AVO Multimeter survey

[mick]

Hi,
Just got a model 48a in box with shunts etc,Ser No12473 943,has the brass plate on lid of box with details but no serial No on or in the meter that i can see,just got to get it zeroed in as it will not do it on the face,am i right in assuming it is the rectangular magnet? with a slot above the movement?

[pmmunro]

Your description does not quite match a 48A. These were all made before 1948 as far as I know and at that time all full-size Avometers had horseshoe magnet movements.

What you describe sounds like a later movement with a block magnet from a Model 40. If that's the case it would explain the lack of a serial number. It could well be a legitimate repair but Avo or one of their official agents would usually transfer the serial number to the new scale plate.

If your movement has a block magnet, the slotted item on top of the magnet assembly is a magnetic shunt used to set the basic sensitivity of the movement.

Failure to zero may be caused by:

Movement out of balance - the pointer position should be within 1% of the set value whether the movement is horizontal or vertical in any palne;

Tangled hairsprings - with very careful use of a needle or similar these can be untangled by tracing gently from the centre outwards;

Sticking movement due to dirt, contamination, distortion of the pivots or damaged bearing jewels;

Looseness of one or both of the bearing locknuts.

Please see other threads on Avometer faultfinding and repair for more detail and some repairers who may be able to help.

Admiralty Pattern Avometers, 47A and 48A, are not uncommon on eBay and not usually very expensive but please try to find a good use for any parts, or someone who can use them as no new parts are available.

Peter M. Munro

[FRANK.C]

Hi
I have got an AVO model 7 . It has a serial no on the scale plate which is 21862-A-150.
I got it in 1988. If I remember correctly it was at an auction in Mayo where a TV shop was closing down. The battery compartment cover was missing, but I made one for it and there was a problem with the "p" and "q" pots which was easily remedied. It has been in use ever sense.

Regards
Frank

[mick]

Peter,
I had a closer look at the meter and you are correct that it is a Model 40 movement although the case has the 48a logo on it,should have researched a little bit more before getting it,will get a model 8 and dispose of this one,thanks for the info,Mick.

[Brased]

With the museum now closed, except for the obligatory 'Santa Days' during December , we have started the 'spring clean'. This resulted in the recovery of a very dusty Avo7 serial no. 71047-A40969 to add to the list.

Edward

[Jimbo]

I found a Model 40 Avo at a boot fair and brought it home [for the princely sum of a quid] because it seemed an undignified end for it to be laying in the mud!

Serial number is 408952 - 1045 and the bottom left hand end of the scale plate also has the lettering 'AVM 314'. The case is complete if rather dirty and the white lettering in the engraved switch-knobs has all but disappeared.

Someone had soldered in a 'C' cell for the lo-ohms range but it is absolutely flat and the twin 4.5v hi-ohms batteries are missing. So far all I have managed to prove to be working are the DC volts ranges up to 1.2v, but there's no sign of life above that.

I've followed up some of pmmunro's and SPCh's threads on the subject of Avos but there doesn't seem to be as much noted about the Model 40 as some of the others.

Are these models worthwhile resurrecting, and if so can you point me in the direction of a good source of repair information?

Alternatively if someone is looking for a 'donor' set to cannibalise for parts I'd be happy to pass it on to a good home.

I'd be interested to learn the reason for the Voltage and Current ranges having top values as multiples of 12 rather than the normal 10 and would be grateful if someone could clarify that particular bit of history.

Thank you

Jim

[SPCh]

Thanks for the inputs.

The reason the Model 40 doesn't get so much coverage is that it's not so popular in our world as the later Models 7 and 8. It was introduced at the start of WWII, and although old doesn't mean bad, it was already too insensitive for the "new" electronics.

All AVOs have to be seen in context - and that includes their range sequences. In the very early days they were targeted at telephony and telegraphy, a world of electromagnetic relays,operating with battery voltages. The only significant AC was the mains (though not everywhere) and ringing voltages and currents.

With the advent of "real" electronics, greater sensitivity was needed (though not yet achievable) and higher and lower voltage and current ranges.

