VCM163 Question

[kkyahoo]

Gents,

A couple of questions for you:

If you switch on the VCM163 tester with the gm switch in the cal position does the gm meter go hard over to the right and stay there? On mine a gentle tap on the meter brings the needle back to the cal mark.

If you switch the gm switch from cal to say the 0-60mA range does the meter go back to zero. Mine is one and a half small divisions up from zero.

Thanks

Kevin

[Dekatron]

On all of my VCM163s the meter moves like yours but it doesn't stay to the left , it settles at the calibration mark after just a moment, nor does it stop before reaching zero.

It seems like your meter movement needs to be cleaned and adjusted.

[kkyahoo]

Thanks martin,

I decided to gently apply some compressed air from a can to clear any debris. This just made things worse. The meter then lost it's zero point and the cal was at 4 on the meter.

It turns out that I had dislodged a manufacturer's sticky label that had fallen into the back of the movement. Now fixed and the meter is ok but still sticks at FSD+. I guess I could move the end stop to stop it going into the sticky zone but it really needs a service.

I called Herts meters and they will refurb it for 100 gbp, expensive but probably worth it.

Cheers

Kevin

[Dekatron]

Yes, a refurb is sometimes the best solution on these meters as it will sometimes clean them up even better than they were when they were new.

[kkyahoo]

Interesting, now I have moved the bit of paper from the meter movement it looks reasonably ok. It still sticks on switch on, but you can switch on without it being in cal mode to avoid this.

I noticed that when you switch to the 0-60ma/V range from cal the meter goes down to one small divison above zero. If you then go to the ma switch and put it in the meter short position (the blob!) the meter then goes to zero so I think the amp is slightly noisy. Is this a common problem?

Thanks

Kevin

[Dekatron]

There are a few reasons for these meters behaving like yours do, some of these are:

*. Corrosion and/or rust not just on the internal parts but also on the posts where the lugs are screwed resulting in high contact resistance, that resistance sometimes varies with moisture in the air

*. Static electricity affecting the movement as it can stick at several points along the scale

*. Corroded needle bearings, worn bearings, corroded hair springs, small pieces inside the movement like metal pieces and also corrosion that brakes the moving coil, corrosion between parts that carry the current from the posts to the movement

*. Nearby equipment can induce a signal on the mains that makes the amplifier react, solar power converters are known to do this as they work at relatively low frequencies

To really see how your amplifier behaves at the zero point you'll have to short the inputs and see what output you get on your meter.

Beware of using high pressure air as it can skew the moving coil itself so it touches the magnet braking the movement or it can twist the hair springs also resulting in a skewed moving of the moving coil.

All of the reasons for a bad movement above is in my opinion a good reason for having the movement refurbished.

[MotorBikeLes]

I have never seen a VCM163 meter movement close up, so this may not be relevant. However, on the high sensitivity thermocouple meters of 50+ years ago, there used to be a card flag at each end which acted as a stop to a fast moving needle (pointer).
Sometimes, the needle would stick to the card. The solution was a red hot screwdriver or similar to re-char the edge of the card, which is how they were produced in manufacture.
A wet finger on the glass would stop static affecting the needle for a while.
Les.

[Dekatron]

All VCM163 movements that I have seen have a springy type of metal with a ceramic roller threaded over them as a stopper on each end.

You can clean the perspex and spray it with for instance PRF 8-88 antistatic spray if you want to remove the antistatic from the perspex but don't spray it on the scale as it will dissolve the printed text, on the GM meter you will also have to remove the adjustable scale from the perspex before spraying it.

I've included a photo of a corroded movement where the end stops can be seen.