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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 15th May 2018, 10:30 pm   #1661
Sinewave
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Default Re: AVO Multimeter survey

Quote:
Originally Posted by Pamphonica View Post
By enormous good luck, NVCF turned up for a total outlay of less than 30 no less than three 36-range ("Model 5") Avos. Two are working (one a bit down on cal, one faulty on ac) plus one that's definitely a parts-only machine as it's missing terminals, fuse and has a totally stuck and badly re-painted movement. But a spares source is always useful...

No-one seemed to want them. Two sat on the Bring & Buy for hours, and the last one I bought as the show was closing.

They rather nicely complete my pre-war line-up. I had no 36-Range meters at all before.

I'll add them to the database shortly.
I just noticed that they are 1 month apart and 100 serial numbers apart. A production of 100 meters a month, roughly. Not bad!

Jeremy
Very lucky! I missed out on those. I probably miss out on a lot by not attending enough bring and buy events.
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Old 15th May 2018, 11:01 pm   #1662
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Default Re: AVO Multimeter survey

Sinewave, please see posts #29 & 30 here.
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Old 16th May 2018, 7:37 am   #1663
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Default Re: AVO Multimeter survey

Poo

Should have sent me a pm, I'd have snapped these up.

Have no idea how I missed your post, I do check that forum section every few days.
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Old 16th May 2018, 9:14 pm   #1664
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Default Re: AVO Multimeter survey

Having secured a decent low voltage DC reference, i have calibrated the magnetic shunt on my Precision Avometer, which was showing around 1% low at 66% of FSD.

If anyone wants/needs to do this i will do some comments/images, as it is more straightforward than i expected it to be. The time needed is however 3 times more than on the standard avo of the period.

I have now got it accurate at 66% FSD, and 0.12% (of the reading) high, at 83% of FSD. (Comfortably within the spec. printed in the lid.)

This is a heavy old dinosaur, but it interests me as being a good effort to do as much as was possible with an analogue movement, with the shadow of the digital revolution already hanging over it.
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Old 16th May 2018, 9:43 pm   #1665
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Default Re: AVO Multimeter survey

That sounds like it would make an interesting how to guide.
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Old 16th May 2018, 10:18 pm   #1666
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Default Re: AVO Multimeter survey

Yes please! I've adjusted magnetic shunts on standard Avo movements, but at FSD, so I may have been doing it wrong.
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Old 16th May 2018, 10:48 pm   #1667
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Default Re: AVO Multimeter survey

I think you were doing it correctly Phil, it's just that i had to work with set voltages which didn't correspond to FSD! My 10/7.5/5/2.5 ref. is ideal for most meters, but compromises had to be made for the curious ranges on the BIG Avo.
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Old 16th May 2018, 11:17 pm   #1668
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Default Re: AVO Multimeter survey

There is a steel helmet that sits over the rear of the magnet/movement, which was covered with magnetic 'fur'. As well as giving physical/environmental protection, it appears to be able to grab and retain all magnetic matter that might otherwise jam the moving parts. The downside of this is every time the shunt is tweaked the helmet has to be replaced and screwed down, as calibration depends on it being in place.
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Old 17th May 2018, 9:01 am   #1669
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Default Re: AVO Multimeter survey

I only have a 2.500 volt reference, but it seems to work well. For testing higher voltage ranges, I connect the reference in series with a variable DC supply, wind it up to get a suitable voltage indication (say 10 volts) on the meter under test, then switch out the 2.5 volt reference and look for 7.500 volts on the meter.

Calibrating the current ranges is a little less straightforward. I usually apply a known accurate voltage through a known accurate resistor direct into the movement, and adjust the mag shunt to create the correct reading. Once this is done, most of the ranges tend to fall into line, and any that don't can be swiftly identified and fixed.
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Old 17th May 2018, 9:42 pm   #1670
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Default Re: AVO Multimeter survey

I have just rediscovered my late father's AVO Minor, still in the box. It was posted to him on 25 November 1954. Any advice on where to find a serial number?
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Old 17th May 2018, 10:10 pm   #1671
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Default Re: AVO Multimeter survey

Most likely on the printed scale plate, (behind the glass.)

Recommend removing the old battery if not already done so!
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Old 19th May 2018, 12:20 pm   #1672
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Default Re: AVO Multimeter survey

Precision Avometer movement calibration/set-up only.

