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Old 24th Apr 2014, 1:03 am   #1
vwJim58
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Default AVO Model 8 mk3 fault / help needed...

Evening,

Hope someone can help out. I use my Dad's old AVO mainly on 12v electrical circuits / circuit continuity testing while working on older cars, and have done for many years.

But today I found that the AVO no longer works on the continuity test function. I've searched through various threads but not sure if I have same issues others have had.

If I hold the lead contacts together, nothing happens when "ohms divide by 100" selected, (no idea what should happen as never use that position on the dial)

If I hold the lead contacts together, nothing happens when "ohms" selected, (this is what I mainly use the AVO for when hunting for wiring issues, so I would normally expect the needle to full deflect to the right)

If I hold the lead contacts together, the needle jumps off the scale when "ohms x by 100" selected, (no idea what should happen as never use that position on the dial)

I thought initially maybe a flat battery, so replaced the D-cell, and no change. I did then measure the voltage across the D-cell it read 2volts which I thought was strange for a 1.5v battery. Then tested the 15v battery, and that measured very nearly 20v. So not only the resistance measurement function is faulty, but it would appear the voltage calibration is out too.

Are these things likely to be fixable? Either by myself or any companies / individuals out there?

Really don't want it to be dead, as Dad had it from new and we've always used it in the family. Thanks in advance.
Jim
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Old 24th Apr 2014, 9:19 am   #2
pmmunro
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Default Re: AVO Model 8 mk3 fault / help needed...

Jim,

First you can be fairly sure that most faults in Avometers can be fixed, some of them very simply.

What you need is a circuit diagram and a clean working area free of dust and especially ferrous metallic particles. Depending on your experience of instruments of this type, you will also need some servicing information as in the AVO service manuals, but these are now a bit less appropriate than when the meters were in production as they were written on the assumption that all parts and facilities were currently available.

One important point to be aware of if you decide to open up your meter is that there are long pointed hexagonal posts fixed on the back of the little plates which support the "ohms" and "ohms x 100" zero adjusters. These posts connect to spring leaves which connect to the resistors, attached to the battery box, for the 2500V ranges. The length of these hexagonal posts means it is very easy to put an unintended force on them which will break the plates they are fixed to.

In order to address these issues and the many questions which arise on repair and servicing these meters, I am drawing up a service manual which I hope will be appropriate to the present day but it is a considerable undertaking and is not yet complete.

The ranges on these meters are selected by the rotary switches as will be obvious, but it is less obvious that there are leaf switches operated by Bakelite face cams fixed by screws to the end of the switch spindles. These leaf switches set up the circuit arrangement needed for each measurement function. To fully explain how these switches operate, you need appropriate diagrams. The fault you describe could be caused by the cam on the AC switch being loose on the spindle or by the switch spring leaves being out of adjustment.

When you select any of the three "ohms" ranges and connect the leads together, the pointer should move towards the right hand end of the scale to the 0 ohms position (on the top scale). The exact zero is then set using the corresponding adjuster.

If you feel up to fixing this fault yourself, send me a private message and I will send you further information.

PMM
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Old 24th Apr 2014, 12:45 pm   #3
David G4EBT
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Default Re: AVO Model 8 mk3 fault / help needed...

Quote:
Originally Posted by vwJim58 View Post
If I hold the lead contacts together, nothing happens when "ohms divide by 100" selected, (no idea what should happen as never use that position on the dial)

If I hold the lead contacts together, nothing happens when "ohms" selected, (this is what I mainly use the AVO for when hunting for wiring issues, so I would normally expect the needle to full deflect to the right)

If I hold the lead contacts together, the needle jumps off the scale when "ohms x by 100" selected, (no idea what should happen as never use that position on the dial)
Firstly, the 'Ohms Divide by 100' position will make no difference as it requires a ’Resistance Extension Unit' – an external accessory which enables the meter to read both high and low resistances beyond the two basic meter ranges of Ohms and Ohms x 100. In the LR position the Extension Unit divides by 100, with a full scale resistance of 2.5 Ohms. In the HR position it extends the resistance range to 200 Meg Ohms. (The manual explains more about that).

When you say that on the Ohms x 100 range the needle goes off the scale, that shows that the 15V battery, (BY1 on the circuit diagram), which is only used on that range, is in good condition. With the test leads shorted you should be able to zero the needle by adjusting the zero pot.

If there is no deflection at all in the Ohms position, that is the only range that uses the 1.5V battery (BY2 on the circuit). It also has an in-line 1 AMP fuse to protect the meter movement. Assuming that you have a good battery in place, you need to check that the fuse hasn't blown, which would happen if you tried to measure voltage on the Ohms range - an all too common inadvertent error. Hopefully, that's all it is, otherwise it couple be the switch leaf contacts not closing, which I doubt.

I've attached the circuit diagram, and switch position table which shows which switches are open/closed on the various ranges.

This reply isn't exhaustive, but it might help a bit.

I'm not personally into AVOs but there are many AVO on the forum who will be happy to help further.

(If you would like the full 27-page manual (8.4mB), please drop me a PM and I can e-mail it to you).

Good luck with it.
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Old 24th Apr 2014, 2:42 pm   #4
pmmunro
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Default Re: AVO Model 8 mk3 fault / help needed...

David,

Your point about the fuse in series with the 1.5 cell is a good one - it's certainly worth checking. However, I think you are confusing the "Ohms div by 100" with the LR range. Only the LR and INS ranges need the extension unit.

PMM
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Old 24th Apr 2014, 3:21 pm   #5
David G4EBT
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Default Re: AVO Model 8 mk3 fault / help needed...

Thanks for pointing that out PMM. As I said earlier, I'm not an AVO8 fan and don't own one - I simply looked at the manual, and in looking at the 'divide by 100' setting it seems only to parallel the Ohms setting, but clearly there is something else going on in the circuit 'switchery' which eludes me, but which enables divide by 100 facility. Fingers crossed for Jim that the fault he is experiencing will just be the fuse blown, since the AVO appears to be working on the Ohms x 100 setting.
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Old 26th Apr 2014, 8:18 pm   #6
Herald1360
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Default Re: AVO Model 8 mk3 fault / help needed...

The fact that it's reading high on volts makes me wonder whether the universal shunt for the dc current ranges may be open circuit somewhere. This will increase the sensitivity of the meter somewhat.
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Old 27th Apr 2014, 8:45 pm   #7
vwJim58
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Default Re: AVO Model 8 mk3 fault / help needed...

Evening guys,

Thanks for the replies. I was expecting email notifications that I'd had replies but not seen any so need to check my forum settings / junk mail folders.

I've got the booklet that came with the unit back in the 60's when Dad bought the AVO, I've had the AVO out of it's box, and it looks "near perfect" inside, amazing for something that is so old and been used all its life.

I put the voltage settings across a new 1.5v battery and it reads 2.1volts. The fuse is fine.

Looking at the action of the contacts as the dials are turned, again look perfect, and I see no burning / buildup or anything outwardly wrong.

I spoke Meggar and they weren't interested. Then I spoke to http://www.glanfordelectronics.co.uk/avo.html and they seemed to think anything wrong with a mk3 would be terminal, as no parts available. Which seems such a shame.

The guy from Glanford did mention something about a "universal shunt", but complex electronics is like speaking French to me, ie I only get about every tenth word.... I'm more a nuts and bolts person than electronics.

I'm more than happy to pay someone to test / identify the problem, and if replacement parts are available then that would be great.

As well as the voltage ranges seeming to read high, I don't seem to be able to make any difference to the ohms ranges (that work) using the zero adjuster. Maybe these are all linked or maybe different issues.

thanks
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