Making a new meter for the AVO VCM Mk III, Mk IV, CT160 and CT160A.

[Dekatron]

Hi All!

I am currently making new meters for all of my AVO VCMs that have broken meters and I am using the Simpson 29T "Rectangular" 25µA Taut Band meter as it is a very tough meter compared to regular moving coil meters and it almost looks like the original meters.

The Simpson 29T meter fits almost perfectly in the hole where the old meter is put, I just needed to carve four small indentations in the front panel to let the four case screws through. The old rubber gasket can be used if it is still soft and can be stretched somewhat, otherwise a new one will have to be made. The meter can be fastened by four large washers on each case screw, that way it will stay flush to the front.

A small component board can be screwed to the meter connectors and then two cable ties can be used to fasten the old component board inside the VCM to the new meter board, so it will not swing loosely inside the VCM. The old meter wires can either be screwed or soldered to the new meter board. On this new board I have fitted two diodes and a capacitor like AVO did in the AVO Mk IV VCM, one diode in each direction as meter protection.

A datasheet for the Simpson 29T "rectangular" meter can be found here: http://www.simpsonelectric.com/uploa..._datasheet.pdf

The Simpson 29T 25µA Taut Band meter has an internal resistance of 1960 Ohm which means that it only needs one resistor in series with the meter and one in parallel with the meter to make it behave like the AVO 30µA meter which has an internal resistance of 3250 Ohm (1940 Ohm in series and 19500 Ohm in parallel).

Simpson manufactures the same 25µA Taut Band meter in many different versions but the rectangular type is the one looking most like the original meters. The Taut Band meter is the only one that can be used as it has the necessary low internal resistance to make it possible to use resistors in series and in parallel to make it behave like the original AVO meters.

I had to redraw the scale as the Simpson meters use 100 degree deflection whereas the original AVO meter uses 90 degree deflection. This means that you can not use the Simpson movement with the original AVO scale as they do not line up. I tried to fit the Simpson movement inside the original AVO meter but I could not find an easy solution so I decided to keep the Simpson housing and make a new scale. I have made new scales that I have laminated and put in my meters and they work excellently.

I might put some more time in the future to fit the Simpson movement inside an AVO meter, but I do not know when that will be.

I still have the job to make the window for the "over current" lamp in the AVO Mk III & CT160 / CT160A. Since I used a laminated scale I can easily cut a window in the original simpson metal scale and also make a window at the back of the meter case and glue a thin piece of plastic or glass there to let the light through and keep the meter case sealed. I have used a glossy plastic sheet for the lamination but I am looking for a plastic sheet that has a matted surface, but have not found any so far.

The scale in the picture is not the final scale as I have changed the leakage scale so it fits better with the pointer on the needles on my meters. I will also redesign the CT160 / CT160A scale to fit better.

I will write more in this thread when I have finished the scales for all meters, and I'll also show pictures of how I mounted my meters. I'll also publish the scales here for free download so you can print them and use them yourself if you want to.

[Nicko]

Nice work - what s/w are you using to draw the scales?

Cheers

[Dekatron]

Thank you Nicko!

I am using PaintShopPRO 7.04, a really old Windows software, but I know how to use it so well that I do not bother to upgrade as long as it runs on my computer. I guess more modern softwares would have many functions to stretch and manipulate pictures that would help me but I simply do not have the time to learn about all those features.

I am using a mix of bitmap images and vector graphics and a lot of image manipulation as I mainly use scans from original scales and adapt them to the new scale.

I try to do as little as possible to the original scale, but these scales were a real challange as I had to remake them from 90 dgrees to 100 degrees deflection, so there were a lot of vector lines added so I could measure where every line should be. The tests I made afterwards showed that the lines were in the correct positions.

I have made new scales for my VCM163 where I made them from the bottom up since the scales in my orignal meters were so faint that they were only used as a background while making the new ones. I used a pair of Kyoritsu KM-118 meters with a totally different size of the scale so it was easy to choose to do new scales when the originals were faint to.

