technically that is the meter movement. Depending on what the jewels are, they may be dry & need a drop of "clock" oil. There is the possibility of mechanical distortion of the armature cage holding the windings, due to impact, or other cause, including debris causing the cage of the armature to drag on the core. On AVO 7X the balsam often failed, causing that.

The jewels may be too tight, but this is not an area to be playing with, if you do not understand the mechanics of it. It is not dissimilar to a clock jewel, but damage one of those hair springs & it all has to be re-calibrated.

I cannot see from those photos any form of dampener.

Marc


 
 Return to top of page · Post #: 9 · Written at 1:32:22 PM on 10 October 2014.
STC830's avatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 558

Dug out a reference book: if there is no visible form of damping such as an aluminium vane fixed to the pointer operating in a magnetic field, or a piston or vane in a closed chamber, then the damping would be by generation of eddy currents in the metal former of the moving coil.

There are some suspicious scratches on the brass adjusting screw locknut of the bearing - wonder if that screw has been overtightened.

Noting what Marcc says, and if none of the possible causes that he mentions are apparent, it might nevertheless be worthwhile carefully noting the position of the adjusting screw, loosen the lock nut and backing off the screw a fraction of a turn. If the screw is too tight it would add frictional damping that would not diminish as the pointer slows approaching zero, as the eddy current damping would diminish.


 
 Return to top of page · Post #: 10 · Written at 4:43:44 PM on 10 October 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 2747

Forgot to mention; If it only has dampening problems with the scale on, bent pointer likely.

Marc


 
 Return to top of page · Post #: 11 · Written at 8:08:56 PM on 12 October 2014.
Kxdniu's avatar
 Location: Melbourne, VIC
 Member since 19 May 2014
 Member #: 1577
 Postcount: 92

Thanks everyone for the advices.

I did some Googling and haven't get much info regarding analogy movement repair.

I will leave this alone for now until I can find more info about it.

Regards

Kevin


 
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