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Old 31st Mar 2017, 11:51 pm   #1
Spencervs
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Default AVO Meter 8 mkII

Does anyone know if an old 1960s avo such as this is likely to work/be accurate still? I've just got one today and I am hoping it will be usable. Also where can I replace the scale? I ave chipped paint work on it which ruins some of the amps.

Could someone also advise me on the sort of battery?
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Old 1st Apr 2017, 12:37 am   #2
Johnbarn
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Default Re: AVO Meter 8 mkII

You only require the batteries for the resistance ranges. For basic ohms range you need one 1.5 volt U2 or HP2 torch battery. That's the easy one. The other one is a 15 volt battery for the other ohms ranges and used to be type B121 but I haven't seen one of them for a long time. I have fitted an RS 591-821 but I'm not sure if they are still available either. Check the catalogues. Whether the meter works or not will be governed by the continuity and suspension of the moving coil meter movement: the accuracy will be determined by the accuracy of the internal resistance elements but it's a great quality meter and well worth spending a few pounds on. I much prefer it to my newer digital meter.
John
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Old 1st Apr 2017, 10:01 am   #3
Boater Sam
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Default Re: AVO Meter 8 mkII

You can check the movement accuracy on DC with a stack of batteries for a rough check.
Compare it with a DMM should be close enough for hobby use.
The 15v battery is easily made with a stack of five 3v Button cells CR2025s or CR2032s from the Poundshop, taped together with a conducting spacer. Watch the polarity, + to the middle.
Use a decent HP2 cell, leakers can do massive damage.
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Old 1st Apr 2017, 10:41 am   #4
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Default Re: AVO Meter 8 mkII

Fortunately there is a lot more (widely available) information on refurbishment now than there was a decade or so ago; because of a) the internet b) this website, richard's radios website, peter vis' website, etc etc. c) people appreciating them and repairing them.

The needle on yours is sitting in a sensible place so it's likely that it's perfectly ok apart from the peeling paint. Accuracy is usually quite good, and the sensitivity of the meter is quite good (at least by 1960's standards)so there is less tendency to disrupt the circuit that is being checked. Bear in mind that the basic design is quite old though so there are certain things it won't be able to do with electronics. The avometer is not accurate on the ohms ranges by modern standards, but it's plenty good enough for getting a general idea. If you have old lightbulbs with the legend rubbed off it will certainly be able to sort the 100watts from the 60's from the 40's.

You will definitely find a scale plate (on the usual auction websites) but it will usually have a whole meter attached so you are down for a tenner in postage plus maybe £7 or £9 for a broken meter. The scale is printed so accuracy should be maintained with a better looking scale plate installed.

Test it with a small battery before you pay out for second hand bits, maybe.

Your battery compartment looks quite clean; often they aren't. This is a good start.
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Old 1st Apr 2017, 12:43 pm   #5
John M0GLN
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Default Re: AVO Meter 8 mkII

If you're in luck they can be very accurate, I have an Avo Mk8 which although looking almost brand new has suffered sometime in the past and is about 30% out on the ranges I have checked, however I have an older 'Heavy Duty' Avo which is so accurate I hope no one thinks I've been fiddling the results, this is a test I've just done against a Fluke 73 Mk 111,
the test voltages came from the mains, a power supply and a PP3 battery, of course had I been measuring voltages in circuits the results may have been very different and in the Flukes favour.

Fluke 6.87 VAC Avo 6.5 VAC
Fluke 246.8 VAC Avo 247 VAC

Fluke 8.95 VDC Avo 8.75 VAC
Fluke 100.4 VDC Avo 100 VDC
Fluke 126.1 VDC Avo 125 VDC
Fluke 200 VDC Avo 200 VDC
Fluke 360 VDC Avo 360 VDC

The Avo results were the closest I could read from the mirror scale with a magnifying glass, its Serial No is 17669 - 164A

John
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Old 1st Apr 2017, 3:37 pm   #6
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Default Re: AVO Meter 8 mkII

Oh im keen to give this a try. Do you guys know where I can get the leads from?
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Old 1st Apr 2017, 7:14 pm   #7
David G4EBT
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Default Re: AVO Meter 8 mkII

Quote:
Originally Posted by Johnbarn View Post
You only require the batteries for the resistance ranges. For basic ohms range you need one 1.5 volt U2 or HP2 torch battery. That's the easy one. The other one is a 15 volt battery for the other ohms ranges and used to be type B121 but I haven't seen one of them for a long time.
John
They're available from the Small Battery Company, but sadly, at a price that's more than what many AVO 8s are worth:

http://www.smallbattery.company.org.uk/sbc_a411.htm

As to the scale, the best chance is from a scrap AVO 8 with, for example, a duff meter movement or smashed case etc. They do crop up from time to time.

