AVO Multimeter survey

[Phil G4SPZ]

194-C151 must be one of the earliest Model 8s. Nice!

[pmmunro]

Richard, Phil,

I would agree that 194-151 must be a very early serial number - without a detailed search I would think it is the earliest we have heard about.

The serial number dates it to January 1951 - four months before the official release for sale. This is the second instrument to be reported here with a serial number predating May 1951 so it is possible that production began earlier to build up stocks.

As I have mentioned before May 1951 coincided with the Festival of Britain which could be linked to marketing strategy.


PMM

[Phil G4SPZ]

I vaguely recall that an early Model 8 passed through my workshop a while back, and it had plain black Bakelite terminals like the Model 7 Mk 1. Am I correct, or is my memory playing me up again? Richard, I wonder if you know what terminals are fitted to Roger's Model 8?

[Barnmead]

I have asked him to let me know and have sent him this picture of my HRM and 8Mk1 terminals.

[HamishBoxer]

My AVO 8 is a MK4 but no serial number in or out of the case.

Also I have a MultiMinor which my father got me new in 1962 still in its leather case with instructions inside.Were they just over £9 in those days?

[Barnmead]

Here are two pictures of the 8Mk1 194-151. As can be seen it has terminals like my 8 Mk1 and Model 7 Mk1.

The second interestingly shows G&H in the RH corner of the scale plate. Any guesses?

[Phil G4SPZ]

I have a 1955 Model 8 and that also has "G&H" in the same place. No idea what it stands for, though

[Phil G4SPZ]

I've been asked to look at a Model 40 for a colleague at the museum. Its serial number is 4861-641 which I assume dates it to June 1941. It has the horseshoe magnet. Picture of innards attached.

Two multipliers are o/c as well as one or more of the resistance range components, and the movement is sticking. The instrument is filthy and looks like it has been dragged around the floor on its front! The 4.5 volt battery contacts are missing completely, as are the main terminal screws. The poor thing has clearly had a hard life.

I suspect that, when I tell him how long it will take to restore, the owner will lose interest

Phil

[pthomas]

I also have an early Avometer 8 Mark II that has G&H on the dial plate.

[Phil G4SPZ]

Avo MultiMinor Mk 5, serial number 84592. Remarkably, this is the first MultiMinor to enter the collection. It was kindly donated by an ex-colleague. The only fault was an open-circuit 2.25 ohm 1% resistor, which I replaced with a 2.2 ohm 5% component. All ranges now appear to work OK.

Can anyone please advise me how to date this instrument from the serial number?

[Audiophile_AU]

Hello all,
I have just acquired another AVO, an 8 Mk III, serial 76804 368. Clearly built in March 1968.
It came from Chelmsford via Ebay, so is now 'lost' to the UK - sorry :-(
The low voltage DC and ohms ranges work - I have yet to check the remainder. It is moderately grubby - but hopefully will clean up like my Model 9 IV.
It is missing some of the screws holding the case together - does anyone know what the thread size and type is? I assume 6 BA countersunk head, as it is 2.7mm diameter by 3/8"(10.1mm) long.
Anyway, as someone said in a very early thread, it is not possible to have only one AVO...
regards from Down Under
David

[pmmunro]

David,

Thanks for the information. Don't worry too much about this latter day export; as you'll see from eBay there are still plenty about.

You're right about the screws. All threaded fasteners in your meter have BA threads and the screws are plated brass - nothing magnetic - to avoid spurious attraction to the movement magnet.

Most of the case screws are raised countersunk (instrument) screws which are even rarer than ordinary countersunk screws. There should be one ordinary countersunk screw originally covered by a a wax seal on the left side. Your best hope is a cache of old stock.

PMM

[Audiophile_AU]

Hi PMM,
thanks for the information. Even though I work among loudspeakers and compression drivers with strong stray magnetic fields, it didn't quite occur to me that external bits of ferrous stuff might affect the internals.
But than I am not normally worried about a <<1% change in linearity!

I will make very sure I purchase brass screw things!
I think one of the receptacles might have a stripped thread, which is another complication...
Many thanks
David

[pmmunro]

David,

Sorry if I misled you; I think the reason for the brass screws is to avoid stray screws/nuts/washers being attracted to the magnet, not the influence on linearity of steel screws fitted in place of brass ones.

PMM

[Goldie99]

Just to add one more to the list - 'Model 40 Universal', serial number 978-B-448

Seems to be working on most settings (not all).

Rgds,
Alan

[Phil G4SPZ]

I'm overhauling a Model 40 for a colleague, and bought a very scruffy but complete 40 as a spares donor at the NVCF for £5. Serial numbers are interesting; 4861-641 and 40493-442. Internally, they differ only in the large resistor between the upper and lower resistance boards, the single large blue resistor being on the earlier instrument.

Did Avo really make some 35,000 Model 40s in the ten months between June 1941 and April 1942? It seems unlikely...

Phil

[pmmunro]

Phil,

There seem to have been at least four serial number ranges for the Model 40, over the c40 years of production.

