AVOmeter 8 mk1

[juanafati]

Hello everybody

I have just received a multimeter AVO 8, MK1. Wished for a long time.

There are several missing pieces and I would like to ask for advice to the members of the forum to be able to restore it and make it work.

The missing pieces are:
- Top cover of the battery compartment. (??)
- Screws that hold the 2 parts of the housing. (From RS or others ?)
- One of the nuts-and-bolts which holds a rubber leg and one of the upper lateral ones.
- The battery itself or maybe batteries? (Megger Co. or make it myself.)
- The rubber legs of the back. (??)
- Leads and probes. (From RS, Farnell, etc.)

I have read "AVO model 8 FAQ", "AVO multm. survey" and quite a lot of posts about our device, but I have some doubts yet.

The problem I have about the screws is that their thead is not similar to the usual one in my place. So, could anyone tell me the type of thread they have, please? That way I can look for them at RS or similar sites.

Another problem is, in my case, I have never seen a complete AVO mkI, so the setting of the metal plates / plugs in the battery compartment do not give me a hint on the way I can fit the home made battery (with some litium ones) in either of the the 2 sections of that battery compartment. Nor a real original battery. A photo of its positioning would be really welcome.

NB: I have written to Megger company about the missing parts, but I have not obtained an answer from them yet.

I suppose some problems will arose when I start it, but we will fight them later.

Thanks in advance for reading.

[Technomaniac]

Hi. I have five or six Model 8s, two or three operational. My first was bought new around 1967 for $110 Australian. Others have come from auctions and garage sales. The model 8 has two 'batteries' , a 1.5v D Cell which operates the two lower resistance ranges, and a 15 volt for the top range. It is Eveready No.411. I find them expensive and sometimes difficult to obtain in Australia so I just don't use that top resistance range. Digital multimeters are inexpensive. The battery lid would be easy to make as it's a flat rectangle of Aluminium with rounded-off corners, and two keyhole slots for the retaining screws, and there are rubber blocks glued to it in the appropriate places to hold the batteries firmly in place. The Avo 8 has an overload feature which appears on the front as a round button that pops up if the needle goes past the top of scale, particularly if it is moving at a high speed. That overload button will normally stay down when the meter is flat on the workbench, but will release if you try to use the meter vertically. The meter is to be used only horizontally. Check the glass. I have had to reglue one or more of mine, it is important, because dust will get in and ruin the scale. There are actually THREE screws holding the front to the case, the two are halfway along, and the third is close to a nearer corner, and normally is covered with red sealing wax or red nail polish, to show that the meter has not been tampered with. Someone else may be able to help you to identify the screws, they are probably BA and we do not see them much over here, and have a hard time finding replacements. You need to keep the connectors on the test leads polished up, as with age, they tarnish and give trouble, particularly on low ohms. That's the banana plugs and sockets, I doubt that you'd have much trouble with the spade ends that some leads have, that go under the knurled knobs of the terminals. They have been a great meter and are still very useful, I know that a new Model 8 was still available about ten years ago, and may still be. There were leather cases made for them, I think I have only one, but I don't usually use it as you have to remove the meter completely from it to operate. I bought a 50A shunt for my first one, you use a shunt for DC and a current transformer for AC. I never found out why the shunt wouldn't work for AC as well. If you don't have a leather case that the meter can live in, try to find at least a corrugated cardboard box of suitable size to give it SOME protection. Cheers!

[ronbryan]

I have made a drawing of the Avo 8 MkII battery cover if it is any use to you. It would be easy to make from sheet aluminium and paint black. Imperial dimensions though!

I think there was an earlier, moulded Bakelite battery cover that had a housing for probes. This one is just a flat sheet of aluminium.

Ron

[micheal]

Hi, the nuts and bolts for the battery compartment are 6 BA, there are also some 4 BA and 2 BA for the meter movement.

You are unlikely to get any help from AVO

MIKE

[juanafati]

Thank you very much "Technomaniac", Ron and Mike for your information.
I will use all this tips and data step by step.
Technomaniac, all your extended advice are already kept in a safe place to be taken into account when needed. Toguether with the design of the battery cover.

I am waiting for some info about the screws which fasten both parts of the meter. All of them are missing. Maybe they have something in common with the ones described by Mike.

And, as Mike says, I don't expect much from AVO Co. We'll see.

Thanks again.

[pmmunro]

Juanafati,

All the screws and nuts used in these Avometers were BA (British Association) types and were nickel plated brass or, in the case of some nuts, plain brass. The case screws were 6 BA raised countersunk slotted screws except for the one at the lower left of the case which was a plain countersunk screw which was sealed over with wax. Until the 1970s, BA screws were the usual choice for electrical and electronic equipment made in the UK. Only the even numbered sizes, 0, 2, 4, 6 and 8 were commonly used, except in telephone equipment where some odd numbered sizes were chosen. This supplier, for example, publishes some information (https://www.ba-bolts.co.uk/faq.html).

These screws have a 47.5 degree V-angle and the thread pitch is based om a millimeter scale, although in most thread charts is expressed in decimal parts of an inch. BA screws can still be bought from some specialist suppliers, but nickel plated ones are not easy to find.

I don't know for sure why brass was used in Avometers, but in some parts of the movement ferrous screws might have had the effect of disturbing the magnetic field. It is equally likely that it was done to avoid the hazard of stray parts being drawn to the magnet.

The rubber feet for the back of the case were probably once very common and might have been available from specialist manufacturers of such parts but I have been unable to find an exact match for the rather flat profile. For the quantity required, an eBay search might be useful to find something similar.

[The Philpott]

May be best to confirm the meter actually works at this stage before spending too much time on it. The missing screws tell that someone has been working on it..which is a worry.

Dave

[juanafati]

Philpott
Yo are completely right. I am waiting for its 15 V. battery to arrive and have my first try, but... meanwhile I do not want to loose hope and get disappointed and (very) sad.

As a matter of fact I am very stubborn on these subjects and I am nearly sure I will get through it sooner or later. I do not mind spending the neccessary time (and perhaps money) to reach the target, even though I am not a technician.

As far as people who give accurate and, sometimes, wide explanations don't stop contributing to our request for help, I keep my faith on that.

The last comment is dedicated to all posters and specially to "Technomaniac" and "pmmunro" for their wide, deep statements.

Maybe in the future I even dare to ask "pmmunro" for a document of his to service the AVO 8. Who knows? Let's future to come.

Juanjo

[pmmunro]

Juanjo,

Send me a private message with your email address and I will send you a copy of the Avo service and user manuals.

PMM