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Old 9th Jan 2022, 1:16 pm   #1
Barwick Green
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Default AVO 7 Mk 2 advice please

I've just bought an AVO 7 Mk 2, generally looks good cosmetically but for £20, as expected, it needs some work. I've removed the badly corroded 1.5v battery and cleaned the battery contacts. A new D Cell sees the low Ohms ranges working but it isn't possible to get both to zero without adjusting when switching ranges. Two 4.5v batteries and some cheap test leads are on order.

The next problem is that I can't get in to it, I've undone the countersunk screws all round, but the front is firmly stuck I'll take a plastic spudger to it and try to get it moving, but what's this curious extra screw likely to be? It looks as if somebody has drilled a hole, fitted a metal ring (which turns) and through it there's a rather mullered screw which doesn't.

Many thanks.

Rupert
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Old 9th Jan 2022, 2:05 pm   #2
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Default Re: AVO 7 Mk 2 advice please

That looks like the "secret" case screw, normally filled with black plastic or similar to hide it.
It needs to come out to open the case. Most people never bother re-fitting it.
I have seen examples where this screw has been drilled out so the screw not turning must be a common problem.

Peter
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Old 9th Jan 2022, 2:20 pm   #3
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Default Re: AVO 7 Mk 2 advice please

Yeah, that anti-tamper grub screws needs to come out. It's only brass so shouldn't be too traumatic to drill out (if you have a left-handed small drill bit and a reversible cordless drill it should come out that way....low speed and quite a lot of pressure is the way to go.) If it just spins you could try supergluing a small screwdriver to the head. (afterwards, acetone/nail varnish remover gets superglue residue off.)

Low and medium resistance ranges should behave better (and synchronise their zeroing) if the P & R potentiometers are inspected. Unlike the Model 8 they're BOTH wire wound so at least you can get to the contact surfaces. The R pot. has a brass shaft which sometimes goes dull and causes a bad contact with the leaf contact which presses onto it's side. Future corrosion or dulling can be mitigated by a tiny smear of petroleum jelly.

High resistance range- the trend now is to not bother with a pair of 4.5v batteries, they're overkill capacity wise. I adapt mine to use an alkaline 9v PP3, which is adequate.

Watch out for the leather strap hardening and cracking up, especially where the rivets hold it to the case.

The aluminium case you have difficulty removing is slightly tapered, (having been pressed from sheet) and this is a common problem. If you can wheedle the rubber o-ring out gently, this avoids it getting damaged during the following: Get a blunt blade in at various points around the periphery and ease it off. This procedure is accompanied by a series of pops, creaks and bangs, but it will come off squarely eventually- the key is to keep moving round. Avoid any white powder from corroded aluminium falling into the meter.

Dave
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Old 9th Jan 2022, 9:43 pm   #4
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Default Re: AVO 7 Mk 2 advice please.

Thank you, that's excellent help.

Thank you, very detailed and helpful. I'll get the old girl going!
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Old 10th Jan 2022, 11:49 am   #5
Barwick Green
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Default Re: AVO 7 Mk 2 advice please

Thanks to the advice from Peter and Dave, I'm in! It looks clean with no obvious bodges. Looks as if I have to undo the two big screws and lift out the movement to get to R and clean it? I tend to use a pencil eraser to clean contacts, or good quality white card slipped in. This has got P & Q moving smoothly but I still can't get the Ohms scale to zero on both low ranges. I wonder if a modern Mallory D Cell has such a low internal impedance it's "out of range"? If not, I guess one of the fixed resistors has gone out of range. More investigation required but I won't get chance to do much until next weekend.
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Old 10th Jan 2022, 1:05 pm   #6
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Default Re: AVO 7 Mk 2 advice please

R pot. can be cleaned without removing the movement, i believe....using tweezers (if the tweezers are ferrous they shouldn't be allowed to get pulled onto the magnet as it's flux can be degraded by this- which is very bad news!)

There's probably a couple of crinkle washers that will try and lose themselves when the screw/wiper are undone.

Once the R knob/shaft has been removed the leaf contact arm will have it's tension removed and will move a third or a half way across the hole....if it doesn't do this you can always bend it over more to ensure it makes proper contact on reassembly.

The whole meter may of course have accumulated traces of dampness while idle, and the resistance ranges might improve now it's got some air up it's kilt. I've never had a problem using modern D cells for the low and mid range..

One thing i do recall is that any lack of continuity in the Q pot (even it's tiny silver-gold arrow contact in the end of the 'prod' can come loose) messes up ALL of the resistance ranges. I have a feeling some meters were discarded or set aside with a fault in this area as it's not immediately obvious.

There's some white spots on one of the shunt boards- is it mould or just debris/varnish..?

There's something amiss with the capacitance range on my Model 7, so after trying it once i've bravely left that range alone!

The meters with aluminium cases have shorter casing screws than bakelite cased meters, one has to look out for oversize screws fitted later on which could reduce the safety factor at higher voltages. The insulating sheets inside the ally case eventually come loose, but araldite does a good job of reattaching them in the name of safety.

Dave
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Old 10th Jan 2022, 1:17 pm   #7
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Default Re: AVO 7 Mk 2 advice please

Thanks Dave, that's really helpful. I think it's mould, the leather case it was in has signs of damp. A good drying will doubtless help. I've just discovered that pressing on the front panel with my finger makes the meter wander so there's clearly a dogey connection somewhere, perhaps the brass strips for the battery box to the pointed contacts on the front panel, I'll have a look at those when I get chance. For now, i have to get on with earning a crust!
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Old 11th Jan 2022, 11:54 am   #8
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Default Re: AVO 7 Mk 2 advice please

Thanks everyone for the help. A bit of a strip down and thorough clean has the low ohms ranges zeroing properly, and the glass and mirror nice and shiny! Now just needs a bit of tidying up.
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Old 11th Jan 2022, 2:57 pm   #9
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Default Re: AVO 7 Mk 2 advice please

Ah, I spoke to soon. Having received the batteries for the high ohms range I found it dead, the 4922 ohm resistor mounted on the battery box is open circuit. I carefully unwound the thick wire wrapped around the surface of the coil for termination and the problem isn't there, it must be at the termination at the centre of the bobbin. I suppose in theory I could unwind the wire until I get to the centre, solder the connection back and rewind the wire, but seeing how thin it is I think that's beyond me. Before I order some modern high stability resistors to make up 4922 ohsm, are there any other suggestions, please??
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Old 11th Jan 2022, 7:08 pm   #10
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Default Re: AVO 7 Mk 2 advice please

I repaired an open-circuit fault on my AVO 7 with modern resistors - this thread details it https://www.vintage-radio.net/forum/...d.php?t=126747. I still use it regularly now.
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Old 13th Jan 2022, 7:59 pm   #11
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Default Re: AVO 7 Mk 2 advice please

-Yes, i must admit that when one of those bobbins goes o/c i replace it with component resistor/s. Finding where the bobbin wire has rotted is too much hassle for me, that's if you can even find the break under the (cotton?) insulation.

(6.2k and 24k in parallel is a close match)

Dave

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