AVO VCM-163 meter “static”

[Craig Sawyers]

Looking at the service manual for the 163, they say:

"Use the 500V megger, test the insulation between:

(1) Zero line and mains wire (Red - line, Black - neutral
(2) Zero line and the frame of the instrument

(a convention point on the zero line is the junction of LP1 and LP2)

With the instrument supply switch at "on" the insulation should be greater than 100Mohm"

Confirming that the instrument is truly floating inside the earthed casework.

[Brian Beck]

I’m not sure why there has to be in excess of 100 Mohms of isolation except as a verification of there being no short. Maybe to prevent ground loops from interfering with measurements when there is other connected equipment which is also plugged into AC power. Normally there would be no other path to ground when only a valve under test is plugged in.

In some high-end audio amplifiers, ground loops (hum) are broken up by floating the electronics common from ground, like the AVO. To prevent a shock hazard should a power wire fail and contact the electronics, beefy back-to-back diodes are connected in parallel from electronics common to power ground, clamping any voltage in a failure to no more than about +/-1 volt before the circuit breaker blows. But in normal use the electronics are essentially floating because the diodes are not conducting. A resistor of 100 ohms to 1000 ohms is sometimes added across the diodes to establish a zero-voltage float reference for the electronics, but since that resistor value is far higher than the cable resistances that can cause ground loops, such loops are broken. Sometimes a small cap is also put across these components to keep the electronics from behaving as an antenna to RF signals. The cap’s impedance at 50 or 60 Hz is negligible.

I don’t know that any of this would help with the meter static issue, when built-up charges could generate thousands of volts between the polystyrene cover and the needle. I think the needle and its armature must be connected, even through a large value resistor, to the a cover, which has been made slightly conductive by some anti-static coating. It’s the relative voltage between these two parts that matters, irrespective of ground - I think...

[newlite4]

We have exactly this problem with our Metrix rubber cased analog multimeters, we have about 20 of them. Whenever the plastic meter window is rubbed, the needle goes mad. Anti-static spray helps. Spray it on but don't wipe it off, for best results.
Neil

[Dekatron]

Use the spray on the inside of the glass, it will last longer there as it can't be rubbed off, but be careful so you don't spray on the scale. It is more cumbersome to spray the inside but it works better that way.

[Bazz4CQJ]

Martin, I'd guess that you probably have more experience of the 163 than most other people. What is it your evaluation as to how common this problem is? I don't recall ever having seen any mention of it previously, nor do I believe it affects my 163.

Isn't there an external factor here - the extent to which Brian is acquiring static charge, after all, no charge - no problem? Have you ever tried moving the 163 to different locations, especially to somewhere like the garage with a concrete floor to see if the problem remains?

B

[Dekatron]

Moving coil meter manufacturers treat their meters with anti static measures so that static electricity won’t affect repeatability, linearity and other errors when the meters are used. This can sometimes be read in datasheets.

Since temperature and humidity affects how static electricity is built up and how it affects meters some of you might never see the effects but some will be affected almost always.

It also seems that meters that are old and which have been stored poorly, which have corroded internals, are more susceptible to static electricity buildup - at least that is so in my experience.

[Brian Beck]

Originally Posted by Dekatron

Since temperature and humidity affects how static electricity is built up and how it affects meters some of you might never see the effects but some will be affected almost always.

As I mentioned in my first post, the irony in my case is that I live in coastal South Florida, a place notorious for its high humidity. We never experience static “sparks” here. Granted the 163 enjoys the same continually air-conditioned space that I do, with temps held near 75 deg F and with relative humidity in the range of 40 to 45%. But that’s hardly dry like Arizona or Sweden in the wintertime.

I think that this is just a design flaw in the otherwise great 163. I have many items of test equipment with analog meters and I have never experienced this problem before.

I am determined to fix it, because I find the 163 most useful. The suggestions here have been very helpful.

[Bazz4CQJ]

I have an old Farnell L30 power supply, which I guess I've had for at least 40 years and I've never seen a static charge effect on the meter...until today.

The plastic cover on the meter had acquired a slightly hazy appearance, which I think was just a the cumulative effects of fine scratches rather than oxidation, so I decided to lightly polish it with some high quality (Meguiars) car polishing compound, and this did quite a good job in terms of improving transparency.

But wow, is that thing static-sensitive now! I wonder if there could have been an anti-static coating on the plastic which the compound has removed?

B

[G6Tanuki]

Static is a big issue with some meters: the signal-strength meter on my Racal RA217 can be made to go all over the place by rubbing the front.

In the middle of the last century, before antistatic sprays became easily/cheaply available, wiping meter-glasses over with washing-up-liquid then wiping it off (so only a very thin, invisible film remains) was the accepted trick. The film left behind is slightly conductive so drains away the static.

[Bazz4CQJ]

Curiously, the meter steadily returned to its "static immune" condition over a period of some hours. I'm not sure how to explain that, unless it is purely due to adsorption of atmospheric water vapour?

B