AVO VCM 163 Meters

[waran2005]

Hi all,

Just doing a few checks on my VCM 163, after removing the meters from the VCM and checking current required for FSD, the Ia meter indicates FSD at 21 uA and the mA/V meter indicates FSD is at 35 uA - am I missing something obvious here? These are not doing what I expect, shouldn't FSD be at 50 uA for both these meters - any ideas?

Meter resistance values are correct, the low uA required for FSd would also explain why the meters are off scale when selected to check AC, MA/V.

I have replaced the set AC / current meter with a 2.4k resistor as I don't want to damage it during circuit testing, the current measured is around 45 uA which is I believe required for the meter pointer to reach the set AC / calibration position

Any help appreciated.

[Bazz4CQJ]

The two meters are not quite the same. The Gm meter is straight forward, but the Ia meter has another component (a thermistor?) built inside it. My recollection is that both meters are 50uA. It's not uncommon for them to have shorted turns and read low, but getting FSD at 35uA is unusual.

There's stack's of info in previous threads.

B

[waran2005]

Hi Bazz,

It is unusual, not reaching FSD I can understand as meters age, but this is very strange. I did do a search, I will look again.

Cheers.

[Dekatron]

I'm not sure what's happening here but lately there have been a few, at least three that I know of, VCM 163's that have been sold with meters in similar state with to low FSD current. Maybe storage has affected the magnet in the movement from external magnetic fields or people have tried to glue extra Neodymium magnets to the movement magnet.

Yours is the first where we might have a chance to find out what's happening.

Do you have the possibility to put some photos of the meters here on the forum? If so please take photos of any stickers on them.

It would be even better if you could open them up and remove the scales to take some photos of the internal movement so we can see if they have been modified, they might not even be original movements.

However to get them to work in your tester it should be enough to add a resistor in parallel with the movement and then one in series with them - so you shouldn't need to open them up, unless you want to hide the extra resistors inside the housing.

Whatever you do make sure that you don't get any metal swarf in the movement nor get any rosin from the solder splattering the movement as both will affect the movement negatively and also make sure that you don't damage the needle or hair-springs. Shorting the movement will dampen its swing during handling - just like the switch on the front of the VCM 163 does when wet to the black dot position.

The parallel resistor would carry the extra current needed to get to 50uA (29uA for the 21uA movement & 15uA for the 35uA movement). The series resistor would compensate for the total resistance. The left IA-meter should have a total resistance of 2400 Ohm, the internal movement should be close to 1500 Ohm and it is temperature compensated with one ordinary resistor, one NTC and one wire wound resistor, the right gm-movement should have an internal resistance of 1500 Ohm as it is not compensated.

[waran2005]

Hi Martin,

Thanks for the detailed reply, and I'm glad I am not the only one to have experienced this meter movement anomaly. I belive the meter movements to be original and the resistance for current meter is 2.4k. I have taken some photos of the Ia meter removed from the case, there does appear to be corrosion and the set zero adjustment was hard over to the left as viewed from the front, that is the only way it would zero correctly.

Something makes me wonder if the hair springs are working correctly, tipping the meter on its side alone makes the needle move right accross to the FSD position by gravity alone, is that normal for one of these? The way the needle appears to move is that there are no springs in place at all, or extremely weak ones.

Please see the photos I've taken, I will take more when I get a chance.

Look forward to some feed back,

Thanks Martin.

[waran2005]

More photos

[waran2005]

More photos.

[waran2005]

Hi, I have figured out that the needle had been bent, so in the wrong place compared to the moving coil and hence the difficulty in zeroing the meter and why the adjuster was originally hard to the left. The movement now appears to work really nicely, but it still requires a lot less than the 50 uA for FSD. I have no idea why it is doing this, both movements appear to be fine, I'm going to do what you suggest Martin and correct using resistors, at least for now, so I can carry on with trying to get it to function properly.

I'm going to strip the thumb wheels, check them, clean and lubricate - none appear damaged so that's good. I'm also going to replace eloctrolytics and repair the knobs the way Martin has suggested as a few are damaged and are missing the covers / caps.

Unfortunately this VCM will never be immaculate, it has suffered some corrosion and the paint is bubbling in places, but inside it is 100% original and clean. I think with the meters reading correctly it will work to some extent until fully calibrated.

What do you guys use to remove / prevent static charge build up on meter glazing?

Cheers,

Waran.

