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Old 11th Apr 2022, 12:17 pm   #1
retailer
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Default New movement fitted into AVO CT160 meter case

The meter of my fixer-upper CT160 was in a sad state - the meter case was cracked in the usual spot, the rear glass had come loose and broken while in transit and the coil of the movement was open circuit. On taking the movement apart I could see that a few of the inner turns the coil which is not wound on a former had come loose and had been rubbing on the central pole eventually breaking the wire - it is very fine. With a transplant in mind I found a suitable donor in my meter drawer, the pointer was the right length and the movement itself was a reasonable fit into the CT160 meter case, I just needed to fabricate 2 brackets - one to hold the movement in the case and another to support the original Avo plastic scale, this way I could keep the original appearance of the meter.

Late last year I converted my manual milling machine to CNC and while it is not yet fully finished it is operational so I took the time to measure the CT160 case, the movement and the scale and draw up a 3D model of the two brackets in Fusion360 to be machined in the CNC mill - I did end up machining 3 of each in total, as I had to fine tune the measurements - the 'new' movement fits nicely it's 200uA fsd and needed a meter amp to bring it up to spec at 30uA fsd, the meter amp board tracks were cnc milled - once again Fusio360 was used to design the board and route the tracks. The plan was to cut the PCB to exact size and drill holes in the tester aluminium framing to mount it but it struck me that I could just mount it directly on the meter, there is room for it and turned out to be a good solution - if I had thought of that sooner I could have designed the board so it fitted exactly behind the meter case with no overhang - the PCB is the subject of a separate post in the diy section.

I used fine steel wool to clean up the meter case and soon found one shouldn't let steel wool and meter magnets share the same room - I spent the best part of an hour with a length of masking tape that I rolled into a tube sticky side out and then flattened so it could be slipped down between the central pole and magnet to capture all those fine bits of steel wool.
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Old 11th Apr 2022, 3:17 pm   #2
The Philpott
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Default Re: New movement fitted into AVO CT160 meter case

Nice one. It's a challenge just to avoid dinging the needle once the scale plate is detached (much more so on the longer needle of the standard Avometer)
Steel wool does moult badly!

Dave
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Old 12th Apr 2022, 9:20 am   #3
woodchips
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Default Re: New movement fitted into AVO CT160 meter case

What circuit for the meter amplifier did you use?

Sure I am going to have to do the same job one day!
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Old 12th Apr 2022, 10:27 am   #4
Craig Sawyers
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Default Re: New movement fitted into AVO CT160 meter case

Here you go. Ignore the blue lettering - I was using this for a different application.

Anyway R1 is what the tester originally wants to see (in my case 1k).

Anyway, the way it works is that the voltage across R2 has to be the same as that across R1. R2 sets the current through the meter.

In my case the donor meter was ten times less sensitive than the (open circuit) original. So the voltage that the original saw was 0.1v. So 0.1V across 100 ohms is 1mA - the current sensitivity of the donor meter.

RV1 sets the zero, R3 is a current limiter to prevent the donor meter burning out if there is something wrong somewhere - it limits the maximum current to 2mA - so a x2 maximum overload. The rest is just power supply stuff.

Craig
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Old 12th Apr 2022, 1:14 pm   #5
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Default Re: New movement fitted into AVO CT160 meter case

I used what I think is the Crowthorne tubes circuit - it isn't anything special and is quite commonly seen in opamp application text books - it is much the same as the one posted by Craig, he covers the operation of it nicely - in place of R2 I've used a 1K trim pot - R1 in my case is 3250 ohms so I went through my stock of resistors and found a 220k and 3k3 that my Fluke tells me makes 3249 ohms when in parallel. My movement is 200uA fsd at 750 ohms - out of the dozen or so meters I have it is the only one that fitted inside the Avo case and also has a pointer long enough to cover the Avo scale
Even though I have mine made and working I'm going to make a second one that fits exactly on the rear of the original Avo case, this will do away with a PCB hanging out to the side - it will give me some more experience with Fusion360 and CNC PCB board milling, I'm still on the steep part of the learning curve with these.
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Old 14th Apr 2022, 10:55 am   #6
woodchips
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Default Re: New movement fitted into AVO CT160 meter case

Thank you for that, archived!
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