Author Topic: AVO multimeters - which one, and why?  (Read 427 times)

Offline AdeV

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AVO multimeters - which one, and why?
« on: October 09, 2020, 01:57:59 PM »
Over here, there's quite a lively discussion going on about AVO multimeters.

Having never belonged to the AVO owner's club, I'm curious:

1) Why are they so sought after, compared to any modern analogue multimeter?

2) If I were to peruse eBay, there seem to be dozens for sale. Which is considered "the" one to have, assuming one wants to use it as a general purpose multimeter?

I'm not (necessarily!) proposing to buy one.... I have 2 working DMMs here right now; although I fried my last analogue one many moons ago.
Cheers!
Ade.
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Offline Fergus OMore

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Re: AVO multimeters - which one, and why?
« Reply #1 on: October 09, 2020, 02:17:32 PM »
I saw one on one of these restoration programmes being made into a radio.

Maybe the answer?

Cheers

Norm

Offline Sea.dog

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Re: AVO multimeters - which one, and why?
« Reply #2 on: October 09, 2020, 02:20:28 PM »
"I'm not (necessarily!) proposing to buy one.... I have 2 working DMMs here right now; although I fried my last analogue one many moons ago."

You're clearly a beginner  :D

I've three Beckmans, all over 30 years old, a TMK500 from the 1960s and a very nice AVO 8 in the original leather case. To say nothing of three or four clamp meters and a capacitance meter. Analogue meters are better at averaging noisy readings. DMMs have a (usually) fast sample rate so the reading can jump all over the place, whereas the response of an analogue meter is damped.

Offline John Rudd

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Re: AVO multimeters - which one, and why?
« Reply #3 on: October 09, 2020, 03:43:49 PM »
I worked in the tv trade during my working life, my bestie m8 when I was at school, his dad worked for NEEB.....during those time periods, the AVO 8 was the 'one' to have....

I now have 3 UT50 (x) dvm's at my disposal....not an analog meter in sight...
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Online awemawson

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Re: AVO multimeters - which one, and why?
« Reply #4 on: October 09, 2020, 04:59:29 PM »
An analogue meter is dramatically easier to use than a digital if the measured quantity is varying as in the case of motor current in a servo loop for instance. There are many other situations where the good old Universal Avo triumphs over a digital, but both have their place.
Andrew Mawson
East Sussex

Offline Jo

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Re: AVO multimeters - which one, and why?
« Reply #5 on: October 10, 2020, 03:41:06 AM »
I used to have a very nice Mk 8 but my ex-husband felt the need to take it with him  :wack: 

As Andrew says there are times when an analogue meter is better than a digital read out to enable you to see a moving value - like with the rev counter and speedo in a car.  I have been able to use one of those cheap as chips (<4) chinese digital meters for everything for the last 20 years, even though I have a much more expensive Fluke digital carefully squirrelled away somewhere  :scratch:

Jo

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Offline Pete W.

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Re: AVO multimeters - which one, and why?
« Reply #6 on: October 10, 2020, 06:24:19 AM »
According to my Senior Engineer when I was a mere Junior, regarding comparison of the AVO 7 and the AVO 8:

The AVO 8 is more sensitive because its movement's full scale deflection (FSD) is 50 μA (microamps).  It consequently needs to 'rob' only a little current from the circuit under test.  This will minimise the difference between the indicated and true voltages when the circuit under test presents a high source resistance.   This low value of FSD requires high Ohmic values for the AVO 8 multiplier resistors and other internal circuitry.  These high Ohmic values require, for reasons of economical manufacture, the use of metal film or hi-stab carbon resistors which are less intrinsically stable than wirewound resistors.

The AVO 7, on the other hand, is less sensitive because its movement has an FSD of (if I remember correctly) 1 mA (milliamp).  This means that the AVO 7 needs to 'rob' more current from the circuit under test leading to more disturbance of the circuit and a greater difference between true and indicated values, the more so if the circuit under test presents a high source resistance.  However, the higher FSD of the AVO 7 movement leads to lower Ohmic values for its multiplier resistors and other internal circuitry.  These lower Ohmic values are consistent with the economic use of wirewound resistors.  These are intrinsically more stable than metal film or hi-stab carbon resistors so the AVO 7 is more precise than the AVO 8.

Best regards,

Pete W.