The "cleverest" AVO (in my opinion) was the Model 7 introduced in August 1936. It had a sensitive (for then) movement of just 1 mA, a range sequence based on 1/10/100 (for the first time ever), and switched ranges for power - in watts and decibels - and capacitance. The latter were clever applications of the "Q-knob" (which offered variable sensitivity on both AC and DC). There was also a comprehensive range of accessories to extend its capability and measuring applications. In the hands of someone who could drive it (and in those days people thought about what they were measuring and how) the Model 7 was, and still is, a very useful instrument. The targeted applications were very clearly valve radios and amplifiers, and the power reference values (=0dB) were arbitrary (200mW in 500, 5k and 50k ohms); the days of 1mW in 600 ohms were still to come.

The Model 8 was the first (volume production) high sensitivity (50uA) AVO; it was introduced in 1951 and targeted at the "modern" radio and television repair world. The ranges were based on a 1/2.5/10 sequence, which gave wider resolution by introducing intermediate scales between 1 & 10, and between 10 & 100, but this made the decibel step between adjacent ranges messy at +8/+12/+8/+12...

The Model 8 Mk V, with an even more sensitive movement, used the range sequence 1/3/10 which was rather neater for the "intermediate" ranges, and would have cleaned up the decibel steps to a tidy +10/+10/+10, except that the decibel scale was inexplicably deleted ! (I suspect that by the 1970s few radio and television fixers understood decibels, whilst the hi-fi freaks thought they were about window-rattling volume, or used VUs).

To complete the story, there was a Heavy Duty AVO introduced for the railway signalling market, and the Model 12 for the automotive industry (so, 6 12 and 24 volt applications covered by a 9/18/36 range sequence).

I think I may have more than answered the question; hope it helps/is of interest.

[paulsherwin]

A few months ago I was kindly given what looks like a late model 7 by a forum member. I can't find an obvious serial number The rear of the meter movement is marked with what look like calibration dates (3/8/77 and 14/10/82) and there are 2 numbers on sticky labels, 814578 and 022369. The left of the scale has 'PART No 3284-144' and the right has '69847-7475'. There are 2 Exide 4.5V batteries fitted dated best before 1990, which I guess is some indication of when the meter went out of use. The 1.5V battery which is supposed to be in there seems to be missing - it looks as if it may have been a D cell. I have no idea of its history.

Paul

[plumbweiss]

In daily use, earning my living, model 7 number 5XX8-A-648

Black crackle finish, original sticker on the side saying Farnell of Wetherby .

[plumbweiss]

I repair AVo 40's, 7's and 8's of all ages for a local calibration firm who usually send them in because they're out of 2006 NAMAS requirements for 16th edition instruments. Yes, they're still used by electricians, power supply authorities, schools and colleges all over the world.

The worst for repairability is the 70's version with the blue flexi pcb, these go open circuit or a minor overload can frazzle them. The cutouts go over-sensitive, the dust guards go brittle and jam swarf inside the movements. The hairsprings get buckled. Later versions have trimmed shunts and multipliers and these overload easily, germanium diodes go leaky, ac voltage transformers go open circuit. Switch contact arms fracture. The ohm zero pots go very noisy and are a pig to replace. Movement coils go open circuit. Later 8's had an optional DC-DC converter to produce 15V from the 1.5V battery, it fits where the 15V battery should go.

The older wirewound units suffer frazzled internal resistances, dry joints on the current shunts, open circuit oxide rectifiers, poor accuracy and weak magnets, fallen-in glass, bad tolerance resistance wire, broken trip springs/lost bearing jewels, dirty rheostats, poor battery spring contacts etc etc.

Not that any of them are unreliable, I've just repaired them as part of my living for years...as many as 10 a week for the last 18 years. We have all the diagrams for all the 40/7/8's .

Megger Limited (formerly AVO) still manufacture the model 8 but only to order, they are handbuilt and a snip at over 600 quid a piece.

[pmmunro]

Plumbweiss,

Your list of defects is very intersting and reflects my own, considerable more restricted, experience of Avometers repairs.

(I suspect however that this discussion is off-topic for the survey and the moderators will want to move it to a thread on repairs).

When you mention lost jewels, are you referring to the pallet stone on the cut-out latching pawl? (I haven't seen a movement pivot jewel missing). If so, what method(s) have you found for refixing the pallet stones?

Would you agree that, while all these defects are common, and many could or should have been corrected in production, the greatest cause of defects is abuse, either in use or due to inappropriate storage?

It's also worth remembering how many Avometers were made and how long many have been in service.

