Sticky AVO VCM meter movement needing repair

[Ricardo]

Here is a photo of the front of the original AVO VCM meter removed from my valve tester. It has been replaced with a new Simpson 29T meter, as described in a separate thread.



The main issue with this meter is that the needle tends to stick mostly at around 80%. Otherwise it seems fairly accurate. I have however noticed what I presume are a couple of magnets fitted by a previous owner; I have circled these in red on the photo.

If I can do so without too much likelihood of causing further damage, I am considering trying to clean any magnetic particles inside the coil gap, inspecting the hairsprings, etc, but it isn’t obvious to me how to safely proceed to remove the movement from the case.

I would be grateful of advice or recommendations from anyone with past experience of working on these meter movements on how to do this. What are my chances of a successful repair? Is it just too risky?

Richard

[Station X]

The movement is held in the case by its terminal screws. Remove the nuts from the screws and the movement can be withdrawn from the case.

[Diabolical Artificer]

I had the same issue recently and managed to fix it by very carefully removing tiny detritus by slotting a slim strip of double sided tape, only one side of sticky, backing tape in place, down between the magnet and fixed coil. It's mighty tricky though, bit like breaking into safe breaking whilst the diplomat next door is having a kip. Tongue at the right angle, hold your breath an all that. Also loosened the screw a tad that holds the moving meter shaft a tad.

Andy.

[Radio Wrangler]

I've also added paper shimming between the coil former and the centre pole by both bearings to take any force off of the bearings if I caught the coil in my cleaning operations. It just makes things a bit less fragile. Non-magnetic tweezers are definitely required.

David

[Dekatron]

When I used to clean meters like these I hooked them up to the measuring circuit which I described in a thread here on the forum many years ago, a simple circuit to vary the current through the meter for checking of FSD and internal resistance. By doing that I could adjust the current to whatever value I want so that I can turn the moving coil into a position so I can reach the swarfs more easily. This way I don't need to touch the moving coil at all and it can be left in the housing in most occasions.

Then I actually used small sewing needles made out of magnetic metal, but I heated them over a flame to make them non magnetic, or hit them with a hammer on a small vice to get the same result - I tried different demagnitizers but heating and banging them was easier. I use other methods to but this was easiest. Then when you put them into the movement you use the sharp point to touch the swarf and by the small magnetism induced in the needle at that moment the swarf will stick to the sharp end of the needle and can be lifted from the magnet. I read about this in a very old article but I can't find that article now as I lost it in an SSD crash I had some years ago, but it was published in one of the common UK/US magazines. If you are careful you can use the needle even if it is still slightly magnetic as that makes the swarf almost jump to the sharp tip, but it also makes it easier for the needle to smash into the movement.

[Ricardo]

Thanks for all the comments and suggestions to date.

So far I am still struggling at the first hurdle of simply withdrawing the movement from the case!

The nuts on the rear appear to be somewhat recessed into the case and I can't seem to turn the nuts or the connection bolts within these nuts. The connection bolts don't obviously align with something that can be undone from inside the case.

Here are a couple of photos of the offending nuts and connection bolts.




Richard

[Dekatron]

You can't remove these screws from the outside.

Before you remove the movement from its housing I suggest using the meter test circuit, I've included the circuit here again, you can do most of the cleaning with the meter movement still in its housing.

Before removing the meter movement, if you go that route you should get hold of two standoffs with British BA threads that you can use to screw into the positions for the hexagonal standoffs that you remove. These extra standoffs should be at least 5-6 cm long.

If you look at your first picture I'll make references to that one, also remember to take a lot of photos when you do this so you know where each piece should go and at what position it was at before dismantling, also keep the scale in place as that will protect the needle and front hairspring while out of the housing - do rememeber to not touch the meter scale as the paint will easily come off:

1. First unsolder the black wire at the bottom going to the swamp resistor on the bottom right.

2. Now unscrew the two small screws holding the small end stops for the needle, the right screw also holds the magnetic shunt used for magnetic calibration, if that arm is not glued you'll ned to use a marker pen to draw its edge so you can refit it at the same position

3. Now put the meter housing flat on its back and secure it in a vice or something similar, you can clamp down on the two screws if you protect their threads with wor instance the nuts. Now you will see two hexagonal standoffs, one on each side - you now need to unscrew both of these - keep hold of the meter movement so it won't fall in and damage the rear needle bearing and rear hairspring.

4. Now that the meter movement is loose you can lift it out of its housing and unsolder the rear black wire.

5. If you got hold of the long standoffs I suggested above you can now screw them back into the housing and then put the meter movement on top of these and fasten it with the two short hexaginal standoffs you just unscrewed from the meter - by doing like this you will have a steady movement to work on, either by just laying it on the table on either side you wish, the scale will work as a stand on three sides and the bottom of the meter movement as a stand on the fourth side. Or you can keep it clamped in the vice now with a steady way of cleaning it.

Now the meter is out of its housing. Just do all of this in reverse to put it back in.

Take a moment to see if you can leave it in the housing and clean it from swarfs without removing it.

[Ricardo]

Martin,

Many thanks for the detailed description.

It certainly seems more difficult to remove the movement to examine it properly than I had hoped.

I built a version of the meter test circuit to check the accuracy and behaviour of the movement. That allows me to set the needle to various positions in a controlled manner, so as you have suggested I can try to clean any metal particles from the coil gap from the front without removing the meter.

I think there might be an issue with the front hairspring. There seems to be kink towards the end which is attached to the zero adjuster and there might be a slight twist in it also. I have tried to take some photos. 1) – Shows the right side of the hairspring connected to the zero adjuster with the meter reading near zero, 2) Shows the left side of the hairspring again with the meter near zero, 3) Shows the right side of the hairspring with the meter reading about 80%.



I have also taken a photo showing the screws (A and B) which I understand from your description need to be removed before unscrewing the hexagonal standoffs if the meter is going to be removed from the case. Does that mean that the fine wires, presumably connecting to the movement, via the end stops would need to be unsoldered from the tags where shown?



Regards,
Richard

[Dekatron]

That kink doesn't look good as it restricts the hair spring coiling/uncoiling as it looks like the turns touch - could someone have been at it before, trying to adjust it?

You don't need to unsolder the stops, just remove the pieces they are soldered to. Either take good photos of their position or draw their outline with a marker pen so you know what position they were in.