When Model 40 production started, serial numbers were in the format my-nnnn for the 36-range Universal and 7my-nnnn for the model 7. The new Model 40 seems to have changed that pattern to nnnn-m(m)yy.

By 1942/3 the pattern had changed to 40(n)nnn-m(m)yy, later rolling over to become 41nnnn-m(m)yy, presumably after 10000 meters had been allocated a serial number of this type. (The corresponding Model 7 range seems to have retained the original pattern of serial number until 1947/8)

Around 1947/8, the Model 40 serial numbers took the form nnnn-A or B-(m)myy.

Finally, in the early 1970s, the serial number moved to a self-adhesive label on the side and the serial number pattern was nnnn 40/(m)m/yy.

The earliest model I have is 2438-640, which is a year to the month before your 4861-641. This would suggest that 2423 Model 40 meters were made that year - roughly 200 a month. At the time the other full-size Avometers in production would have been the Model 7, the Air Ministry Type 'D' and the Admiralty Pattern 47A and 48A with all in considerable demand by the three services. If we assume that the Admiralty types shared production allocation, that makes 4 types, and if production was roughly equal for each type would suggest about 800 meters per month, 200 per week, 33 per day. This is probably a bit on the low side, but possibly one of the four types had priority.

The other of your pair, 40493-442 would compare with my 404941-543 giving 4448 meters in 13 months; 342 per month, which is a rate 50% up on the previous year.

ACWEECO had a shadow factory inn the early 1940s, but I know very little detail and it may have been smaller than the Vauxhall Bridge Road factory.

"That resistor" in the Model 40 seems to have given a great deal of trouble; the 120k/1200V multiplier. The corresponding multiplier in the preceding 36-range Universal Avometer was a large "pile" wound coil of very fine wire, several hundred metres long. It must have been expensive to make and the very fine wire made it vulnerable to damage.

Some Model 40s were fitted with a multiplier fitted with a glass cylinder wound with two unequal sections of extremely fine wire, possibly 52 swg, and this too was easily damaged - I don't think I've seen more than one which was intact. (These are marked "Patent NOS 658052 and 651070) Later, the glass cylinder was replaced by an SRBP board carrying four 39k high stability resistors with a small carbon resistor to make up the last 3 or 4 kilohms. This last pattern was retained, but with more modern resistors, to the end of production.

PMM

[Phil G4SPZ]

Peter,

Many thanks for clarifying the fine detail of Model 40 serial numbers. A most erudite treatment, as always. I think I have one Model 40 with the glass resistor somewhere in the collection.

How's the book coming along, by the way?!

Kind regards,

[pthomas]

My list of pristine meters. These are devoid of any components swaps or repair so as close to original as possible:

Avometer 8 Mark V
Nice clean condition but I have noticed that some of the high value internal blue resistors are well out of spec even though the have 0.3% ratings. This is my least accurate AVO in that AC mains reads 3V down for example compared to my Fluke 87 V. There no easy way of fine tuning either.

Avometer 8 Mark I
I will call it Mark I but is simply a very early model without the black and red type terminals. The magnet blocks are grey. It need no work at all and even the trip work perfectly. This had an EMI Laboratories Hayes Middlesex calibration tag. 41354-C-1255


Avometer 8 Mark II
Really nice 975588-C-1059 and accurate on all ranges and no internal repairs. Shunt is wire wound and no irritating fuses to get corroded. Dial slightly yellowed compared to others

Avometer 9SX
Very heavy metal clad and shielded panclimatic MOD/REME meter. In fabulous condition. The glass was loose when I got this so I fixed it back with some black sealant. Fortunately the movement was not damaged. I had to do a fair bit of work to clean and dismantle the trip system to get it to work. Also I rebalanced the movement a little so it zeros regardless of tilt. 5QP/17446 817-15-361 and broad arrow. Unlike later version mine does have fuses.

Avometer 7 Mark 2
83298-A-160 Again needed work to get the trip working plus wirewound potentiometers Q, R and P needed cleaning. This one includes the adaptor for the 1.5V battery to use cylindrical D type. Internally a very large number of wirewound resistors. This meter is very accurate after a small adjustment to the magnetic shunt. This Mark 7 is within a lightweight aluminium (?) case rather than Bakelite

Avometer 9 Mark 2
17693-469. Another nice meter. Mine is like new with fantastic clean dial. Lots of international marking. It has fuse protection for ohms range. Internally it is more like a II rather than III and doesn't use germanium diodes but instead has a square rectifier block on the top board. Its accurate but needed (again) work to clean and reset the trip

Avometer 8 Mark III
Two versions of this meter: 56847-467 where the bottom board has several blue high precision resistors fitted. 24789-965 where there a more wire wound resistors. Both in good condition with no batter corrosion. AC performance extends to 15KHz and above. Both have fuses for the resistance ranges.

[pmmunro]

Thanks for your information pthomas.

It's a pity your Model 8 Mk V is inaccurate. A good version of this model can be well within the specification. If the accuracy on direct current ranges is good, it should worth correcting the out of tolerance multiplier resistors.

PMM