I am going to test my new CT160 / CT160A scale today and also check if the Mk IV scale differs anywhere to the Mk III scale apart from how the plastic has been molded.

The scales included below are not the finished ones but I'd like to share them so you can see what they will look like, I also included a picture of a freshly laminated scale before it was cut out. I've also included the original scans from my own AVO meters that I used as a base for my new scales.

[Colin]

Really excellent work Martin, very well dome!

[Kevin Hoyland]

Hello Martin.
Outstanding work. I have an Avo mk 2 VCM working but seeing a new meter that will fit the AVO VCMs will give hope to all the People with a broken meter.
Thank you Martin for all your work on the AVOs.
Regards Kev.

[Dekatron]

Thank you Kevin and Colin!

If someone can supply me with a 600dpi scan of an original AVO Mk I / II scale I can adapt my scale to fit to the Simpson meters too.

But there will be a need for a meter amplifier anyway as the internal resistance of the AVO Mk I / II meters is much lower than the Simpson 29T (I can't find any other Simpson meter that has the correct FSD and internal resistance either).

[Dekatron]

A few photos of the prototype meter for my AVO Mk III, and the last photo showing that it is fully working as the needle is sitting at the calibration line! The AVO Mk III I tested my meter on is fully calibrated and in fully working order and only needs one more part to be like new, and that is the black bakelite protective bar across the roller switch, sadly it was broken when I bought it.

I have mounted all necessary components on the brown component board at the back of the meter, which I showed in the diagram in the first message. I will make a new turret board when my turrets arrive.

I decided to change the way I mounted the meter from large washers at the back to two U-formed aluminum bars stretching across two of the mounting screws at the back of the meter and then upwards and downwards gripping the front panel at the back. Using washers was not so good because the were not sitting straigth and they were bending scratching the back of the meter house. The meter can't move sideways as the small indentations made in the front panel holds the four screws at the back of the meter in place.

[Richard]

Nice job

[valvekits]

Nice work Martin.
I took a photo of an old mark I several years ago, don't know if it will help the cause.


Eddie

[Dekatron]

Thank you Richard!

Here are some photos of the prototype with a window at the back. Since the scale is held in place with two small screws at the top there was not much space to make the hole. The round protruding part containing the movement also takes a lot of space so the window could only be made approximately 15mm high by 60mm wide, but it is enough to let light through from a lamp!

Since I had to remove the movement from the meter house I was trying to see how I should align it once it was put back again. Fortuanetely that had a very easy solution! It turned out that the metal scale is held by four screws, two on the movement and two in the housing. The two screws on the movement are made in such a fashion that they align the scale to the movement and when the second pair of screws are tightened in the housing the movement and scale are aligned nicely! Thank you Simpson Meters for making it work like that! Without that nice feature it would have been harder aligning all parts after the movement had been removed for all drilling, sawing and grinding. This makes it really easy to make these meters compared to how it could have been if there was no easy way to align all parts again.

I'll also make some tests with a red led placed somewhere inside the meter house, but out of sight, shining sideways onto the scale. It will not be "original" any more then but it might actually look and work better, and also be so much easier to make than drilling, sawing and grinding the very hard bakelite material.

The first photo shows the window with white light from my desk lamp, in the second photo I have put a red lens at the back of the meter to simulate the red lamp from the VCMs and in the third photo you can see the window with a clear plastic piece taped to protect the insides of the meter from dust.

[Dekatron]

Thank you Eddie!

Your photo must be from an AVO Two Panel Valve Tester like this one: http://www.richardsradios.co.uk/valvetester.html

The models I meant look like this one: http://www.radiomuseum.org/r/avo_val...istic_met.html

The scale for the AVO Mk I / II is almost identical to the AVO Mk III but the leakage scale is different.

[valvekits]

Thanks for that Martin. I had always assumed that the two panel tester was a MkI and I was beginning to think that you had suddenly become a black belt in Photoshop considering the rectangular shape of the two panel scale!

