AVO Multimeter survey

[Phil G4SPZ]

Left - February 1934, right - January 1936. Somewhere between these dates the 'flange' appeared but the meters are otherwise identical! Maybe it was done to increase the strength of the Bakelite moulding and prevent pieces breaking off, like another one I've got

[Paul Adams]

Another Model ‘D’ (Air Ministry) rescued and put back to original condition.

When I obtained this meter it had been ‘got at’ someone had removed the ‘P’ rheostat, and fitted a ¼” pot, opening up the original hole in the process, as shown here:



I wanted to put this back to it’s original condition, so a quick sort through the junk box revealed all of the necessary parts for this including the threaded post that fits onto the ’P’ rheostat wiper assembly.

The problem encountered was that when someone enlarged this hole to just over ¼”, they also removed the molded Bakelite spacer section from the front for the ’P’ adjustment. Not one to be deterred, another rummage through the junk box revealed a short plastic spacer of the same inside diameter of the ‘P’ knob shaft, this was fitted to the enlarged hole and araldited into place, allowing the knob to be spaced away from the front panel correctly, so that the mechanical stop for the knob assembly still worked.

I also re-painted the switches with a cream colored paint which seemed more period for this meter, rather than the usual white.



Oh the serial: No. D. 19470-445. 1945.

Paul.

[pthomas]

I have just scored another Avometer 8. This time its a Mark 1 Serial No. 41354 C 1255. I have opened it up and it is perfectly original with no repairs. It has some scribbles on the back of the scale plate with the last one 1988. Interestingly it has a service tag attached - EMI Service, Electronic Repair and Calibration Service, Hayes, Middlesex. I am left wondering about who used this!

I have tested it out on most ranges and it is very accurate (i.e. around 1% or better compared to my Fluke 87). Inside the movement is larger than other Avometers I have and the magnets painted grey. The only problem I had was that the spring loaded plunger for the trip was sticking due to oily residues - so I removed it and cleaned it. I didn't need to adjust the trip afterwards and it worked as intended afterwards. The battery compartment is completely free of corrosion also. I am thinking about not really cleaning and re-finishing the facia and just leaving it in the original aged state. The screw down connections are bit of a pain - the original leads are perished

[Sparky_McCrunch]

Avometer 8 Mark 5 Serial No. 06826 8V/5/76, a former resident of the South London College, now lives in the Austin, Texas area. The case is in excellent condition except where it has been engraved with SLC's information. The blade portion of the meter pointer was bent to the right by three minor divisions of the outermost scale, this has been fixed. The balance is off by 2 minor divisions and the direction of the imbalance is consistent with lower pointer spar being bent by the same accident that bent the upper pointer. The glass is cracked cleanly from top to bottom. The balance and glass will be fixed as time permits. There was mild corrosion in the battery compartment where the D-cell had leaked. This has been repaired. Checking the linearity of the meter movement using the 0-10 scale shows the worst errors to be about +0.5% at full scale and about -0.2% at the "1" mark. The errors at the "2" through "9" marks are less that a pointer-width. DC voltage indications at full scale are +0.5% on all ranges and lower indications match with the linearity errors. The 3000v range reads about 2% low and I suspect the 40M R24 may have aged high. With fresh batteries, the ohm ranges all zero but the locations on all three pots where the wipers spent most of their time are a bit jumpy. The resistance indications at the major divisions are well within one-half of the smallest adjacent minor division. The AC linearity and full-scale checks are waiting for me to put the test rig back together. Having read comments in this and other Avometer-related threads, this remarkable performance seems to be the norm for this line of equipment. Amazing.

Does anyone have a template for the glass in the Avometer 8 Mark 5?

[pmmunro]

Another fine rescue Paul. Avometers seem to inspire this kind of attention; it's part of their adictive nature.

Pthomas,

Your latest acquisition shows how well a carefully used Avometer can perform after 60 years.

