A teaser on what a new AVO VCM meter can look like - solution for back-light!!

[Dekatron]

I've been working on a solution to replace AVO VCM Meters for some time, but only in short bursts when I have had some time, as some of you here know it was a long time since I posted anything on this. Time has been the biggest limit on this, and a few more things like: money and also how do you get a nice solution to the back-light without having to cut open the meter case (cutting in the case means that you first have to remove the meter movement so you don't damage it, then cutting and drilling and then mounting it back), and I've also tried to find a material to print the scale on that would withstand anti-static spray and also be as little static in itself as possible, I only found that a few months ago.

Well, I finally thought of a solution for the back-light recently, make some laser cut pieces of perspex, mount them behind the meter on top of the front panel and place some LEDs inside the perspex to have it light up like those signs you see everywhere nowadays. As you see in the first photo it lights up quite nice. I have just sanded the surface of the perspex to get a smoother shine of the bright LED light, but I am planning on having the perspex treated while it is being laser cut to increase the shine. It is also possible to use perspex which is designed for light guide purposes, but that is a lot more expensive than sanding or laser treating the surface.

I still have to build the module that connects to the AVO cut-out relay so it lights up the LEDs, but that is not far away as the design has been finished. I will have some help with testing it all out as all of my AVOs are in storage. The only part I could dig out from my cellar storage was a front panel from an AVO Mk III.

I also designed a circuit board that fits behind the meter which is used both as a mounting bracket for the four mounting screws of the meter as well as it is used for the components that protect the meter and also for the resistors/trimmers necessary to adjust the 25uA Simpson 29T Taut Band movement so it becomes either a 30uA/3250 Ohm or 39.75uA/2452.8 Ohm movement (the latter is just made by adding the 10 shunt on the board) and it will also house the LEDs and possibly part of the electronics driving them, but not all since some of it is connected to the live mains wires. Jumpers make it possible to select what component you want to use like diodes, capacitor and which adjustment for FSD you need. The board uses SMD capacitors which makes it possible to add 100uF like Karsten did in his design which he wrote about here on the forum, or you can hang a large 8uF polypropylene capacitor beneath the meter and strap it to the board if you want the original component value.

I also intended to place the holes for the connection to the AVO wires at the correct distance so they could use standoffs to screw the original lamp with its brown perspex to those standoffs - but I forgot about it when designing the PCB so that will have to be added in a future version.

I hope you enjoy the photos below (some sharp eyed people will notice the wrong orientation of the } character, that was due to a fault in the ripper software and has been corrected).

I've had help with this from two persons without which this would have been impossible: Mike Endacott with design of the scales in Galva, and Markus Lindblom for testing the perspex and help with the LED driver building and testing.

I hope I can release it for free personal use under the Open-source hardware (OSH) license https://www.oshwa.org/open-source-hardware-logo/ (if that is the right license to publish it under) this year, but you never know with my lack of time.

I'll upload all of the CAD-files for the circuit board, the schematics, DXF-files for the laser cutting of the perspex pieces and so on, but you still need to be able to calibrate the meter by yourself as every meter has a somewhat different FSD-current and internal resistance due to manufacturing tolerances.

[sven_swe]

Since I have been fortunate enough to test these meters from Martin, I just wanted to chime in and give some feedback about the meters, which in my opinion are an upgrade to the old original meters. With built-in meter protection, very nice "swing",dampening and very good accuracy.

Since my meter in my AVO MK3 was "pivoty" and not working as it should, I was looking for a replacement and Martin had a solution that is working perfectly.

Also, since a major German courier that shall remain unnamed lost my meter for my AVO CT160 in transit to Herts Meter in England for service, I was looking for a replacement for that unit as well.

Now I have the new meters mounted in each machine and they are working really well so far. I only have the calibration of the MK3 pending.

Now the final touch will be the back-lighting, which is about to be finalised - I am really looking forward to this.

Very happy with the new meters and thanks to Martin for making this possible!

I have attached some pictures with the new meters mounted in the machines.

Regards,
Sven Markus Lindblom