[Tony DF]

You could try dryer sheets, wiped over the surface of the meter to dissipate the static.
Tony

[Dekatron]

Nice that you found the reason for the zeroing problem.

If you test the meters out of their plastic housing do they deflect the same, electrostatic charge as Tony mentions, can be a reason for strange deflection too, and sometimes for sticking too.

Test the moving coil now that it is out of its housing and also clean the bolts, nuts and lugs that they connect to as they sometimes gives rise to high contact resistance.

Be very careful if you are going to dismantle the thumb wheels as they crack easily and are unobtainable today. They once contained a white lubricant but even though some people have tried to discern what type it was no conclusion has been reached as far as I know. I once repaired a Yamaha Clavinova piano that had sticking keys and then I used a special compound used for lubricating plastic gears from Dow Corning named Molykote EM30L (I also tried YM-102 & 103 but used EM30L in the end) and I suppose that could be used here too. Use any lubricant sparingly as it will accumulate dust and grit and it should only be placed inside each of the thumb wheels in the slotted holes on one side where the metal springs can be seen. It is hard to get grease to the places where it needs to be but that is the best place to apply it, you might have to lift that flat metal spring very carefully to place grease underneath it too as there is a steel ball beneath the center there. You can see what each thumb wheel looks like inside in the photo where I have pointed out where all parts fit.

When desoldering the wires on the thumb wheel switches, if you decide to do so, use a cooling clamp on the metal piece from the thumb wheel as well as on the wires as you might damage the plastic with too much heat and the plastic sheath on the wires will melt.

There is an anti-static spray named PRF 8-88 that is used on meter perspex and plastic details. Spray it on the inside of the perspex and it will stay there for a long time but don't spray on the scale nor the movement, just the perspex and the housing. Handle them with care afterwards so you don't rub it off. You might have to clean the perspex with IPA before spraying and you might have to clean it of and respray as it sometimes forms bubbles and small dots that don't look to well.

[waran2005]

Hi Martin,

Thank you so much for all the advice you have given, you are an AVO VCM legend.

Meter deflection made no difference with or without the glazing, still not happy with the meters really, I'd like to get to the bottom of the problem, feels as though I've stuck a plaster on a wound.

I have made boards (as you suggested) to correct the meter errors, both now are within tolerance. After setting up on the bench and refitting into the 163 they appear to be responding normally when selected to the set mains voltage / mA/V calibration positions - see photo.

I have not done any further testing but there appears to be hope for the 163, I'm tempted to replace the Ia meter movement with something more durable, maybe a 1 mA movent driven by an op amp, I'll see how the rest of the repair / calibration procedure goes first.

I have taken everything you have said about the thumb wheels on board - thank you. I will take my time, hopefully nothing will break!

Cheers

Waran.

[waran2005]

Unfortunately the Ia meter is not working as it should, on further testing I found the movement is not linear, it is 50uA for FSD and bang on at the set AC position, but the lower the value the more inaccurate the meter becomes. for example, 50% FSD only require 19uA, so has a massive error.

I think this may be due to possible damage at the bearings, additional magnetism in one spot, spring damage - who knows. Does anyone have any idea how to correct or repair this? I could possly add a magnet to the movement somewhere but I doubt it will ever function as it should.

I have an old 1mA Sifam meter, I'm going to do some checks to confirm accuracy and maybe transplant the movement and drive it via an op amp. Any op amp designs / ideas would be appreciated as I've not done anything like this before.

Dussapointing though as the original meter has no damage to the coil, resistance measurement check was spot on. I think any repair I attempt will probably destroy the original movement, so probably best just to remove it and keep it safely stored away - oh well.

Martin, just a quickie regarding the thumb wheels, I didn't realise they are clipped together, only noticed after looking at the photo you attached. I guess they could break when seperating them due to the age of the plastic. I thought they would be a similar design to the bakelite ones, not clipped together. I can't see how you can truly clean and lubricate them without stripping them down, what do you normally do with them? I also noticed on my 163 that the nylon Bush where the pins go through at one end appears burnt / melted in one spot, so not sure if this is from a previous repair attempt or that a rod has got very hot!

[Longespee]

Have you looked at the weights, there are three points where they add it, I can see the glued spring weight opposite end of the needle.

