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Old 19th Nov 2018, 6:44 pm   #1
stevehertz
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Default Strange metal AVO casing

I've just bought this AVO 8 Mk? and was surprised to see that it is housed in an additional case made of metal and the case has a connection terminal at the top end in the middle - see photo. I'm guessing this terminal is for earthing (safety) purposes but why the metal case in the first place? For added strength in an industrial situation? Some form of shielding? And was the metal case an option or a designated model of AVO in its own right ?

Also, there are holes in the metal case where the heads of the screws holding the inner case in place can be seen. As far as I can see, the only thing holding the metal case in place is the two small screws on the underside? Is it bonded to the plastic case at the factory?

I've tested a few ranges and it appears to work fine on those. In any case I'm not doing serious lab work, so I don't need that level of ultimate accuracy. I can compare it with the readings on my other AVO 8 that in turn I can compare with my digital MM and if the two AVOs concur then all's well. I've given the front panel an initial clean to remove ingrained grime and - what else but?! - white emulsion from it but I can go over the metal case with some kind of thinners to remove that tape gum and the 'next calibration due in 1983' sticker..

AVOs are just great workhorses aren't they? I used my first one at work as a test engineer in '72 and even now at home it is my 'go to' instrument nine times out of ten. A moving needle tells me a lot more than a confused digital display when the reading is not constant or I'm having difficulty making a connection.
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Old 19th Nov 2018, 6:50 pm   #2
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Default Re: Strange metal AVO casing

Test Set No.1 High Sensitivity, ie a child of the early Model 8/9 series, with a suit of armour for military/MOD use. Substantially heavier than the standard article- and that steel case can be a pain to remove after all this time-lets hope you don't need to.

The spots of white/magnolia emulsion are compulsory- they help denote you have the genuine article..

Dave
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Old 19th Nov 2018, 7:04 pm   #3
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Default Re: Strange metal AVO casing

Yes Dave, I've been doing my own research on Richards Radios where they have some comprehensive info and it is indeed a Test Set No.1 High sensitivity - thanks!

Here the blurb:

Test set No.1 (a military version of Model 8/9 Mark 2) Essentially the same as Model 8 Mark 2 apart from 3-10 ranges instead of 2.5-10; no dB scale; resistance on bottom of scale plate instead of top; no LR or Ins. positions. At some point added fuse on low ohms ranges.

This model is housed in a protective steel case.
No dB scale; resistance on bottom of scale plate instead of top, no LR or Ins. positions

DC 3, 10, 30, 100, 300, 600, 1000, 3000 volts - sensitivity 50 uA
DC 0.05, 0.3, 1, 10, 100 mA, 1, 10 A
AC 3, 10, 30, 100, 300, 1000, 3000 volts -
sensitivity: 1 mA on 100+ volts, 3 mA on 30, 10 mA on 10, 30 mA on 3
AC 10, 100 mA, 1, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)

Later versions of 'Test Set No. 1' were essentially identical to the Mk 5 (to 1986)
and Mk 6 (from 1986 to end of Avo use).

It doesn't mention the screw terminal at the top though, maybe that was added by the end user?
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Old 19th Nov 2018, 7:04 pm   #4
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Default Re: Strange metal AVO casing

Sort of 'thankfully' I've never had a metal case over the top of case AVO, I've had the AVO Bridges which can be a devil but just how do you remove a case like on Steves one if it's stuck?

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Old 19th Nov 2018, 7:38 pm   #5
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Default Re: Strange metal AVO casing

(That earthing terminal, i assumed,was part of the MOD spec.)

The question of siezed steel cases has come up before, things like differential thermal expansion (hot water or a hairdryer) and lubricants were suggested. I think one of these meters was so bad the owner had to remove the back, drill subtle holes in the bakelite, shove a drift through and drive the two shells apart.
Invisible once reassembled.

Certainly excellent workhorses. I had a problem with a bakelite component that had started tracking recently, and of course a DMM was just throwing numbers at me, and the trouble couldn't be provoked,visualised and located until an Avo was used. A significant feature that i appreciate is that a good analogue meter tries to tell you when it's sick, whereas a DMM tends to keep it to itself.

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Old 19th Nov 2018, 7:50 pm   #6
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Default Re: Strange metal AVO casing

I've got one. Ex-Forces, with the metal case over the Bakelite moulding - all in a leather carrier with flip-round lid. So, 3 layers of encasing!
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Old 19th Nov 2018, 8:15 pm   #7
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Default Re: Strange metal AVO casing

I've one of these, too- large, heavy but very appealing. From a large hopper-full long ago at the recently burnt-down Nottingham branch of Anchor Supplies, it was a case of take-your-pick and maybe get lucky- I wouldn't be surprised if they had been shovelled in off the back of a 4-tonner! Still, I found a good one, calibrated until May 1991, courtesy of RAF Henlow.

What attracted me was the 1-3-10 (being pedantic, 1-3.16-10 or 1-(square root 10)-10) scaling, it struck me as making more sense than the common 1-2.5-10 format, I'd previously bought a half-price Micronta 22-203B with the same scheme. Maybe the military also felt that good resolution on generator-running 24V systems made sense.