1. Check against known voltage reference to see if it needs attention!
Slide lid from hinges, remove chrome plated facia screws, carefully invert meter and lift box off. Check integrity of box bottom, as it is rather thin plywood and can tend to warp away from the sides, which are much more substantial.
Remove loom clip screw (Pic A) and 4 main screws holding the wooden shunt frame to it's hex pillars. De-solder red and black wires from protection rectifier (Pic B). Note green earth wire solder tag under corner of shunt frame. (Pic C) The tag may also need to be de-soldered from the GN wire to allow removal of the white polythene movement dust cover.

2. Hoist (or get an assistant to hold) the shunt frame up at an angle (Pic D) and remove the screws and shakepruf washers holding the (polythene?) dust cover to the facia. RD GN and BK wires now have to be fed through the circular holes as the dust cover is taken off.

3. You will note at this point (Pic E) the temptation to extend the holes in the dust cover to the edge, to facilitate future removal without use of a soldering iron. The polythene on the subject meter is high quality, but the thickness is variable and it is almost translucent in places.

4.Lower shunt frame down and attach with one screw at either end. The steel 'helmet' covering the rear of the movement should be wiped over thoroughly with blu-tack to remove ferrous/magnetic/magnetised material (Pic F)

5. Carefully remove helmet, (set aside GN wire) and wipe inside with blu-tack. As per (Pic G) the magnetic shunt is now best accessed with a long screwdriver. What is not recommended is a conventional spanner, as access from the side is limited and a mistake could occur.

6. Unfortunately the movement will show some 6% more deflection with the helmet removed, so every time the meter is tested it must be replaced for a correct reading! There is a certain and inevitable amount of non-linearity which means- if you set the meter correct at FSD it will lag a little behind at lower deflections. I set mine up at 66% FSD due to lack of a suitable voltage reference for FSD, but it all worked out quite well.

7. Reassembly is reverse of disassembly, but note that before the meter is finally fitted back into it's box, there is very good access to the rotary switches and leaf switches. if the meter has been locked away in the warm for decades with rubber test leads inside the lid, these leaf switches are likely to be blackened.

-Mods feel free to shuffle this summary (complete with pics which follow) elsewhere if it would make more sense.

Dave
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Old 19th May 2018, 12:25 pm   #1673
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Default Re: AVO Multimeter survey

Precision avometer

A,B,C,D,E
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Old 19th May 2018, 12:32 pm   #1674
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Default Re: AVO Multimeter survey

Precision avometer

F,G

-There are some sequential anomalies within the images, as they were done once the job had been completed. It should all fall into place when you proceed!
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Old 19th May 2018, 3:35 pm   #1675
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Default Re: AVO Multimeter survey

Thanks for the step-by-step 'how-to' guide, Dave. If I should ever be lucky enough to own one of these, I'll know what to do! I'll keep my eyes open for one.

I guess that the majority of Precision Avometers are still in reasonable condition, having spent their time in laboratory or test workshop conditions rather than facing the rough handling that most ordinary Avos seem to have suffered.
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Old 19th May 2018, 5:15 pm   #1676
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Default Re: AVO Multimeter survey

You rarely see these two evolutions in the same photo Phil, which doesn't help with assessing scale- it's difficult to appreciate until it's actually sitting in front of you! Even so I would be the last to discourage people from buying one.

It had to be fixed; being 1% low across the board was glaringly obvious and it was being made to look stupid by a Model D!

Dave
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Old 20th May 2018, 8:55 am   #1677
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Default Re: AVO Multimeter survey

I'd need a bigger shed! Crikey, it's huge. Not likely to fit into an electrician's toolbox.

Thanks for the photo.
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Old 21st May 2018, 9:15 pm   #1678
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Default Re: AVO Multimeter survey

Thanks Dave, the number is U23187-36. the battery has been removed, I'm not sure what size it takes, I seem to remember my dad used half a No.8, and it has probably not been used since No.8's were discontinued.
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Old 22nd May 2018, 2:15 pm   #1679
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Default Re: AVO Multimeter survey

Probably a March 1936 meter then. An AA cell can be used, it's length can be increased (for example) by stacking three or four 5p pieces at the + end and it's girth increased by using a short bit of fuel or coolant hose.. slit from end to end if required.
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Old 23rd May 2018, 11:24 pm   #1680
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Default Re: AVO Multimeter survey

Just picked up a few rare ones I'd been after for some time. An EM272 in original Avo packing box, works fine on all ranges. Also a 72, which does work, but is a bit wild on the DC mA ranges and resistance ranges, so may need some work doing to it, but the movement appears ok.
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