The marks on the back of the scaleplate are most likely to be the signatures of repairers so it probably means that each repair was done carefully rather than there having been no repairs. The Model 8 Mark I used an Alnico magnet which is deeper than the Alcomax types which superseded it.

PMM

[martiR2R]

Avo model 7 S/N 7747-36893
Poor condition, with metal case, silver/grey hammerite type finish, horseshoe magnet.
Pencilled dates inside, I guess from when it was repaired, also Farnell sticker from a repair.

Regards
Martin

[pmmunro]

Thanks for the information Martin. It's useful to have details of Model 7s from around 1947/8 to try to track the date when the design changed from horseshoe magnet to block magnet.

I'm surprised though that your meter has a silver hammered painted metal case which I would associate more with the mid 1950s onwards. It could possibly be a replacement for a broken Bakelite case.

PMM

[thomass]

Avo model 8 Mk.V S/N 07709 8V/2/74

Good condition, looks like it's been hardly used. I got it from our Science department during a recent clear out.

[pmmunro]

Thomass,

Use and cherish it and be thankful not everyone appreciates the fine qualities of these meters.

Thanks for the information.

PMM

[avocollector]

Been away for a long time but back now - fantastic to see how the thread has grown since 2010 (p30 was I think my last visit). Have now got a dedicated 3 x 3m shed for avos - and a few other things. Store each avo in a click clack type box to keep dry. Side line is does anyone use those DC to DC convertors for supplying the 15 volt battery?

[ex 2 Base]

I have a DC to DC converter and works OK. Simple to fit, but I think they are now sought. after

[Prakash]

Hello,
I found 2 out of 3 AVO 8 MK6, one says d.o.m 3/85 , 1089 M90095, M0D 8V1, Part number: 5172-271.

The other one's just an Avometer 8, Part number:5170-211.

No other labels

Cheers

Prakash

[martiR2R]

Avo 7 S/N 7344 - 14156
Bought for spares 'cause it has a good case, Golborne this afternoon, £2!
Ooops nearly forgot - horseshoe magnet.
Regards
Martin

[Phil G4SPZ]

That historic instrument might be considered too good to be used "for spares"! I managed to restore a Model 7 of similar vintage which was in basket case condition, by scavenging parts from later, less-desirable meters. But well done on rescuing it, the £2 price tag was a bargain!

[pmmunro]

I share Phil's view that a Model 7 of this age is worthy of a better fate than to be just a parts donor. It's unfortunate that Bakelite back- cases are easily damaged and virtually uncontainable but apart from that there are few parts which would be useful as replacements in more modern Model 7s.

However, if you mean a leather carrying case, the old Model 7 may be content not to do too much travelling in retirement.

Thanks for the information.

PMM

[martiR2R]

Fear not, I don't intend "stripping" it just swopping it out of it's nice undamaged bakelite case and one day I'll get round to looking at whats' wrong with it.
Sadly it didn't come in a leather case, I'm still looking for one of those.
Martin

[pthomas]

I now have an Avometer 7 MK II. I wasn't really looking for another Avometer having restored and sold on quite a few over the last year of the model 8 type. This was in a local antiques emporium looking a bit lost. I managed to get it for £20 - not bad considering it looked like new with the battery compartment completely free of corrosion. On test I noticed the movement not returning to zero without a tap so I cleaned up the little levers under the tiny plate to solve this. Also meter read low but I soon found a contact under the Q button needed more spring pressure. Cut out needed adjustment also. The No is 83298-A-160. The back plate (almost completely legible like new) says Avo Limited Dover and also part of the Thorn group. The casing is of light metal alloy grey in colour (aluminium?). The internal parts have may wire wound components etc but the movement and trip look very much like those of the Model * Mark II. The movement has red magnet blocks. It has a converter to allow the standard U2 battery size. I am not sure what to do with this one yet since I already have 2 x Model 8 III, 1 x Model 9 II and 1 x Model 8 I all in A1 condition!

