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Old 15th Apr 2018, 11:26 pm   #21
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Default Re: Avo 7 Mk 2 Repair

Quote:
Originally Posted by The Philpott View Post
(The ac current ranges are reliant on the transformer windings, not the dc windings in the pic.)
Ah of course. I'll have to get my 10A dc supply set up.
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Old 20th May 2018, 11:54 pm   #22
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Default Re: Avo 7 Mk 2 Repair

Hello.

I picked this project up again this evening, still boggled over the intermittent ability to be within spec at FSD.

I took the current transformer out to allow me access to clean the switch contacts underneath, as I hadn't cleaned those ones yet, only the DC ranges are the ones I had cleaned.

I appear to have found two poor connections, highlighted in blue.

One is a wire which goes to the movement from a resistor junction and the other is where a wire is soldered to the small thin wire of a wound resistor. What's left on that wound resistor is some flux residue.

I've remade those connections and things are looking rather good.
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Old 21st May 2018, 10:56 am   #23
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Default Re: Avo 7 Mk 2 Repair

I notice one of the magnets (and also the stabiliser/keeper bar across the magnets) looks wonky. I have not encountered this before..and it is highly unlikely to have come out of the factory like that. I don't know what can/can't be done about that, as it is verboten to undo those two nuts. Undoing them slightly and straightening things out is likely to be a death or glory scenario. if you can get the meter to behave without touching them this would be ideal!
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Old 21st May 2018, 11:25 am   #24
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Default Re: Avo 7 Mk 2 Repair

Well spotted.

I don't know it's the angle of the dangle of my camera which has made it appear like that or not, so I'll take a look at it later.

But so far, any measurements which have been taken are linear and are within % of FSD.
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Old 21st May 2018, 5:34 pm   #25
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Default Re: Avo 7 Mk 2 Repair

Speaking of repair and restoration, another meter needs some new rubber feet which sit at the back of the meter.

I wonder if you'd know what would make suitable replacement rubber feet for an Avo?
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Old 21st May 2018, 6:40 pm   #26
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Default Re: Avo 7 Mk 2 Repair

I had universal avominor feet printed off by 'Ti Pwun' but never got round to sending him a big avo foot to copy. Big Avo feet are composite insofar as they have a layer of woven fabric to prevent them getting pulled off or squishing out of shape under pressure from the rivet. The plastic that he used for the 'minor feet would be sufficiently hard to prevent this happening. You may have to renew the rivets as the plating originally used makes them prone to cracks in old age.
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Old 21st May 2018, 9:27 pm   #27
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Default Re: Avo 7 Mk 2 Repair

So it's almost back together and still looking good.

The only other observation to make is that when 50% of FSD, when I press the divide by 2 button, the needle will shoot slightly past 100% rather than read the exact x2 of 50% FSD.

Regarding the magnet not being quite straight, it does appear at a slight angle, but I daren't touch it at this stage.
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Old 21st May 2018, 10:17 pm   #28
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Default Re: Avo 7 Mk 2 Repair

Yes, i have noticed this, it's unavoidable non-linearity characteristic of this type of movement. More noticeable as you get beyond 85% and head up to 100% FSD, they get slightly optimistic. Stability from Zero to 50% is usually quite good...but the manual hints that voltages should be measured towards the high end of the scale if possible!
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Old 22nd May 2018, 8:09 pm   #29
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Default Re: Avo 7 Mk 2 Repair

Right. Back together and reading good.

It still needs to be taken through a full calibration and I need to order some 4.5v batteries so that I can test the resistance ranges. But the unit is back to consistancy. Happy with that.
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Old 22nd May 2018, 9:02 pm   #30
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Default Re: Avo 7 Mk 2 Repair

A new alkaline PP3 would be stable enough to test the high resistance range if you have one lying around.
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Old 22nd May 2018, 9:55 pm   #31
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Default Re: Avo 7 Mk 2 Repair

Even those I need to order. I've got some in some digital meters, but even those would probably drain quite quickly as they're old but fine for running a digital meter.
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Old 23rd May 2018, 12:17 am   #32
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Default Re: Avo 7 Mk 2 Repair

Just a quick check on the resistance ranges which use the 1.5v battery, reads a little high. Wonder how easy this would be to bring back into cal.
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Old 23rd May 2018, 6:22 pm   #33
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Default Re: Avo 7 Mk 2 Repair

Actually, no it's alright. I was doing something wrong.

I were on the 10k and 100k ranges and using the Q dial. I should be only using P and R on these ranges. Only have ever used a model 8 in the past for resistance which doesn't require any other dial apart from the obvious range zeros.

This turns out to be a very nice unit so far. I've got another 4 or 5 of them somewhere, so at some stage will run calibration on the lot and have just been given another one with a cracked and broken rear casing.
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Old 31st May 2018, 9:49 pm   #34
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Default Re: Avo 7 Mk 2 Repair

I revisited this one tonight to adjust the shunt by 10uA FSD. Hardly required, but still.

Anyway, the circuit diagram I have for the Mk 2 I noticed says 'Model 7 Avometer with letter "A" in serial number'.

Now, I beleive that would be one of the early 7 Mks, identifiable by the mushroom headed fork only type terminals at the bottom and the serial written in the movement, so say a 1952 onwards model or so into the 60s where they had a change to the later type of terminals we've come to know and still had aluminium rear casing before changing to plastic rear casing.

This model I'm working on is made in 1971, plastic rear casing, serial and date on the side of the case on a sticker. No letter A in the movement.

I can't find any other different drawings for the Mark 2's, does someone here have one at all?

Thanks.
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