AVO Multimeter survey

[Phil G4SPZ]

Have you got a circuit diagram for that 1939 meter, Dave? The 1943 Model 7 circuit that I've seen definitely has the 200k resistor linking the 1.5 volt battery to the movement via leaf switch contacts 'e' and 'f' on AC ranges only. I was just interested to know whether yours has no such resistor fitted, or perhaps it's gone o/c sometime over the past 79 years!

[The Philpott]

Phil, I see the resistor you mention in the circuit diagram, my (pdf) instruction book is quite early, but the circuit diagram does specify that it relates to-

'Model 7 (with letter 'A' in serial number)'

I have opened the meter up and see that coming from contact 'e' there is only one wire, which goes straight to + on the rectifier. Consulting my notes this whole switch stack from 'a' to 'f' was changed for a later one, as the original switches were worn beyond adjustment.

My scruffy notes from this indicate that the (later) switch stack had the stubs of two wires coming from 'e', so this, i think, indicates that the 1939 meter lacks this rectifier biasing circuitry.

I wanted to take the meter out of it's box anyway as the Q bodge i detailed on pages 72/73 of the survey has suffered from a loosening nut, stopping the wiper from rotating.

Medium strength Loctite and possibly a star washer needed!

[Phil G4SPZ]

Very interesting, Dave. It seems that you have pinpointed the date when rectifier biasing was introduced, to somewhere between 1939 and 1943.

Does the "add half a division..." instruction date from 1939 or earlier, or was it continued post-1943? The quest for improved AC accuracy was probably driven by the radio trade.

[The Philpott]

Indeed, i think it was Pete who commented that the Mod 7 went through a lot of detail changes over the years, none of which was represented by a change in nomenclature.
I think that even the evolution from Mk I to Mk II was muddied by the early option of PF terminals, and retention of the old style main terminals for a short time.

Not sure when 'add half a notch to press-button AC ranges' got phased out- it certainly applied to the Models 7/40/47/48 in the mid 'forties but i don't have a meter from the 'fifties to compare.

[Pmark34]

Hi latest in the collection a Heavy Duty AVO BR (W) No 568-748 so July 48.I think I read on here somewhere that the W stands for Western region, after the change from GWR. I noticed that the blue battery connection is on a plug and socket arrangement, would this be original? The meter seems to work nicely on the settings checked so far. Has a nice set of Avo leads but they have screw in ends not push in, not seen that before although the Croc clips look fairly new. The calibration sticker on the back has a date of 10.4.86 penned on it.

[Phil G4SPZ]

Yes, BR(W) was the designation for the Western Region of the then very newly-created British Railways. That's a nice instrument, I'd only ever seen them with a Bakelite front panel before.

I think the combination screw-down and 4mm terminals came in with the later MkII versions of the Model 7 and 40 in the early 1950s, but I stand to be corrected.

[The Philpott]

I think you're right on the terminals for the 7 & 40 Phil.

The terminals on the Heavy Duty Avo pictured look identical to the type that is seen on certain voltage multipliers and low resistance range extenders designed for the Model 7 and 8. The only other meter i have seen them on is the High Resistance Avo from the late 'forties/early 'fifties (effectively a 4 terminal prototype of the Model 8, but without a transformer)

These terminals are rather spindly and fragile when extended, it is comparatively easy to ding them and bend them. This would probably be why they were superceded by the more versatile (and stronger) red and black terminals. One suspects they had a lot of unused stock of the 'spindlys' and this could be why they turn up on avo accessories.

Dave

[The Philpott]

1. Terminals from left to right, intermediate, early and late.

The early ones would benefit from the Bakelite being a larger diameter, allowing more torque.

2. Previously sheared Q assy. on Model 7 with studding sharpened and araldited into rear, nut loctited in place and araldite blob across nut and spike. Star washer used. The low and medium resistance ranges will not work unless the spike makes contact with the leaf when Q knob is home. Q wiper must be set at correct orientation on reassembly and must not turn independently of the shaft (unlike P & R which can usually slip without consequence)
This repair works so far but is not recommended..

[Phil G4SPZ]

A neat fix, Dave.

The terminal screws on the earlier 36-range Avometer, the predecessor of the Model 7, have an even smaller diameter thread.

[Sinewave]

Late model 8 mark 7s have gone for serious money on that auction website recently.

[The Philpott]

Indeed, some odd asking prices are coming up on the interweb. If you want to know how much something eventually goes for, add it to the appropriate 'watch' list. I am currently watching a busted Model 7 which is up for >500GBP, with great amusement. One assumes it's a mistake with the decimal point.

[Sinewave]

I do love a 'spares or repair' going for more money than I've ever paid for a lovely working one.

They never sell, yet are constantly resisted at the same eye watering price, whilst others are being sold for more reasonable money.

[The Philpott]

Type E Air Ministry Test Meter: E-21060-1945

In exceptional cosmetic condition, hardly appears to have been used. Even the switch is positive; they are usually quite mushy.
Usual age related glitches: loose glass, o/c bobbins, dust and varnish flakes inside and -just for a change- the swamp card was o/c.

Particularly of interest is the E type fuse cartridge, which has the black seal intact, but measures about 13 ohms compared to between 11 and 11.5 ohm which is what i was expecting (and had documented here previously)

Conveniently the (o/c) resistance range bobbin is supposed to be 470 ohm or thereabouts, so a piggy-back resistor of common value was used.

[MelMeter]

Universal Test Set Multi Range No 1 High Sensitivity 6625-99-105-7049
Multiminor Model 1 027428
Both in excellent cases, the Universal belonged to REME last calibrated 1984.
Both with original test leads and manual.
Perfect working condition.

[Sinewave]

Today I've just given a new home to my first Post Office MMR14C Avo 8 Mk5.

Readings are nice and accurate. I ought to input my data onto the survey.

I'd been after one of these Post Office ones, for collecting purposes, no other reason really, as it's the same as my other mark 5's

[Sinewave]

I wonder if it's of interest, to have another entry in the data base for the date of the AVO? I know this is in some of the serial numbers, but the later models it is separate from the serial, though it's on the same sticker.

[Pamphonica]

Good idea, I have added a year field (optional). We now have over 60 entries in the database.
(See first post in this thread for links to the database)

[The Philpott]

This is not a new entry, but I cleaned the switches of the Low Resistance Ohmmeter 370-441 recently and this is a view which i hadn't circulated before. The battery box is removed, which reveals:

The Low Range switch has two pairs of contacts and the High Range has one pair. The circular marks on the inside of the case betray where mould inserts were used to plug up the terminal locations used on the standard DC Avominor.

The weak point is the dimpled bronze spring leaf sitting on the zeroing pot, it must be shiny underneath (then tiny dob of vaseline for future proofing) to make the meter work properly.

[bill knox]

Hi

I have just bought an AVO Heavy Duty meter in absolutely cracking condition complete with a brilliant leather carrying also with what looks like an almost new let of genuine AVO leads, prods and croc clips.
All I had to do was change the battery and away it went.

I have it on the AC and DC ranges and also the ohms ranges and it is OK.

I have never seen one of these before, it looks like a 3/4 size AVO 8 but with only 1 selection knob, it's serial number is- 8272-971 which I believe means that it was made in September 1971.

I have no idea what it's worth but if someone could give me an idea.

Regards

Bill

[The Philpott]

Could we have piccies?

I doubt it is rare but potentially scarce in good condition, as they get dropped and chipped quite a lot.

As regards the value it is safe to say you are under your household insurance' single item limit in any case!

Dave