[pthomas]

I tested my new Avo 7 this afternoon against my fluke on both AC and DC voltage and some of the current ranges. It agrees to better than 1% compared to my Fluke 87 V on all ranges. Ohms is OK also but harder to resolve plus I only have the 1,5 v battery for the high range.
Do I infer from the serial number that this is 1960? It seems newer and says made in Dover. Why did they make this when they already had the Avometer 8?

[pmmunro]

pthomas,

Congratulations on you new Model 7 acquisition. It's very reassuring when they test within the original specification as yours does.

There is an anomaly over the instruction plate and the serial number however. Your useful description of the internal details is correct for a meter made in January 1960, but the move to Dover did not take place until 1964 after which there was simultaneous production in London and the new factory for a few years.

It would look like your meter has been given a replacement case or perhaps just the wrong one. Model 7s originally used aluminium cases but during the Second World War this was a premium material and this may be the reason phenolic resin moulded cases were introduced the 1940s. Later both types were used but most of the final production seems to have been with moulded cases. Moulded cases did not stand the abuse that pressed aluminium ones would. (Strangely, the Heavy Duty Avometer orginally lived up to its name by having a cast aluminium case, later replaced by a pressed alumium one but latterly a moulded case which is seldom found intact now. The explanation may be that the original was designed specifically at the request of the Great Western Railway, for whom technical merit was always paramount. Later models were supplied to British Railways where first cost obsessed economics may have been a higher priority).

The Model 7 established itself as a standard meter for radio work from 1936 onwards and was specified on many service sheets and service manuals. It's often stated that it was a "power engineer's" meter but while it may have come to be regarded as such in later years, the Model 40 would more closely fitted that role.

Even the Model 40 wasn't designed specifically for such use. It was intended as a general purpose successor to the 36-range Universal Avometer (Universal in the then accepted sense of having AC & DC ranges - the universality of use was secondary, though probably implied to encourage sales).

The merit of a high "ohms per volt" figure was not given the importance it later gained until meters were needed to suit television and radar circuitry. Against this, a passive circuitry multimeter design has to trade off high sensitivity on voltage ranges against a high resistance, and hence significant voltage drop, on current ranges. In low impedance circuits, low resistance on current ranges may be the prior consideration.

Until the Dover factory opened, demand for Avometers always exceed supply and there were long delays in delivering orders. It follows that the manufacturer was supplying what customers wanted at that time so there was good reason to continue making all the models which sold. Despite the lively home market, the company put considerable effort into export sales and designed models specifically for overseas customers, some of which do not appear to have been offered on the home market.

PMM

[Phil G4SPZ]

Originally Posted by pthomas

Why did they make [the Model 7] when they already had the Avometer 8?

As Peter has said, the Model 7 was cited in the vast majority of service sheets for radio sets over a considerable period of time up till the end of the valve era. Service departments dealing with valved equipment would have still found the Model 7 useful if not essential; it also had a capacitance range, a power range and the variable-sensitivity feature offered by the Q-knob, all of which were missing from the Model 8.

My guess is that the Model 8 came into its own for transistor circuitry, whilst the Model 7 remained favourite for valve circuits. Out of all the Avometers, my humble opinion is that the Model 7 Mk II was probably the easiest to use and most versatile of them all. The switches are far easier to turn than those fitted to the Model 8, especially with greasy or gloved hands.

Do try to get hold of a couple of 4.5 volt 1289 flashlight batteries (still readily available) which will get the 1 Megohm range working on your Model 7. Whilst an internal 1 Megohm range was probably a bit restrictive even then, remember that the service engineer of the late 1930s to the 1950s would have had easy access to 90-volt dry batteries, which could be used with the Model 7 to create a 10 Megohm range, which would have been adequate for most purposes.

I'm confident that, at some future date, Peter will publish much of his research and we'll be able to see how the quantities manufactured of the various models changed over time. Oh, and by the way, it is impossible to have too many Avometers...