AVO VCM 163 open circuit meter coil.

[bud allen]

The mA/V meter has a bad open coil. There is literally no service in the USA for repairs. I have taken the guts out of a bad meter before and replaced it with new good guts from another, often with "surgery" to make it fit, and adjust for current and voltage.

However, the internals of this meter are a bit different. A straight forward coil of 1500 ohms (according to Martin's schematic notes) at 50ua, no internal resistors. Also, I don't know if the original meter is linier or not?

Is there a service in Europe which can re-wind the coil or has an acceptable replace meter or the guts I can fit to this shell to keep the original look.

I have several Simpson meters, but they are pre 1960's and not candidates. I think Martin noted he might look into have replacement meters made, but I don't see any links.

Any guidance is appreciated.

bud

[retailer]

A bad meter is not good news, while I haven't seen a 163 meter I'm quite sure that moving coil meters are linear, this is a characteristic of the moving coil meter, so if you did manage to find a suitable 'guts' to fit, linearity should not be an issue.

[Radio Wrangler]

Originally Posted by retailer

I'm quite sure that moving coil meters are linear, this is a characteristic of the moving coil meter,

This is one of the things that catches people out in the rare moments when they come across a non-linear one.

Meters have been made to have all sorts of characteristics by shaping the pole pieces and the central core. I've seen sinusoidal characteristics and custom ones to linearise sensors.

HP used meters with logarithmic laws in some audio level peters (The Psophometer or P***-off meter as it was called up the manufacturing lines )

Log meters have no zero volts/current/Watts on the scale of course, so without power, their pointer is rammed off scale at the left end. They're used to give a linearly graduated scale of decibels. Almost everyone who sees one for the first time assumes it's bust.

David

[bud allen]

I have worked on the Triplett 3444A, the transistor type. I had a friend at Triplett then, and although no one seems to know it there were three revisions of the "type" three.
The problems were two fold. The meter they used created a non linear feedback to the transistor circuit, and visa versa. The 3444 had no such issue. Mike told me, that Triplett basically gave up trying to make the readings linear, and with tubes losing ground in audio, they didn't see a reason to continue making the 3444A. The final revision is close to the 3444, less so at the low end of the scale. With a modern IC, it would have been easy.

[bud allen]

Looking into some possible can't-I-dates.

[retailer]

Radio Wrangler - your comments about nonlinear meter movements are interesting I had heard of movements with 4 magnetic poles and dynamometer type movements but not about shaped poles. I recall seeing a HP level meter with a linear dB scale in an electronics salvage store a few years back and I assumed that HP had used non linear electronics to drive a standard moving coil meter and so achieve a linear dB scale where in fact the movement itself is non linear, I guess one never stops learning.

[Radio Wrangler]

HP used both shaped pole and electronic logging at various times.

With the power off, the magnetic logger meters pointers are rammed off the left side of the scale by the spring. The electronic logger instruments use a plain linear meter that sits on the left end of the scale when the instrument isn't powered.

I've find it distrssing that AVO of all companies have made meters so easily damaged. I have a transistor characteristic meter with an open circuit coil and I managed to photograph the fused break in the wire (on the outside layer of the coil)

David

[Dekatron]

AVO didn't manufacture the meters for the VCM163 series of testers themselves, ICE in Milano, Italy made these meters and sold them to several companies as well as used them in their own series of analog multimeters. I've included a closeup of the Logo pressed into the inside back of the meter housing for an VCM163 meter.

The meters in the earlier series of VCM's were usually manufactured by AVO what I understand from reading documents at the IET. For their later models of transistor testers and some transistor testers for the military they used Sifam meters and one more brand that looks very similar to Sifam (which I can't remember the name of right now). This change of meters seems to have happened some time after the AVO Mk IV VCM was at its end of life when the new series with grey instruments were introduced, which in some cases can be found marked as Marconi on the front - like the AVO TT166 which is the same as Marconi TF2703. It was probably easier to buy cheap meters from other manufacturers than make them yourself at that time.

