AVO VCM-163 meter “static”

[Brian Beck]

Hello,

My first post here. I am a happy owner of a pristine AVO VCM-163. I have a number of tube (OK, “valve”) analyzers, but the 163 is my favorite daily-use analyzer.

My unit does have one quirk though, and I’m hoping that someone here will have an answer for it. At times the meters seem to move erratically, giving different readings for the same test conditions. I found that brushing my hand on the plastic meter covers causes the needles to move either toward or away from my hand. I suspect that static charges are building up on the plastic meter covers. This was verified by then buffing the covers with a dry cloth that made the needle positions even more erratic. This situation is odd because I live in humid Florida, although in a dry climate controlled home. The meter movements seem to be free and not sticky. I have not encountered this before in any of the other items in my rather large test equipment collection.

I sprayed an anti-static compound on the meter covers but that seemed to have only a slight and temporary effect.

Has anyone else observed this?

[trh01uk]

".....the 163 is my favorite daily-use analyzer"

Wow, you must be busy! I barely use my valve tester once a month!

Richard

[trh01uk]

Do these meter movements you describe, happen when the tester is switched off?

[Brian Beck]

A figure of speech obviously. But I do use the AVO regularly in my role as a consulting design engineer; perhaps weekly is more accurate.

My point is that the more sophisticated true analyzers that I have are not as easy or convenient to use as the solid-state 163 for routine tube checking. These include the Daystrom Weston CA-1630, New London 901A and RCA WT-100A. They’re fun but rarely needed.

Any thoughts on the meter static issue?

Originally Posted by trh01uk

Do these meter movements you describe, happen when the tester is switched off?

That’s a great question and the answer is yes. It is affected whether power is on or off.

Here’s a short clip showing the phenomenon. Power is off in this video. Lest one might be tempted to say “Just stop wiping the meters”, I should add that this problem sometimes seems to occur with the least provocation. Moving my hand nearby when plugging in valves or the mere close-by presence of a piece of paper will cause this problem.

https://youtu.be/sz69OJqQawk

[Brian Beck]

I have also wondered if perhaps the meter movement (not the coil itself) is floating. I could ground the movement, but I don’t hold out much hope for that and I am loathe to open irreplaceable and otherwise working meters.

[Bazz4CQJ]

I've owned a 163 for ~15 years and I don't think that I ever felt that the meters are sensitive to static. Those meters do not enjoy the best reputation for durability, but I don't believe I've heard a suggestion of problems with static previously. Of course, when it gets cold in here in winter, on goes the heating, and the relative humidity in many homes is pretty low, so it should show up then.

B

[Brian Beck]

Thanks for replying. Since my first post I have tried a couple of things:

A friend suggested covering the meter in metal screen. That’s terribly unwieldy, but it might work. When I flatten a piece of aluminum foil over the cover it does seem to help (peeking in from the side to see the needle). As soon as it’s pulled away, the needle starts wandering again.

I tried another experiment. I grabbed onto metal parts of the chassis with my left hand while I rubbed the meter cover with a cloth in my right hand. Still does it. I think a piece of cloth could have opposite charges on either side of it, so my body’s potential is mostly irrelevant.

Then I pried off the plastic cover, which makes me nervous. I attached a length of very skinny wire-wrap wire to a stationary piece of the meter movement and was able to snap the cover back on with the wire coming out. I attached the free end of this wire to the chassis. Nope, it still does it. It occurs to me now that the moveable armature and needle might be electrically isolated by the jewels from the stationary part. There’s a watch-style spring in there that should be grounded on one end, but who knows? And you can’t just hang a grounding wire on that delicate armature.

My next step might be to find a more aggressive anti-static compound and spray both inner and outer surfaces of the cover with it. A chemical screen, if you will. The covers feel like polystyrene, which is about as staticky a material as exists. Maybe applying a better antistatic (conductive) film would help.

[Dekatron]

Those meters are unfortunately sensitive to static electricity, and seems to become more sensitive over the years as the hair springs loose their strength.

If you ever spray them with anti-static spray keep the spray well away from the scales as the paint on the scales is dissolved by most of these sprays.

I've done a few things to make these meters behave better. First I removed the movement and the scale from the case and then cleaned the case and perspex in lukewarm water with Yes dish-washing soap, the one without any perfume, then I have used IPO or ethanol to remove anything that the dish-washing soap didn't get rid of, sometimes the cleaning has to be done more than one time to get rid of all dirt. After cleaning them properly I have sprayed them with PRF 8-88/220 anti-static spray inside and out. Using rubber gloves when doing this is good so you don't put any fingerprints on them after having cleaned them. Some movements have had a line of grease at the bottom back, which I replaced, as this is used for catching dust and scrap that gets into the case hopefully keeping it from getting to the movement (I've only seen a few meters with this grease so I am not sure it was AVO who placed it there in the first place).

This has worked in almost all cases, but in some cases the hair springs have lost a lot of their spring strength over the years so the needle will swing easily even for small amounts of static. These have had to have their hair springs replaced with new ones.

Lastly I put a small bag of silica desiccant inside the meter case, taping it to the bottom of the case, beneath the meter movement, not on the back of the movement.

You can always send your meters to Herts Meters to have them cleaned and treated (they will look at the hair springs and needle mounts too): http://www.hertsmeter.com/

[Brian Beck]

Martin,

Thank you for a thorough and apparently conclusive response!

I will try the PRF 8-88/220 formula.