Good luck with it.
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Old 1st Apr 2017, 7:21 pm   #8
The Philpott
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Default Re: AVO Meter 8 mkII

Spencer, Leads come up on ebay on a regular basis, but the 'correct' ones in perfect and unused condition can fetch in excess of £40-£50 a set, and they don't appear too often. Used ones in reasonable condition are more affordable, but can still fetch half that. The reason is simple; the meters age much better than the leads do. Some people just use odd test leads or bits of wire that they have to hand, provided they aren't measuring high voltages or high currents.

John, is your Model 8 30% low or 30% high? If it's LOW, it may well be the leaf switch stacks are not set up correctly.
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Old 1st Apr 2017, 8:19 pm   #9
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Default Re: AVO Meter 8 mkII

Talking of accuracy, i have just made a voltage reference device out of a (tiny) break-out board from Pimoroni which provides 2.048v and 4.096v supposedly to an accuracy of 0.1% after a 30 minute warm-up. I have powered it from a PP3. I have tested several avometers for basic accuracy on low voltage DC and they show up very well, and the ones that have 'aged' certainly haven't drifted beyond the compensation provided on the meter magnet.

John's claims (above) don't look unreasonable!
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Old 2nd Apr 2017, 8:31 am   #10
John M0GLN
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Default Re: AVO Meter 8 mkII

Quote:
Originally Posted by The Philpott View Post
John, is your Model 8 30% low or 30% high? If it's LOW, it may well be the leaf switch stacks are not set up correctly.
Thanks for the suggestion but it's reading high, I've just rechecked and the error is nearer 20% than 30%, still useless though, a look inside reveals that the PCB on the top right behind the movement has been seriously overloaded in the past.

John
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Old 2nd Apr 2017, 11:48 am   #11
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Default Re: AVO Meter 8 mkII

Oh crikey, yes that's much more modern than the Avo 8/III meters that i found the switching fault on.
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Old 9th Apr 2017, 7:33 am   #12
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Default Re: AVO Meter 8 mkII

So I had a moment and bought some button cells which I taped together to make 15 volts and replaced my 1.5 volt battery but have had no luck... am I doing something wrong? Or is it just not working?
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Old 9th Apr 2017, 10:06 pm   #13
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Default Re: AVO Meter 8 mkII

To answer your original question, yes these meters can be very useful and as accurate as the day they were made. However, they may need some repair and servicing but both are readily possible.

You need to understand how the basic moving coil movement is adapted to each of the function of measuring direct and alternating current and voltage and resistance. For the specific case of the Model 8, a service manual will be invaluable.

The first attachment will give you a good grounding in the basic theory, in terms of the Avometer Model 7. The Model 8 is designed around the considerably more sensitive 50 microampère movement but the principle is the same.

The second and third attachments are simplified diagrams of the ranges of the Model 9, a derivative of the Model 8 with 0 - 3 - 10 scale sequence rather than the 1 - 2.5 -10 sequence of the Model 8.

There is a great deal of information on Model 8 repair and servicing in other threads on this forum and I can send you user and service manuals by email if you need them.

The scaleplates of Avometers were individually matched to the movement and are not therefore interchangeable without a serious risk of loss of accuracy. Earlier models were hand calibrated, later ones used pre-printed scales selected from a set of 10 or 12 "Scaleshapes". The selected scaleplates were used on later Model 8 Mark III instruments; look for a bold digit or pair of digits at the bottom centre of the scaleplate. Scaleplates with the same scaleshape number are interchangeable.

To make a basic test of your meter, start with the 50 micro-amp (DC) range. A 1.5 volt cell in series with a 27kohm resistor and the Avometer should give nearly full-scale deflection. If that works, check the sensitivity against another reliable meter if you have one. If not, you may need to check the basic movement, but for that you need servicing instructions or risk irreparable damage. If the movement is sound, it should be possible to repair the meter.

With a bit of effort and research, you will have an interesting and enlightening project and end up with a very worthwhile and useful instrument but if your other commitments do not allow time for this, please pass the meter on to someone else as these meters are too good to waste. I'm sure another forum contributor would be interested.

PMM.
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Old 9th Apr 2017, 10:35 pm   #14
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Default Re: AVO Meter 8 mkII

I thought all the printed scales were identical- never cease to be amazed at the information that comes to light..

Both my Mod 8/III's have a concealed number '5' showing at the centre/bottom.
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Old 9th Apr 2017, 11:42 pm   #15
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Default Re: AVO Meter 8 mkII

Scaleshape 5 does seem to be common.

I have a table somewhere defining the scaleshapes; if anyone is interested I could have a look and see if scaleshape 5 corresponds to a scale which is linear throughout the range.

PMM.
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