[bud allen]

Interesting that when you go to a manufacturers site, so many important specs are missing on meters. They will post the ua but not the resistance. I did find two possible meters.
Here my question for the real experts on this tester. The choice is a standard meter at 1800 ohms or a Taut-band at 960 ohms. I know I can add to the 960, but how critical is the original coil resistance of 1500 ohms? I can see ways to up the current output but C8 and C9 look to be a part of a feedback circuit to vt2 via C7, thus my question on the coil resistance affects.

[Craig Sawyers]

You can make any meter resistance and sensitivity work by using opamps. As far as the internal AVO circuit is concerned it sees the correct resistance, and if the gain is correct the meter gives the correct deflection.

I did precisely that with my CT160, and I'm about to do the same with my Mk2.

Craig

[Dekatron]

I like the Taut Band meters best since the ones I have tested have a more damped movement in themselves, however they are usually a bit more expensive.

You can either use an opamp solution or adjust the AVO circuit. It was a long time since I made tests on this but I used 100uA movements with an internal resistance up to 3000 Ohm - but I decided to leave the original circuit and use the Taut Band meters as I liked them more and wanted to keep it simple.

[bud allen]

Originally Posted by Dekatron

I like the Taut Band meters best since the ones I have tested have a more damped movement in themselves, however they are usually a bit more expensive.

You can either use an opamp solution or adjust the AVO circuit. It was a long time since I made tests on this but I used 100uA movements with an internal resistance up to 3000 Ohm - but I decided to leave the original circuit and use the Taut Band meters as I liked them more and wanted to keep it simple.

Thanks Martin... I should have noted both the T-band and analog, are 50ua.

I guess I will go with the 50ua Taut-B. But I will call Simpson tomorrow and see what they say.

[Radio Wrangler]

Once you bite the bullet on the opamp approach, you can use any sensitivity of meter movement you choose. Meters in the 1 to 10mA FSD bracket seem appreciably more rugged than their more sensitive brethren, and if you look at higher FSD currents, you start to find they're shunted.

I'd choose a meter which would look OK in the panel, scales could be made for and of a robust sensitivity, then I'd fix the sensitivity and resistance with some electronic trickery behind the scenes. This would also allow adjustment to accommodate ageing, which is something not too easy with many original movements.

David

[Bazz4CQJ]

Does anyone else remember that a couple of years ago, a guy appeared on the forum (I think he was French and not a regular contributor) who had produced an LCD panel producing the image of a moving coil meter meter as one means of dealing with 163 problems.

It was a really impressive bit of work; don't recall him being back since that post.

B

[Dekatron]

I think you mean the TFT screen for AVO mK IV in this thread: https://www.vintage-radio.net/forum/...d.php?t=121969

I got all the info from him plus a binary file (not the source code though) but never did build it myself.

Many meter companies sell similar meters, but I guess that they are prohibitively expensive if you want to have a custom made display made.

/Martin

[Bazz4CQJ]

Originally Posted by Dekatron

I think you mean the TFT screen for AVO mK IV in this thread: https://www.vintage-radio.net/forum/...d.php?t=121969

/Martin

yes, that's what I was thinking of Martin. Clever, but maybe a little complicated.

B

[retailer]

I missed seeing the original post by the Dutch guy but recently did see on youtube an Arduino/TFT display set up as a moving coil VU meter and thought that it could be done for a Avo VCM, the actual electronics and programming is not that hard, a while back I made a coil/transformer winder with stepper controlled auto traverse - all controlled by 2 microprocessors, but now being at the pointy end of a MKI restore complete with opamp meter driver I'd say the moving coil meter with opamp drive is by far the simpler and quicker way to go.

[woodchips]

Someone better at software than me!

Does anyone know what the resolution of the VGA display is? Is it 1/4 in size or resolution?

I have thrown away several 1/4 size VGA displays from HP test equipment, never checked for how many pixels. Not certain if I now have any, but is anyone interested if anymore turn up?

Must say, the opamp route does seem far easier, or getting an old analogue meter with 30K ohms per volt.

[Radio Wrangler]

The moving pointer meter seems more in-keeping with the VCM.

Both methods should work fine. The opamp route may remain fixable for a longer time.

Some people might think that having digital stuff inside would corrupt every valve tested but then the opamp involves silicon.

David

[Dekatron]

Maybe you could use one or two Philbrick units to keep in style with the testers?