That theory about the meter’s movement spring becoming weaker and therefore more responsive to static forces is interesting. I would think that if a spring has lost enough of its force to become problematic with static, it would also cause readings to be much higher. This is because the meter would then have a higher sensitivity with its weaker spring force to counter the force of the coil current in a magnetic field. Small changes in a meter’s sensitivity can be calibrated out, but big changes probably could not be corrected.

[Radio Wrangler]

THe weakening hair spring idea doesn't seem right to me either, as Brian says, the sensitivity would be dramatically higher unless it was matched by weakening of the magnet.

A good antistatic coat on the inside ought to do the trick, so long as it has a leakage path to cabinert earth potential. THe charge on the outside can do whatever it likes because the pointer sees the field on the inside.

I have a big honkin Farnell power supply (70v, 10A) and the plastic meter fronts, if rubbed can make it read anything you feel like. They're recessed so not rubbing is no problem and solves the issue.

David

[stuarth]

Anti static coatings work by attracting moisture from the air, and it’s the moisture which prevents static build up. So when you wipe off the static spray, it’s a compromise between leaving a coating on meter and leaving it looking “clean”.

Stuart

[Brian Beck]

Originally Posted by Radio Wrangler

if rubbed can make it read anything you feel like.

Well, now we’re getting somewhere! So I guess if I test an old valve and find its transconductance wanting, I can restore it to a new value by rubbing the meter like a genie’s bottle until the right value shows up?

I will try anti-static coatings on the inside of the cover and try to create a leakage path from cover to chassis.

Originally Posted by stuarth

Anti static coatings work by attracting moisture from the air, and it’s the moisture which prevents static build up. So when you wipe off the static spray, it’s a compromise between leaving a coating on meter and leaving it looking “clean”.

This reminds me of the challenges of finding a suitable slightly-conductive coating for Mylar electrostatic loudspeaker diaphragms.

The best solution would probably be vacuum sputtering metal ions into the plastic. But as much as I love the VCM-163 I do have a weakly practical side.

[Dekatron]

Yes, the sensitivity was way out on the meters with the weakened hair springs, so much that I didn't even consider measuring how much.

The needle flew all over the place when you blew just a little on it.

No amount of anti-static spray would have helped there so the hair springs really needed to be replaced before anything could be done to repair those meters - that is where old meters with burned moving coils but with good hair springs are invaluable as spare parts, and of course other parts, that can be salvaged too.

[Bazz4CQJ]

Originally Posted by Brian Beck

The best solution would probably be vacuum sputtering metal ions into the plastic. But as much as I love the VCM-163 I do have a weakly practical side.

Do you know anyone involved with electron microscopes? They often sputter an invisibly fine layer of carbon on to non-conductive specimens so that the beam current can be conducted away, or else it causes charging and the image quality is then poor.

I wonder if it's possible to achieve anything with that really fine graphite powder sold to lubricate locks?

I seem to have lucked out on this one - just never had the problem.

B

[Brian Beck]

Originally Posted by Bazz4CQJ

I wonder if it's possible to achieve anything with that really fine graphite powder sold to lubricate locks

Rubbing graphite powder into Mylar films is a technique used to make electrostatic loudspeaker diaphragms slightly conductive. I don’t know how well polystyrene would accept particles or if an amount sufficient to solve the static would cause visible darkening. Easy to try on a test sample.

I’m fresh out of buddies with electron microscopes sadly.

[trh01uk]

Having just caught up with this thread, I am left wondering exactly how an electrostatic charge can move a meter needle?

I would have thought the needle itself would need to be an insulator to make this at all possible - and as far as I know most meter needles are piece of flattened metal wire, which is electrically connected back to the spring and bearing assembly (and ultimately grounded to the body of the instrument). Is the meter on the AVO VCM163 not like that?

Richard

[Craig Sawyers]

Most AVO VCM's float with respect to chassis/safety ground. The 163, Mk IV, Mk III are like this. Oddly the CT160 has the 0V end of the transformers wired to chassis ground.

Craig

[trh01uk]

Craig,

If I understand your comment correctly, any static charge then would build up between the meter cover/glass and the entire body of the VCM163 (which as you say is not earthed).

I can't at the moment see how earthing the VCM163 would change this situation, since its perfectly possible for an insulator (e.g. the meter glass) to be charged with respect to earth (i.e. the entire planet).

Either way, the meter needle is moving because it is connected to one pole of the electrostatic 2-pole created here - and as a lightly movable item it will respond to the force - just as gold leaf does in a typical electrostatic demo.

Am I understanding this correctly?

Richard

[Radio Wrangler]

Yes, so what is needed is to prevent the build-up or more likely induction (electric induction not magnetic!) of charge on the inside of the glass with respect to the meter pointer. The meter pointer will be connected via the hair springs to the main meter terminals.

Those termilnals are likely to be used floating, depending on the measurements switch setting.

David

[Craig Sawyers]

Originally Posted by trh01uk

Craig,

If I understand your comment correctly, any static charge then would build up between the meter cover/glass and the entire body of the VCM163 (which as you say is not earthed).

Am I understanding this correctly?

Richard

The case metalwork is (correctly) connected to safety ground from the mains input. But the guts are not connected to the chassis, and hence are floating.

The transformers have interwinding screens. For the 163 these are connected to chassis ground. In the Mk IV and MK III they are connected to 0V internally (in fact the 0V end of the Va and Vs transformer)

But apart from where the transformer screens are connected - the guts are floating and have no connection to safety/chassis ground.