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Old 23rd Aug 2019, 10:32 am   #1
woodchips
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Default Avo valve tester meter panel scan

Sorry, can't think of the correct description here.

The meters used in the various Avo valve testers, and others, have custom painted panels, or whatever the bit of metal that the pointer moves over is called!

I have done some searches but can't find any thread where a dud meter has been stripped and this panel scanned at a high resolution to allow for printing of replacements.

Strange that it would be easy to replace the analogue meter with a digital one, but interpreting the results becomes very hard.

I was looking at my Taylor 45A and see that the painted markings are wrinkling and jambing the meter needle, not good.

I have also found a Farnell TM1 AC/DC millivoltmeter and that could do as a replacement meter for something, but needs new scale markings.

One day I will get around to using the valve testers, but it seems that simply sitting doing nothing means they deteriorate. All the other components are replaceable, but the meter isn't quite so easy, need to keep some alternatives on the shelf, just in case.
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Old 23rd Aug 2019, 10:39 am   #2
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Default Re: Avo valve tester meter panel scan

Sorry to hear that - here in NZ I have the problem the sun is so strong that it will actually bleach out meter scales that are left too exposed to it. Most of a red scale on one of my Valve voltmeters is gone after accidentally leaving it a few weeks in full sunlight.
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Old 23rd Aug 2019, 12:18 pm   #3
Ricardo
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Default Re: Avo valve tester meter panel scan

Woodchips,

Here is a scan of the meter scale from my Taylor 45C.

Click image for larger version

Name:	Taylor 45C Meter Dial Plate_5b.jpg
Views:	113
Size:	39.9 KB
ID:	188772

From a quick internet search the meter scale on the earlier 45A model looks much the same?

The attachment is a compressed .JPG version. If you want a higher resolution scan I can send you a .TIF format file. [File size is approx 3.2 MB]


Regards,

Richard
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Old 23rd Aug 2019, 2:15 pm   #4
David Simpson
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Default Re: Avo valve tester meter panel scan

Hello Woodchips,
Yep, Taylor Valve testers would seem prone to loose paint deposits - see my thread post back on 3rd Feb on a 47A Meter.
I've also attached some AVO VCM Meter Scaleplate scans.

Regards, David
Attached Thumbnails
Click image for larger version

Name:	CT160 Scale Plate.jpg
Views:	132
Size:	38.7 KB
ID:	188779  
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Old 23rd Aug 2019, 9:01 pm   #5
factory
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Default Re: Avo valve tester meter panel scan

Quote:
Originally Posted by woodchips View Post
I was looking at my Taylor 45A and see that the painted markings are wrinkling and jambing the meter needle, not good.
Seems to be a common problem, my Taylor 45A meter scale isn't much better but still works OK (I should probably scan it like I did for the 90A scale that was requested a few years back on here), the A & B pots needed replacing.
Click image for larger version

Name:	DSC02554a.jpg
Views:	109
Size:	66.3 KB
ID:	188801

Oddly my Taylor 45 (no A) has a good meter scale, with no flacking paint or fading.

David
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Old 23rd Aug 2019, 10:38 pm   #6
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Default Re: Avo valve tester meter panel scan

Quote:
Originally Posted by David Simpson View Post
Hello Woodchips,
Yep, Taylor Valve testers would seem prone to loose paint deposits - see my thread post back on 3rd Feb on a 47A Meter.
I've also attached some AVO VCM Meter Scaleplate scans.

Regards, David
Hi David,

Do you have these scans in a higher resolution? Perhaps the original scan the you could send?

I have a Taylor 45C that I could also scan if someone needs it, let me know if so.
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Old 24th Aug 2019, 7:51 am   #7
Ricardo
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Default Re: Avo valve tester meter panel scan

Just to clarify, the 'scan' of my Taylor 45C meter scale plate in post #3 was touched-up slightly using Photoshop.

Here is the original scan of the plate, again in .JPG format and scaled to fit the allowed forum attachment size.

Click image for larger version

Name:	Taylor 45C Meter Dial Plate_b.jpg
Views:	123
Size:	57.0 KB
ID:	188809

There is some minor paint loss in the red and green areas and the plate itself is an off-white (creamy) colour.

Regards,

Richard
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Old 24th Aug 2019, 2:55 pm   #8
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Default Re: Avo valve tester meter panel scan

Those AVO meter scans that David show are from an article my friend Euan McKenzie wrote where I made the scales for Simpson 29T meters and they don't fit in original AVO movements. It was published in Radio Bygones issue 141 Feb/March 2013.

Nowadays I have had help to make better meter scales with the help of the Galva program made by Jean-Paul Gendner (F5BU) which you can find here: http://www.f5bu.fr/galva-about/, Michael Endacott helped me with the programming of Galva to make the new scales.

The old scales from that article were hand drawn by me in Paintshop Pro.

