Avo MK4 meter

[mikko rintala]

Hi. I have Simpson 29 25uA panel meter and that is problem.
The needle swings forward and backward.Whether that can be fixed.

Regairds Mikko

[WME_bill]

If the needle swings wildly, it suggests that the meter is un-damped.

What circuit is it being used in.
In most Avometers such as the Avo8, the meter is shunted by a resistor chain from which you can select the current ranges. Movement of the coil generates a back emf which flows around the resistor chain amd damps the movement.
In many other meters the coil inside is wound on a copper former, so the eddy current produced on the coil as it moves in the magnetic field stops it swinging. As in the GEC Selectest Super 50.

One final test, short the terminals on the meter itself. If it still swings, then I'm afraid the coil is open circuit, and your meter is useless.
This is a very good check if you are buying a radio rally or a junk shop, short across the terminals with a paper clip or penknife blade. Twist your wrist, holding the case scale up. If the needle swings, put the meter back. It is failed.
wme_bill

[David Simpson]

I think Mikka is referring to an AVO VCM MK4, which has had it's original 30uA meter replaced by a modern Simpson 29.5 uA meter.

Regards, David

[WME_bill]

Avo VCM Mk4 meter swinging.
Thanks to David Simpson for identifying what instrument.
The meter of the VCM is shunted by a resistor and the capacitor, so should surely be reasonably well damped, even on the Insulation range.
So I suspect my gloom over the condition of the meter may well be valid.
I await with interest to hear if Mikko has checked the meter movement itself.
A correction to my previous comment about a self damped meter coil. It is wound on an aluminium former usually, not copper. wme_bill

[mikko rintala]

I mean when the gauge is disconnected and in my hand, the needle sways restlessly. It hasn't been plugged into anything yet.

Regairds Mikko

[Radio Wrangler]

That is probably normal.

Such sensitive meters hve to be very lightly sprung.

Short-circuit its terminals by wrapping a length of bare wire around both of them. It does not need to be thick, a small diameter strand is enough. and try moving it again.

If the meter is OK, you should notice that the pointer moves a lot less. Any motion of the coil in the magnet gap generates voltage and tries to pass current around the circuit you have just made. This means that there is self-generated current in the meter's coil, and this current produces a magnetic field, which by Lenz's law, opposes the motion which creates it.

If there is a fault in the coil and the wire is broken (the failure mode of the original meters in those AVO VCMs) then current cannot flow and the pointer continues to swing undamped as you have described.

When the meter is mounted in the VCM and connected to its circuits, components in the VCM complete the circuit and provide damping.

Many good multimeters have an 'off' position on their range switch which shorts the meter to damp it to reduce the risk of damage during transport.

David