I had the same problem as you had, a very clean and healthy machine, with internally clean meters, Martin helped me with a great many emails to and fro, eventually we managed to get everything working with shunts, but I was never happy with the result.
I then spoke to and old friend who after me explaining my problem and how can a meter that was basically untouched be so out of spec?, his first question was '' Were there what looked like Bird droppings in the meter when you took it apart? '', '' yes'' I replied, they were more like microscopic egg shell, but still debris, he said '' those bits were your meter weights, we used a kind of Tungsten paint years ago, which is why I asked as it falls off sometimes '', he then explained how to balance check the meter and adding some modelling paint to the points the meters worked perfectly.

[waran2005]

Hi,

Thanks for the advice, it is much appreciated. I had a good look at the movement and cannot find any added weights, they may well have fallen off. What you said makes a lot of sense, but during examination of the movement I noticed that there is a lot of play in the bearings of the moving coil, I suspect this to be the cause of all the errors, possibly in combination with weakened springs from years of use and abuse. I have now decided to replace the movement by using and old movement I have and will drive it via an op amp. So far I have compared the movement with the original and it will fit in the original case (with some additional holes) and its FSD matches the original AVO scale perfectly, so apart from making some new holes to fix it to the movement, it is a straight forward swap. Only thing is the needle is shorter than the original but I'm not too bothered by that. Another bonus apart from the movement sweep matching the original scale perfectly is that it's a taut baud movement, so at 1 mA should be virtually indestructible. I have made a prototype op amp which works well, I'm just waiting for some components so I can make a PCB to mount the additional electronics required. I intend for the op amp board to have a dedicated supply.

After owning this 163 I can't help but wonder what state other peoples movements may be in, every setting had been adjusted in this VCM to try and compensate the meter issues!

When I've had it working it works very well when comparing it to my standardised valve. With the meters in spec, and fully calibrated, this will be an excellent VCM - if not immaculate to look at.

Anyone ever extended needles on METER movements by any chance?

Cheers.

[Bazz4CQJ]

These notes may be useful. In my experience, the "weight" that's needed is only something like a small blob of paint or something; I think we are usually talking milligrams or even micrograms.

I once extended a 163 meter pointer using a pointer salvaged from an Avo Multiminor test meter; we are talking about seriously low cross sections!

B

[waran2005]

Hi Bazz,

That's very useful, thank you. I will give it a try, I may also attempt to adjust the bearings too. I will let you know how I get on, thanks again.

I'm so impressed with the way that taut baud op amp driven movement operates, I may well be tempted to replace the movements in both meters, I have a few of those meters so can experiment a little more before deciding what to do.

[Bazz4CQJ]

Yes, I'm inclined to feel that once those meters have deteriorated to a certain point, fully recovering them to be stable and dependable gets hard and quite time-consuming. And that is even before you deal with the oscillator and amplifier boards. If not already sorted, they will definitely want new electrolytic caps and possibly other attention. I know that Martin has needed replace the original transistors in some units.

B

[Longespee]

Here's an article I found, the meter balance is as Bazz has linked, this goes into a little more detail on other aspects.

https://www.rfcafe.com/references/qs...b-1943-qst.htm

[Craig Sawyers]

I'm not too sure about the filed paperclip as a method of removing magnetic junk from the gap.

I've usually done this with a thin strip of adhesive tape. Being thin and flexible there is less chance of messing up the rotating mechanism. You just fish for the rubbish in the gap.

The information regarding balancing is really interesting though.

Craig

[waran2005]

Well, I gave up with the original meter, I tried to adjust the bearings and weight etc, it just doesn't want to play ball, I think it has had its day.

So yesterday I fitted a new movement and part of its housing into the original. I still have to make a bit of plastic to put in the zero needle hole as it is now adjusted from the rear and the original is missing, and also cover the old holes that were used for mounting the meter face plate.

I thought the easiest and fastest way to transplant the movement was to machine the bottom off the original case and do the same with the new Sifam movement and slot the slightly smaller Sifam housing / movement into the remains of the original case, this also allows for perfect alignet and height adjustment for the meter scale. The new taut band movement would never have fitted into the original case anyway, as much as I didn't like doing it I really didn't have any other options.

Seems to work well enough, I have test fitted and all good, calibration / linearity seems pretty reasonable, rest of the tester is way out though apart from the mA/V readings.

The roller selector / thumb wheels are a nightmare, I really have to get those removed and cleaned and working properly before even considering trying to calibrate it, there are so many dirty contacts, you have to rock each one back and fourth a dozen times before it makes a connection.

I have made a crude op amp, I've not done anything like this before, it seems to work well but is there anything I should also consider doing, incorporating into before permanently fitting it into the VCM?

Cheers all.