I have found that the pre Model 8 Mk V cut-outs often stick due to dirt on the reset button shaft and other parts, again probably due to poor storage conditions.

For pre-war meters (pre-1939), I would add open circuit moving coils due to deterioration of hairspring solder and corrosion around the hairspring collets, both exacerbated, if not caused, by damp.

(I sent you a private mesage).

Peter M. Munro

[SPCh]

Paul,
Your gifted Model 7 has already been entered in the survey by its previous owner. In fact it's a bit of a problem item, because the serial number, 69847-7475, doesn't sound right at all. The date code would imply April 1975, which is far too late for a number in a format which was changed in 1947. It's possible that the number was written with digit reversal - I've seen odd instances of that - but even at May 1947 there's still a problem: in July 1947 the sequence numbers were still "only" in the 36,000s. The sequence number alone would imply late 1956 but, of course, the format would then be nine years adrift.

Clues on the outside are the terminal style (black knurled screws or red and black socketed-screws), and whether there are "P.F." sockets at the top corners. I would never recommend opening a meter out of curiousity, but if you ever have to repair it, I'd be interested to know what type of magnet it has (black horseshoe or a possibly coloured block type), the colour of the plastic sleeving, and what sort of resistors predominate. There may also be a production date on the large 1 microfarad paper capacitor.

This enigma is part of what the survey is about !!!

[paulsherwin]

Originally Posted by SPCh

Clues on the outside are the terminal style (black knurled screws or red and black socketed-screws), and whether there are "P.F." sockets at the top corners.

It has red and black socketed terminals and PF sockets. The whole thing feels very 'late', and even 1956 seems early, though it may have had some internal components replaced of course. The metal instructions panel on the back also has a 60s feel, though I suppose 1956 is possible. I'd be astonished if it dated from the 40s.

http://www.psherwin.demon.co.uk/avo7back.png

Paul

[SPCh]

Thanks Paul,

The "DOVER" on the back panel (assuming it's not a "marriage") would put it post 1962 /1964, which would also be right with the coloured socket terminals. That still leaves the serial number a mystery, though. Never mind, if I didn't have these things to puzzle over, I'd have to invent something else.

[hilitevr]

I recently aquired a DC AVOminor The serial number is 42322-B6
Still has what appear to be original AVO seals covering bottom screws.

Cheers
Trevor

[plumbweiss]

Yes, don't forget our customers are Electrical Supply Contracting companies whose avos rattle around in the back of ladrovers, go up ladders and into substations! Definitely a hard life and frequent fryings !

The jewel on the cutout can be taken off a scrap one, we have loads, I've not heard it called a pallet-stone, we're not that fancy up north.

I'd forgotten about the open-circuit windings, I agree, you find a strange furry, slightly green-white deposit on the movement/hairspring arms when they get damp. Sometimes they get dropped into wells, sumps or chemical vats and this tends to damage them.

Good machines and I prefer the old 7 to the 8 because the needle moves faster when you want to fix a radio in a hurry.

[Brian4radio]

Hi Spch, I have taken my AVO apart but can only find one thing that looks like a serial number and that appears on the meter display itself. So the meter is a Model 8 MK 111 and the serial No is 6xxxx-B67.
I also have another AVO meter which is an ELECTRONIC AVOMETER Model No EA 113 and the serial No is 5xxx.
I do use the Model 8 for alignment but prefere to use my Fluke for most other measurements. Regards Brian.

[dave_n_t]

SPCh,

My Avo 8 Mk. V has
Serial No. 94556 8V/1/76

I haven't omitted any potentially guilty characters, because I don't know its history (it was given to me by a relative some time ago).

dave

[Denis G4DWC]

Herewith some numbers from my stash....

AVO 8 5XXXX-C-771
AVO 8 Mk II 1XXXXX-C-763
AVO 8 Mk IV 4XXXX-84571

AVO Minor 40166 Looks fairly recent. Horrible Beige panel with grey plastic back cover. Still.... it was only 50p at a rally and quite OK to keep in the old car.

I also have an AVO 7 which I can't find at the moment and a Mk 6 somewhere.... will report back when they come to light....

Denis

[tis your junk]

Some more numbers.

Avo 8 S/N 5xxxx-c-257
Avo 7 S/N 2xxxx-a-251
Avo Minor HR S/N HR144-98



Thanks

TYJ