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Offline Joules

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Re: AVO multimeters - which one, and why?
« Reply #7 on: October 10, 2020, 06:31:52 AM »
Hmmm, lets throw in some controversy.  The AVO time is long gone, modern DMM with a bar graph gives you a fast acting visual clue, with the added benefit of much higher impedance.  Your AVO will only indicate an average, if you care about the signal change in detail you should be using an oscilloscope.   These days we have a vast range of affordable DMM's and scopes to hand, sorry guys the AVO age is over, says he who is building a Commodore PET, to relive his YOOF....
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Online awemawson

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Re: AVO multimeters - which one, and why?
« Reply #8 on: October 10, 2020, 06:54:00 AM »
I have a modern 'clamp' equipped Fluke meter that is extremely versatile in that it measures DC current as well as AC giving a true RMS value - it was expensive but is very useful.

I got it when I needed to set up the servo motor on my 4th axis that I added to my Beaver Partmaster CNC milling machine - it did what I asked it to do.

I also used it to good effect setting the current / voltage relationship curve for the field coil driver on the HUGE DC motor on my Beaver TC20S CNC lathe.

So why didn't I use my Avo 8? Well I was convinced by 'those in the know' that the Avo would not give a true RMS reading as the currents involved were complex wave forms - so I laid out the dosh for the Fluke.

Being the sort of chap that I am, in both cases I (once things were set up) put the Avo in series so it was measuring EXACTLY the same variable as the posh digital Fluke and guess what - it indicated PRECISELY the same value - in fact iirc I posted pictures of same here when I was setting up the Mawdsley DC motor on the Beaver lathe.

. . . did I need to buy the Fluke . . go figure !

Andrew Mawson
East Sussex

Offline AdeV

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Re: AVO multimeters - which one, and why?
« Reply #9 on: October 10, 2020, 12:11:50 PM »
Thanks for all the replies, appreciated :)

Yes, I do miss having an analogue MM, although most of what I use it for is digital circuitry, testing resistors (because those colour rings are tiny, and my eyes aren't that good any more!), checking that point X has power to it... and for those uses, a digital is fine.

However... what I really wanted to know was - why an AVO rather than Brand X (Fluke?) analogue MM; and if there's a compelling reason to get an AVO, which one is best - a Mk3, 5, 7 or 8? All of which are available on eBay for varying amounts of wedge...

Ta!
Cheers!
Ade.
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Online awemawson

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Re: AVO multimeters - which one, and why?
« Reply #10 on: October 10, 2020, 12:22:23 PM »
Stout and robustly made, and if in the leather case able to be used port-ably on site.

I carried an Avo 8 (of various marks) for probably 25 years along with a Tektronix 'scope visiting customers sites and resolving problems. As a cost saving exercise we tried many other makes but none stood up to the rigours of life on the road.

There are good reasons why the services and  GPO / BT chose Avo's in vast numbers. Longevity along with accuracy are hard to achieve, but Avo did it !

Andrew Mawson
East Sussex

Offline Pete W.

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Re: AVO multimeters - which one, and why?
« Reply #11 on: October 10, 2020, 02:22:33 PM »

SNIP!

There are good reasons why the services and  GPO / BT chose Avo's in vast numbers. Longevity along with accuracy are hard to achieve, but Avo did it !
 

When the 1939-1945 war broke out, AVO Ltd. agreed to supply large numbers of AVO meters to the RAF (presumably via the Ministry of Supply).  However, AVO Ltd. were cute enough to include a clause in the supply contract which prohibited the Services from releasing these instruments onto the Surplus Market in large numbers when hostilities ceased.

There were various consequences once Peace broke out.  One story is of AVO meters spread edge-to-edge and one AVO deep on a patch of airfield runway about the size of a tennis court while a happy serviceman drove a crawler tractor over them crushing them to smithereens.  At other disposal establishments, personnel were tasked to take hammers to the now surplus AVOs.  The personnel adopted a policy of never hitting two successive AVOs in the same place.  The debris was then sold as surplus by weight  *, conveyed to Lisle Street in tipper lorries and tipped down into certain cellars.  In the cellars, 'instrument mechanics' would rebuild, making one functional AVO from the debris of many.  These Frankenstein AVOs were then offered for sale by the various Government Surplus shops.

*  If I remember correctly, the rate was seven shillings & sixpence per hundredweight!   
« Last Edit: October 11, 2020, 04:43:11 AM by Pete W. »
Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest design change-note!