Avo 8 MK II

[georgeek]

Here is the MK II Avometer I posted about in Loft find last week.

Dusted and with new batteries it seems to work well.

Image search on Google indicates that it is a slightly different MK II. It has a black instead of a red terminal and the middle scale is missing on the right “250V MAX”.

I am used to a modern multimeter and now I have spent time sprucing up the Avo I don’t have clue what to do with it.

[Andrewausfa]

It's an AVO 8 I think, not an AVO 8 mark II.

Always a good meter to have to hand, I have an 8 Mark III and a Model 7 on the bench.

Andrew

[pmmunro]

George,

Your find is not a Model 8 Mark II. It's a Model 8 that dates from before that type of Avometer had a mark number assigned by the maker, although it would now be regarded by most people as a Model 8 Mk I.

The Model 8 was introduced in May 1951 and became the Mk II in 1956 when the terminals were changed to the familiar red and black sockets. The blank range continued a little longer until the 250V range was added around 1958. Why it was not provided earlier is something of a mystery as provision seems to have been there for it.

Although you have an instrument which is nearly 70 years old it is still likely to be able to do exactly what it was designed for i.e. measure current and voltage in either AC or DC circuits and to measure resistance. A 1.5V cell and a 15V battery are needed for the different resistance; for the other ranges it is an entirely passive instrument which does not need a battery. It's all explained in some detail in the manual.

Within the limitations of its ranges it can do all that a digital multimeter can do. However it always needs 50 microamperes for full scale deflection on direct voltage ranges. This can also be expressed as 20,000 ohms per volt. Alternating current ranges need more current as explained in the manual. This meter was designed at at time when most electronic equipment used valves and the current drawn by the meter was not often a problem. It was always wise though to be aware of the current drawn by any measurement.

It's also important to remember that analogue, moving coil, instruments are mechanically and electrically delicate by comparison with modern good quality digital multimeters.

A well cared for Avometer which has been serviced as required to keep it in good order will be as accurate as when it was made and therefore as accurate as is needed for nearly all applications where a portable multimeter is appropriate. It should be as accurate as any 3 1/2 digit multimeter except for the few occasions when its current draw loads the circuit under test too heavily.

If you are not used to using an analogue meter, it you will find it a revelation how useful it is when following trends and varying quantities. With a digital meter you know where you are; with an analogue meter you also have a good idea of where you been and where you're going.

PMM

[The Philpott]

Yes, an original Avo Model 8. The booklet you have for the later Model 8 Mk II should suffice, there are very few practical differences. (You have already noticed the main ones)

If you're wondering what to do with it/use it for, as is often said a digital multimeter can do things that an analogue multimeter cannot....and vice versa.

(The high resistance range is good enough to detect dampness, eg in carpets, which should allow you to avoid buying a moisture meter that you would only use once in a blue moon.)

Watch out for the leather strap weakening from years in the loft- one shortcoming is that they don't survive crash landings very well!

Dave

[Refugee]

All good engineers need an analogue meter some times and an AVO is the best one to have.
I do not have an AVO 8 but do have two AVO 9s.
The ranges are slightly different but in all other respects they are the same.

[Radio Wrangler]

The loading effect has already been mentioned. Your AVO will take a small current, somewhat more than is usual with electronic meters (there are analogue ones as well as digital ones). But most period service sheets for radios and TVs were marked with the voltages that would be given when tested with an AVO 8... The loading effect being taken into account. So you may need to worry about lack of loading from a modern instrument!

That bit of loading also serves to stop you getting perfect readings through dodgy connections and gone-high resistors.

I'm not sure I'd call the AVO 8 the best, it's the most common of the high grade passive analogue meters. The GEC Selectest is every bit as good. Simpson and Triplett made some equally good models in the USA. It has equals, but they're less common. There are spares kicking around for AVOs and many people have donor units, so as said, it's probably the best to have.

AVO made electronically-assisted multimeters as well. I had a CT38 valve-based monster, and I still use an AVO EA113 FET-based one.

David

[The Philpott]

This meter can be damped for transit by setting to 'DC ranges' and '50uA', then shorting the + and - terminals. You will then observe the needle becoming lazy. A good test for a movement that is (very) healthy is to adjust the needle zero (if required), damp the meter for transit as above, set the meter on it's back and give it a twist to move the needle. It should slowly return to a perfect zero....or thereabouts.

Dave