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Old 20th May 2019, 10:43 am   #1
vinrads
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Default AVO 8 mk3 reading high

I have just purchased an AVO 8 from the car boot for a fiver.
When I tested it I found the reading were all high, volts/ohms, any suggestions please, Mick.
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Old 20th May 2019, 12:14 pm   #2
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Default Re: AVO 8 mk3 reading high

Is current reading high also? If not, it could indicate resistors out of tolerance.

A good start would be to check the cleanliness of the range selection contact switches.
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Old 20th May 2019, 5:57 pm   #3
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Default Re: AVO 8 mk3 reading high

Is the pointer well damped and how much higher are the readings than they should be?

What information do you have, such as circuit diagrams, user manual, service manual?

At the risk of telling you what you already know; the Avometer Model 8 has a 37.5 microampere movement with a total resistance for the moving coil and its series swamp of 3333 ohms. The chain of current shunt resistors (known as a "universla shunt") has a total resistance of 10,000 ohms and is permanently connected across the movement. The shunt chain therefore diverts one third as much current as flows through the moving coil, i.e 12.5 microamperes for full-scale deflection.

It is quite common for one or more sections of the shunt to be burnt out making the chain open circuit, increasing the readings by one third and greatly reducing the damping of the pointer. The shunt elements are lengths of resistance wire wound on a length of flat phenolic moulding, except for the 10A shunt which is a coil of flat resistance strip. A replacement shunt resistor needs to be accurately selected for resistance and use low temperature coefficient resistance wire. The high current sections are the most difficult to replace because a very low resistance needs to be carefully set to the correct resistance.

If you take the meter out of its case, you will see that the connections to the battery box are by five long hexagonal pillars with pointed ends. The phenolic plate mouldings to which the pillars are attached are easily broken so care is needed not to apply force to the pillars.

PMM
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Old 20th May 2019, 8:37 pm   #4
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Default Re: AVO 8 mk3 reading high

Thank's for the info chaps , on further investigation I found a 1.2k ohm resistor o/c no sign of stress , a quick clean of the battery connectors got it working , on the DC volts and all the resistance ranges , I will look for the other fault tomorrow which is no dc current range except for the two lowest. time for Mick.
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Old 21st May 2019, 9:07 am   #5
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Default Re: AVO 8 mk3 reading high

I am pleased to report all ranges working well , didn't fancy paying £9 for a 15 volt battery ,so I used two pp3's one full one and four sections from the other , I needed to remove the original holder no problem it is just glued in , work's perfectly . Mick.
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Old 21st May 2019, 10:49 am   #6
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Default Re: AVO 8 mk3 reading high

I used a stack of 5 CR2032 button cells spaced with a 15mm stop end to fit in the original holder. There is no need to modify the battery holder this way.
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Old 21st May 2019, 11:18 am   #7
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Default Re: AVO 8 mk3 reading high

Good idea how much do you pay for the battery's Mick..
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Old 21st May 2019, 11:57 am   #8
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Default Re: AVO 8 mk3 reading high

Most of the time I don't use the high ohms range, but when I do on the bench, I just run a pair of wires to my bench power supply and dial in 15v. Just an idea for bench use.
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Old 21st May 2019, 7:01 pm   #9
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Default Re: AVO 8 mk3 reading high

CR2032's can be had quite cheaply, as can most of the other lithium coin cell sizes (and there are a lot of them..)

I have found a size that SHOULD fit in the original battery shell- haven't got round to making up a prototype yet.

Dave
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Old 22nd May 2019, 1:43 am   #10
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Default Re: AVO 8 mk3 reading high

I just saved the label from an old BLR121.
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Old 22nd May 2019, 9:39 am   #11
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Default Re: AVO 8 mk3 reading high

I have taken advice and ordered some cr2032's the compartments on mine are badly corroded ,thank's for the pictures neat job there. Mick.
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Old 22nd May 2019, 11:13 am   #12
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Default Re: AVO 8 mk3 reading high

15V alkaline replacement BLR121 batteries with a 72mAh capacity, which are the correct oblong shape and size (14mm x 25mm x 35mm), are available for £5.95 post free from a UK supplier:

https://www.ebay.co.uk/itm/AVO-8-MET...gAAOxyxpxQ5~zz

Alternatively, a battery consisting of ten 1.5V cells in heat-shrink plastic tubing can be had for £4.45 post free.

https://www.ebay.co.uk/itm/BLR121-RE...AAAOSwrSdZguwG

Most AVO owners seem to get by with either five 3V lithium cells or ten 1.5V cells.
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Old 22nd May 2019, 12:11 pm   #13
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Default Re: AVO 8 mk3 reading high

Five CR2032s inside a conveniently-sized piece of plastic pipe, with a penny coin at each end, makes a convenient and refillable 15 volt battery that fits the existing slot nicely.
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Old 22nd May 2019, 2:57 pm   #14
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Default Re: AVO 8 mk3 reading high

Five CR2032s work out to be about 2/3 of the length of a BLR151 hence the need for the pipe fitting as a spacer.
PVC tape has proved to be good enough if a plastic pipe the correct size can't be found.
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Old 22nd May 2019, 3:12 pm   #15
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Default Re: AVO 8 mk3 reading high

Ah yes, I remember that now! Thanks. I fitted TWO 1p coins at each end to pad it out a bit and get the length nearer to that of a BLR121.
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Old 22nd May 2019, 3:32 pm   #16
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Default Re: AVO 8 mk3 reading high

It is all down to how springy the battery springs are.
They just don't quite reach 5 CR2032s without some kind of spacer.
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