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Old 20th Nov 2017, 6:51 pm   #1
mark_in_manc
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Default AVO Model 40 Multimeter

Split from this thread: https://www.vintage-radio.net/forum/...d.php?t=140392

I have one of these apart at the mo, and this thread is timely! I took mine apart to cure a cut-out which wouldn't latch; I fixed that, and now find it won't zero on 100,000 ohm range, and is entirely dead on 1000 ohm. 10,000 ohm range seems to work OK. All AC and DC voltage and current ranges seem OK too.

I don't really need it to work for R measurement - but it's so nicely made that now I have it apart, I feel obliged to fix it before putting it back together. I've had a look at the circuit diagram above. If anyone knows the component layout it would save me a bit of tracing out. A resistor attached to the 10k ohm switch position looks rather burnt, for a start - I'll need to study the diagram to work out what value it might once have had.

Circuit diagram: https://www.vintage-radio.net/forum/...7&d=1311627688

cheers
Mark

Last edited by AC/HL; 20th Nov 2017 at 9:58 pm. Reason: Thread split
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Old 20th Nov 2017, 10:55 pm   #2
Phil G4SPZ
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Default Re: AVO Model 40 Multimeter

The 1k and 10k range resistors are connected to the appropriate switch positions. The 100k range resistor is, from memory, a bobbin type fixed to the battery box. Check the values of these resistors against the circuit diagram. It is common to find these resistors burnt out, possibly by someone inadvertently trying to measure the resistance of an AC supply...

It is worth examining and cleaning the leaf switch contacts. These are operated by cams behind the range switches and can be responsible for all manner of strange faults.
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Old 20th Nov 2017, 11:05 pm   #3
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Default Re: AVO Model 40 Multimeter

What year is it? If it's an early one:

The 10k ohm range relies on a resistance of around 180 ohm.. a photo would be a help as i expected this 180 ohm to be a winding but are you saying it's a component resistor.?

The 1000 ohm range uses a winding of around 15 ohm. Although it's failure to function could also be a switching problem. There's an extra pair of contacts that have to close on this particular range, as i recall.

There is (i think) a third winding on the resistance board, which is about 1 ohm (quite a thick one)

The high range (100,000 ohm) can be a nuisance as the Q potentiometer has inherent problems especially in old age- the Q shaft must have continuity with the cleverly shaped brass orifice in the facia. Also there is a little arrow shaped precious metal prod in the end of the Q shaft, and it's not unknown for this to be slightly loose, which can affect all 3 ranges. I posted some images re. this issue so if you search under 'Q shaft' or something like that you will find them in the avometer survey thread, or test equipment thread.

Dave
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Old 20th Nov 2017, 11:08 pm   #4
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Default Re: AVO Model 40 Multimeter

Ah, Phil has reminded me- it is common for the solder tags on the back of the battery box to crack away from each other, as the factory sometimes soldered 2 together without using a link wire, and the solder joint simply couldn't handle the torque when the screws were nipped up. To the naked eye they still can look ok.
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Old 21st Nov 2017, 9:55 am   #5
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Default Re: AVO Model 40 Multimeter

Thanks for your interest, folks, and thanks mods for sorting out my thread error.

Serial no. is 1205-B-267-M

I think this makes it Feb 1967

The circuit diagram does suggest a resistance on the spring contacts of the battery box, but mine does not have one. Instead, the three R ranges connect (amongst other things) to three component resistors encased in something like blue heat-shrink, with their values printed on.

10,000 works (in my first post I missed a zero somewhere and confused things) - 186 ohms

100,000 won't zero, but the resistor is roasted with no visible legend and o/c. I'll go with making up the value suggested in the diagram above - 1977 ohms - and see what happens.

1000 is dead. Thanks Philpott, I spotted the contacts you suggested which close on this range only and cleaned them. Then I worked out which resistor connects to this range - 15 ohms. Looks OK, but is o/c. So OK, another one to make up.
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Old 21st Nov 2017, 10:21 am   #6
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Default Re: AVO Model 40 Multimeter

You're almost there, Mark, well done. You have quite a modern instrument there. Let me know if you need any resistors.
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Old 21st Nov 2017, 7:16 pm   #7
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Default Re: AVO Model 40 Multimeter

All done and working - thanks folks.

Quote:
Originally Posted by Phil G4SPZ View Post
Let me know if you need any resistors.
I'm not going to share a photo of my work. 15 Ohms was easy and quite tidy with three similar 10Rs. 1970-ish was achieved with a rather more exuberant array of things found in the bottom of a cardboard box, whose arrangement tends towards the baroque. You wouldn't like it
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Old 21st Nov 2017, 7:44 pm   #8
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Default Re: AVO Model 40 Multimeter

Quote:
Originally Posted by mark_in_manc View Post
...You wouldn't like it
I bet we would! It sounds like a piece of practical engineering using readily-available components. Nevertheless, once the meter goes back together, no-one will ever know. And it works, which it didn't do before. That sounds like added value, and a success story!
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Old 22nd Nov 2017, 4:22 pm   #9
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Default Re: AVO Model 40 Multimeter

Exactly, if it works it works! You can always go back to it at a later date and tidy to your satisfaction.

Those late Model 40's are certainly very neat and tidy inside, the only problem i had with mine was both of the coil springs were hanging off the (die-cast) cut out, and there was no sign that anything had ever properly held them on. I slid a piece of silicon sleeving onto the peg which sorted it.
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Old 28th Nov 2017, 4:37 pm   #10
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Default Re: AVO Model 40 Multimeter

If I may ask has anyone encountered a distorted former around which Q resistor is wound, if so how did you fix it?
I have one that has been warped somehow and it splays out wards near the 'start' which causes the wiper arm to 'miss' until it has been rotated a bit.
Thanks
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Old 28th Nov 2017, 5:02 pm   #11
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Default Re: AVO Model 40 Multimeter

Are you sure it's distorted, or possibly not seated correctly in the moulding in the inside of the front panel?
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Old 28th Nov 2017, 5:33 pm   #12
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Default Re: AVO Model 40 Multimeter

...or a small foreign object such as a fragment of bakelite jammed in the recess?

I recall that the Q wire is very fine; it's an idea to try and measure/ascertain it's resistance if planning on disturbing it.
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Old 28th Nov 2017, 6:33 pm   #13
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Default Re: AVO Model 40 Multimeter

It was somewhat displaced but it is also distorted
I did wonder about packing pieces but thought that was a bit of a bodge lol.
The wire is indeed 'scarily' fine.
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Old 28th Nov 2017, 6:56 pm   #14
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Default Re: AVO Model 40 Multimeter

fine,and also tricky to repair as any heat from soldering makes some sort of resin leach out of the former and contaminate the joint!
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Old 29th Nov 2017, 12:40 am   #15
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Default Re: AVO Model 40 Multimeter

Ahhh is the former made of resin impregnated paper or something similar? I recollect it looked a bit fibrous but not coarse enough to be glass etc.
If so would it become workable if carefully warmed up with say a fan heater or hair drier and then, if allowed to cool whilst held in shape, take a 'set' ?
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Old 29th Nov 2017, 6:24 pm   #16
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Default Re: AVO Model 40 Multimeter

Sounds feasible to me, but never had to try it..
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