The case is a sort of chocolate-brown rather than near-black, is this perhaps a tropicalised form of Bakelite? Maybe the metal case was also felt to help screen the movement from strong local magnetic fields?
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Old 19th Nov 2018, 8:31 pm   #8
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Default Re: Strange metal AVO casing

Quote:
Originally Posted by turretslug View Post
Maybe the metal case was also felt to help screen the movement from strong local magnetic fields?
That's what I thought - as well as extra strength for reliability/usability 'in battle'. It weighs a ton but I don't hear my workbench moaning and I don't plan to take it on shopping trips or on holidays
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Old 19th Nov 2018, 8:41 pm   #9
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Default Re: Strange metal AVO casing

Quote:
Originally Posted by Andrewausfa View Post
Sort of 'thankfully' I've never had a metal case over the top of case AVO, I've had the AVO Bridges which can be a devil but just how do you remove a case like on Steves one if it's stuck?

Andrew
Perhaps I'm missing something, but surely the metal case comes off with the plastic case underneath - as one? Of course, the plastic cases themselves don't always come away easy, but I can't really see how having an extra layer makes a difference. In fact the edge of the metal case is something to pry against with a wide chisel? It won't fracture like the edge of the plastic case might.

Of course, that's a different thing to attempting to part the plastic and the metal parts of the case. Now that may be difficult, but I'm not sure why you would want to do that? Unless for some reason you just wanted rid of the metal outer part of the case, but why? it's extra protection.
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Old 19th Nov 2018, 8:47 pm   #10
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Default Re: Strange metal AVO casing

Yep, i was referring to separating the two shells, removing them as a whole shouldn't be a problem. The previous thread i recall, someone wanted to separate the two because rust had crept inbetween and the owner wanted to refurb and repaint the steel. Dave
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Old 19th Nov 2018, 9:12 pm   #11
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Default Re: Strange metal AVO casing

You'll find that's not an additional case, it is the actual case. As simple to open as the plastic ones. I've had a few open myself in the past to take a peek inside and I never noticed any other covering other than the steel case once removed.

I've got quite a few lovely meters with that steel casing, about 22 or so of them through the model ranges of Test Set No 1, 8X, 9SX...
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Old 19th Nov 2018, 10:03 pm   #12
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Default Re: Strange metal AVO casing

Quote:
Originally Posted by turretslug View Post
What attracted me was the 1-3-10 (being pedantic, 1-3.16-10 or 1-(square root 10)-10) scaling, it struck me as making more sense than the common 1-2.5-10 format.
Yes. 1_2.5_5_10 would make better sense it also has the advantage that the scale divisions can sit neatly on top of each other which may be why 1_2.5_10 was picked in the first place.
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Old 21st Nov 2018, 9:48 am   #13
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Default Re: Strange metal AVO casing

For me, the scale arrangement is a mild disappointment. I bought the AVO - wrongly described as a model 8 mk3 - to match one that I already own and intended to use as a pair for making stereo adjustments. Hence the preference for the scales to be the same to avoid confusion. The test set model 1 that I now have is probably a rarer, more collectable instrument so I may decide to let it go and get another 8 Mk3.
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Old 21st Nov 2018, 11:51 am   #14
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Default Re: Strange metal AVO casing

I heartily agree- you need two identical for ease of comparative work. But as someone once pointed out to me, if two meters disagree, which one is wrong? You need a cobwebby arbitrator to drag out and identify (with reasonable certainty) the offender!

Why not keep it- they're not actually super-rare, they come up quite frequently. A new strap might be an idea if it's never been changed tho!

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Old 21st Nov 2018, 8:18 pm   #15
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Default Re: Strange metal AVO casing

Well, if AVOs are being used on each of two channels (as in stereo), you're never going to see 'a disagreement' as they are measuring different things anyway. For example setting the quiescent current in each channel. The point is, you have to check the calibration of all your instruments and if there's a discrepancy, sort it out, that's a given. But in any case, like I said before, I'm not doing laboratory testing, just fixing old radios and hifi.

The strap on this one is literally as good as new, so there's no need to change it. As for keeping the AVO itself, I may, but with all my other test kit two AVOs is enough for me, so if I do get another 8 Mk3 I'll probably let the test set 1 go. We'll see, I've nowt against it, I just want two AVOs the same for the reasons given.
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Old 21st Nov 2018, 10:13 pm   #16
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Default Re: Strange metal AVO casing

Instruction leaflet attached.

John
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Old 22nd Nov 2018, 8:38 am   #17
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Default Re: Strange metal AVO casing

Thanks John, appreciated!
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Old 22nd Nov 2018, 10:05 am   #18
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Default Re: Strange metal AVO casing

Currently votexed away from home, but back in the early 1980's I bought one of these from Thackers (Thackers Kn*****s) in Cheslyn Hay. It is self described as an AVO Model 8X Panclimatic.

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Old 22nd Nov 2018, 10:43 am   #19
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Default Re: Strange metal AVO casing

Ah, but what if there is a difference between left and right channels, and the discrepancy follows the meter when the leads are swapped..

(Actually, i see what you mean- the scale you have there doesn't have dB's does it.)

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Old 23rd Nov 2018, 7:58 pm   #20
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Default Re: Strange metal AVO casing

It's slightly ironic that this version omitted the dB scaling, as the 1-(square root 10)- 10 range incrementing mentioned makes it particularly easy to relate higher range readings to the base dB scale- just add 10, 20, 30 etc. to the reading rather than the 6, 14, whatever found with other schemes. OK, only basic arithmetic, but keeping things as simple as possible keeps errors as low as possible.

I daresay that the intended end-users had HM Taxpayer posh power meters with switchable loads from the likes of Marconi, though, so no need to mess around with AVOs!
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