Cheers
Eddie

[Dekatron]

I almost have the Black Belt in PaintShopPro 7 after all these hours editing AVO handbooks, Valve Data Manuals, diagrams and scales!

If I can get a good scan of the Two panel scale I am sure that I can make a very nice copy from it even if the original is poor.

[Nicko]

Have you tried "Galva" from F5BU - http://tk5ep.free.fr/tech/abaqueROS/...lva_185-3L.zip

Its a script-based scale generator - very powerful and as its deterministic, repeatable & accurate too!

(Not that PSP is a bad tool either !)

Cheers

[Dekatron]

No I haven't tried that software, I donwloaded it though and fired it up and was confused immediately. I have a "graphic" mind so it should work easy with this kind of software but the software was not intuitive to me. I'll put some more time into it to see what I can do with it though. If you have any examples that you have made that you can explain to me I might learn to do my own scales faster. Since my mind is very "graphic" in how it works it is usally easier for me to work by editing pictures than describing what I want.

I agree that PSP is a good tool, and I am actually thinking of upgrading, I only hope that it won't be to hard to learn to use the newer versions and that I can upgrade my old graphic files without loosing anything in them!

In the meantime I have made a hybrid scale for the CT160 / CT160A which looks like a Mk III / Mk IV scale, it has the leakage scale from the CT160 and all else from the Mk III / Mk IV. The difference between the sclaes is that the leakage scale in the CT160 is based on the 40V grid voltage and the leakage scale in the MK III / Mk IV is based on the 100V grid voltage, the difference can bee seen in that the 1M and 2.5M positions are positioned more or less stright up and the difference comes from 100V / 40V = 2.5. This is also easily seen at the 10M and 25M points on the leakage scale as they too have the difference of 2.5. Look at 10M on the CT160 scale in my previous post and compare to where that position is on the Mk III scale (25M).

I am also modifying my CT160A to use a 10-trun potentiometer instead of the very special Grid Volts potentiometer and I'll increase the Grid Volts range to 100V maximum so it will fit nicely with the 10-turn potentiometer counting knob. I just hope that there is enough voltage available from the grid volts windings on the transformer, all calculations point to that but I'll have to test it in rela life before I can be sure - then I can use the scale from the AVO Mk IV for the CT160 as the leakage scale is based on the same voltage then. I'll write more about this when I have finished it all, but here are some "teaser" photos.

Changing the maximum Grid Volts available to 100V ofcourse necessitates that I change the gm circuit, but that is easily done as it only relies on the voltage across the gm potentiometer (or the current through the gm circuit) to be the same for whatever voltage is applied over the total circuit. More to come in my AVO Maths document when I have finished fighting with Word Equation Editor, or given up using the equation editor altogether and gone back to simple sub-/super-script in the equations. I've already tested my circuits on the breadboard but I am still unsure if the voltage on the grid volts winding will be enough to let me have 100V grid volts without a voltage doubler and full wave rectification.

[stevefuzz]

Hello

The meter from my MKII VCM went wrong again.
Tired of having it fixed I've already built the pcb with the op-amp to put in a new meter.
Looking at the schematic suggested in the Radiophile magazine article, I can't understand where the connection indicated with "to 0V" from pin 7 of the op-amp goes.
Is there anyone that could explain me this?

Stefano

[glowinganode]

You'll see there's a similar de-coupling capacitor connected to pin 4.
The 0V rail is the centre tap of the transformer, the mid point of the two zener diodes and the right hand input (original meter) terminal.
Rob.

[RAadioloog]

Hello,

I'm from the Netherlands and have a about the AVO MK3 Aweeco panelmeter.
There is a lot of dust in mine and I wonder if there should be a gasket mounted between both meter halves.
It looks like both parts don't fit too well together.

The gasket between the meter and the frontpanel is still there.

Many thanks,

RAadioloog