The new scales for the Simpson 29T meters as well as for the Simpson 1329T meters will be published as soon as I get time enough to finish writing a small article about them and also about the meter replacement and the back lit LED driver that acts as the visual short circuit alarm.
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Old 24th Aug 2019, 4:52 pm   #9
David Simpson
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Default Re: Avo valve tester meter panel scan

I must check my old files on discs. I'm sure I took some life-size scans when I had a couple of CT160's several years ago.
I think those Martin/Euan scans I attached yesterday were floating around the vintage fraternity some time ago. It would be good if there was a set of up to date AVO VCM MK's 1 to 4/ CT160/ VCM163 Scale Plate Scans available to us Forum folk. I'm sure Taylor & Mullard HSVT enthusiasts would like scans also.
Photoshop & other modern scanning software packages are way beyond an old analogue guy like me.
Oh I, just remembered - Benchmark Meters from Ludlow used to supply a modern replacement meter for an AVO 30uA one, and I think they also have AVO Scale Plate copies.

Regards, David
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Old 24th Aug 2019, 6:12 pm   #10
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Default Re: Avo valve tester meter panel scan

The scales I got from Benchmark aren't correct, they differ in where the red/white/green areas as well as where the 1mA/V and calibration lines are drawn compared to where they should be. I pointed this out to them several years ago but unfortunately didn't get any response from them if they were going to correct them or not.

I guess that the Benchmark scales were copied from scales from just two AVO instruments, one Mk IV and one CT160 instrument, and since AVO used scales that were fitted according to the linearity of each instrument these new scales only correspond to those two original instruments. People who know the AVO 8 meters know that the scale is selected to fit the linearity of the instrument.

This means that if you use these scales your measurements will differ from what they should be. The red calibration line in the Benchmark scales I have is more to the right on one scale than on most original AVO scales I've seen, which means that that original AVO instrument had a different sensitivity (swing) than the other scale where the red line is in the center of the black area. Also since the 1mA/V line is more or less at 76% swing compared to the 74% it should be the tube you measure actually has a somewhat higher mA/V value at that point.

Seeing all these different scales from different manufacturers of AVO replacement instruments was what made me delve deep into the AVO VCM's to get at the maths behind how they worked. There are a few different people/companies selling AVO replacement instruments and it seems that most of them haven't checked the maths or read what I have written on this as they still sell scales that are incorrect.

Now this is only a few percent difference but since the calibration line is more important you might have measurements that are more off than you realize if your scale isn't correct.

I've bought a few different AVO replacement meters from different companies/sellers/persons and when I had problems calibrating my AVO's I decided to get to the bottom of why that was so - that was when I discovered that if I used one replacement meter and then swapped it for another replacement meter (which had another scale) they didn't show the same measurements, not even when calibrated against the red line did they show the same measurements!

Linearity still affects newly manufactured meters but they are a lot better compared to old and aged meters. I checked a few of the Simpson 29T and Simpson 1329T that I've used and most of them didn't have any measurable linearity problems when checked with a calibrated current source driving them.

And as a last note I do have high resolution scans of good quality of AVO scales (Mk III, Mk IV, CT160 & VCM163) that I can publish later, I have to clean them up somewhat, but since they have the same problems as the Benchmark scales - they only fit that original AVO meter they came from - I will have to write something in the image explaining that so people understand how to use them properly so they don't get the same problem with calibrating their AVO's as I did.
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Old 25th Aug 2019, 11:14 am   #11
woodchips
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Default Re: Avo valve tester meter panel scan

Thank you to everyone for your response. As guessed all the meters are different so transferring one to another tester won't get the best accuracy.

Next problem is how to archive the scans if they are too large for the forum. Bitsavers? Or even BAMA?

The other thing is I have found a couple of old Farnell TM1 AC/DC millivoltmeters and they have large meters, so the idea of replacing the 30uA meter with something else, much cheaper, is very attractive.
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Old 27th Aug 2019, 10:37 am   #12
David Simpson
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Default Re: Avo valve tester meter panel scan

Woodchips, I wouldn't worry about replacing AVO VCM scale plates. In general, with AVO calibration procedures, there are enough variants anyway. AVO expect users to initially try & keep the needle within the wee black "Set AC" zone before proceeding further. But hey - on a CT160 that black zone covers 3 divisions of the 0 to 100 scale = 3%, or centre black + or - 1.5%.
Back in my RAF CT160 servicing & repair days, as well as replacing whole damaged meters, we occasionally changed the scale plates & this caused no problems whatsoever with the final calibration. We had both old style & new style meter scale plates.
Right enough, one can connect a larger meter with a different & higher FSD - but then an op-amp would be needed. My solution a few years back(there is a thread in "Search" somewhere) was to use an older AVO8 MM on the 50uA DC scale & sellotape an adjusted scan of the CT160's scales over the first 30uA. After a wee bit of delicate fannying about - this worked a treat. A great advantage was that the AVO 8's movement was not prone to the same "whanging to fsd" which CT160's used to suffer from.

Regards, David
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Old 27th Aug 2019, 1:56 pm   #13
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Default Re: Avo valve tester meter panel scan

It was probably ok to swap scales in the day when these meters were like new. But now with meters with weak hairsprings, worn out jewel bearings and other problems due to ageing it is a different thing, this as the meters have a different swing compared to what they should have which affects linearity a lot.

But like with all repairs where you have a moving coil meter you should start by making sure that it works like intended and that includes FSD current, linearity and internal resistance. If all of these things are ok you can replace the scale and check so that the new scale shows the correct values.
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