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Old 16th Feb 2020, 12:48 pm   #1
Glowworm
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Default AVO 8 multimeter inconsistent readings.

My AVO 8 MkIII is accurate when starting from 0V and a voltage is applied. But if the input is increased then reduced again, the meter reads high, e.g. an input of 9V on the 25V scale then reads as about 9.5V. Moving the meter around reduces the error a bit. Disconnecting and reconnecting gives 9V again.
The movement seems reasonably free-moving. It's as if the meter has a small amount of memory of what if did last.
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Old 16th Feb 2020, 9:39 pm   #2
linescan87
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Default Re: AVO 8 multimeter inconsistent readings.

It sounds like its sticking slightly on it's pivots. I heard a cure for this was to hook it up to the indicator lamp in a car boot for a few weeks. The vibration of the car and the on/off action of the indicator on the 10V range freeing it up.

People who know what they're talking about will have better suggestions.
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Old 17th Feb 2020, 12:37 am   #3
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Default Re: AVO 8 multimeter inconsistent readings.

Or flake/s of ferrous debris could be obstructing the movement.
Dave
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Old 17th Feb 2020, 12:47 am   #4
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Default Re: AVO 8 multimeter inconsistent readings.

First I would look into using an oil free solvent to clean out anything from the air that has settled in the pivots. Meths has a dye in it so perhaps FE10 dabbed on with a cocktail stick. Allow the pointer to swing too and fro as it dries off.
I would also be tempted to knock up an oscillator or use a small function generator to deflect it to just touch the stop lightly on each cycle.
Use a diode to half wave rectify it.
Something like 5hz or less would allow it to settle after each half cycle.
10V range on 12 volts would likely stress the pointer too much if it were left pulsing for too long.
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Old 17th Feb 2020, 12:53 am   #5
McMurdo
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Default Re: AVO 8 multimeter inconsistent readings.

check the hairspring isn't rubbing up against itself, so to speak. If the anchorpoint is twisted (eg due to an overload) then adjacent turns can touch and introduce 'stiction' which is remembered by the needle.
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Old 17th Feb 2020, 10:17 am   #6
pmmunro
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Default Re: AVO 8 multimeter inconsistent readings.

As stated in the previous posts, problems like this are usually associated with either debris in the gap between the magnets and the moving coil, hairsprings or pivots.

Ideally, the movement needs full servicing but dismantling is a delicate job and the movement will need to be rebalanced. If you are confident that you can do this and have the facilities, this will be the best course of action. The service manual is readily available.

Check for particles in the gap with a strong light and a magnifying glass or jeweller's eyeglass. Often these can be removed with non-magnetic tweezers and very small pieces of adhesive tape. Blue Tak might also be useful.

If, in the past, any attempt was made to clean switch contacts using a spray cleaner, some droplets may have condensed on the turns of the hairspring, causing them to tend to stick together.

The best way to clean the pivots needs full dismantling, but washing them with a small artist's brush and isopropyl alcohol without dismantling may be successful. The pivots and jewels are not lubricated.

Exceptionally, either the pivots or the pivot jewels could be damaged but this would only be expected if the meter had been very roughly handled.

In all cases, examination or work on the movement is best done withn the movement off the front panel and the cut-out will need to be reset afterwards. The procedure and precautions have been described on the forum many times.

PMM
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Old 17th Feb 2020, 11:08 pm   #7
Glowworm
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Default Re: AVO 8 multimeter inconsistent readings.

Thanks guys, I will look at it carefully. I bought it at a second-hand shop for £12 but have no history for it. On the inside the only thing to notice was that the transformer at the bottom had been getting too hot, maybe it was used for monitoring the mains.
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Old 18th Feb 2020, 3:43 pm   #8
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Default Re: AVO 8 multimeter inconsistent readings.

The transformers are quite robust, that's unlucky. (Could be transplanted if the movement turns out to be fixable.)

Should be possible to ascertain if the transformer's actually damaged by checking resistances and referring to the circuit diagram
Dave
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Old 19th Feb 2020, 12:35 pm   #9
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Default Re: AVO 8 multimeter inconsistent readings.

I had a very similar if not identical fault once - turned out to be the soldered joint of the lead to the +ve input socket had a very fine crack in it - lightest of pressure and the readings swung like crazy. So do check those leads first as an easy option.
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