AVO Multimeter survey

[gingpeakin]

I've got 3 Mk 8s

no.64577-c-857 BBC no.821
no.73506-c-458 BBC no.850
Mk V 22924 8v/10/74 that my uncle bought new in the 1970s. He was instantly dissapointed in the build quality. I agree - it's really nasty compared to the others.

Ging

[LCTorana]

Greetings from Australia!
I've been well and truly bitten by the bug:

Model 7: 4457-A-348
(ex-CSIRO)

Model 8 Mark 5?: 04206 8V/1/74
(on the back instruction panel it says "Meter Multirange 14C/2 AVO/73/6")

Test Set MultiRange #1: 1217-167
(ex-RAAF. Very heavy, with a bakelite case and a second metal shell around that.)

Model 8 Mark 6, New In Box: 1465 M90285 2/86
(destined for the Army, but never go there. Sold to a surplus dealer where I found it online.)

And there's a Model 73 lurking somewhere. Can't lay my hand in it for the serial number.

My Model 7 in particular sees constant use. I know of no other meter, old or young, digital or analogue, from any maker, that comes even close to the low 0.1V burden voltage on the current ranges.

Using an Avometer is a joy that often surpasses even the pleasure of the job that requires you to use the meter in the first place.

[SPCh]

Welcome to the Forum, thanks for your contribution, and congratulations on your enthusiasm.

[pmmunro]

LC,

Your Model 8 Mk V would seem to be a British Post Office (Telephones) version as they designated these meters Type 14C. A picture would help confirm this.

It's good to know someone else appreciates the quality of the instrument and the care that went into the design of the Model 7. Amongst the papers held by the Science Museum Library (London) is an article on the Model 7 to which Donald MacAdie has proudly added a manuscript
note telling himself that this meter was designed by his son Hugh S MacAdie.

PMM.

[LCTorana]

Photos as requested.

[pmmunro]

Yes that is definitely a Model 8 Mk V.

As far as I know, there is no difference other than labeling between the Post Office 14C and any other Model 8 Mark V. As this meter was designed to a NATO specification and to cover all markets for which the earlier Model 8 and 9 meters were intended, including panclimatic versions, no changes should have been necessary.

If anyone knows of any electrical differences between the 14C and the simple Model 8 Mark V, please let us know.

PMM.

[calum]

hi all,

few new additions here; any comments on vintage, etc, much appreciated.

cheers,
calum.


AVO 7 Mk II No 10 009-A-1069-19
Patt 3284-145

AVO 8 Mk V No 001379 CA 8V (label on side)
"reverse MC" works only one way.

AVO 8 No 59804-C-457
Patt No 13574-2

AVO MOdel 40 No 5392-B-451
Patt No 10945-32

Avo 8 No 115073-1160
Patt No 13574-3.A

[russell_w_b]

AVO 8 Mk V No 001379 CA 8V (label on side)
"reverse MC" works only one way.

I had similar trouble with an AVO 8 V reverse M/C, and it turned out that the push-buttons for the cut-out and reverse M/C had been inadvertently exchanged (they are different)! I sorted it out, though - see here.

[n_r_muir]

Hi,

Are the serial numbers of some early AVOMINOR's of interest?

These are the type that has no switch and the range selection is by moving the test lead plugs into the appropriate socket. These have " Douglas Street London" marked on the back.

68061-6422

49632- 19

9121-24

They were supplied in nice wooden cases.

Also a Multiminor Mk 5 in a leather case; this has number 37142 stamped into the meter faceplate.

And one marked "Multiminor" this has no serial number visible.

N R Muir

[SPCh]

Calum,

With the exception of the Model 8 Mark V, yours are easy to date.
In sequence: October 1969, ?, April 1957, April 1951, and November 1960.

The Model 8 Mark Vs are difficult because the numbering system lacks a recognisable consistency. The labels often had a date on, but not always.

NRM: your AvoMinors are also problematical: when they DID have dates embedded in the serial numbers it was usually in the form of (m)myy, but they didn't always, and none of yours looks credible within that system. I don't have enough instances to even guess at dates on the basis of sequence number alone, but they are almost certainly pre-WWII.

[LCTorana]

Found my Model 73:

It has:

5171-998
char.2

on the dial


...and it's dead. Any chance of a circuit diagram, perchance?

[hilitevr]

Not sure if your interested in this model.

I've just aquired an EA113 serial No. 7067 in perfect working order.

Trevor

[calum]

thanks much, Russell W. B. & SPCh

[Steve Browne]

Mine is a model eight Mk. 6, bearing a RS Components Limited label (stock number 611-177 beneath the dial). Separate labels on the side: 3/84, QAL, No. 0001735CE 8 VI. I'm the second owner. It came complete with original leads, probes, operating instructions and leather case. Recent internet purchase. First decent analogue meter I've owned, a real pleasure to use.

[AndrewM]

I have an AVO Model 9 Mk II No. 6720-966.

Sadly although cosmetically in good condition, it is not functional. I suspect the magnets are weak because electrically it tests fine but it will only read around 10% of the true value.

[pmmunro]

Andrew,

If the fault with your meter is caused by weak magnets, I would be quite surprised and I would like to know the cause. It is possible that someone has tried to take the magnet assembly apart but other than that these magnets do not usually loose magnetism, and certainly not to this extent. (It is also possible to have them remagnetised).

I would think it is much more likely that there is a circuit fault. Depending on your skills it should not be too difficult to find out.

These instruments need to be handled with care, but given that, they are not especially difficult to work on. If you have a clean work area, it would be worth checking the basic movement sensitivity which should be 37.5 microamps for full scale deflection.

This can be checked by disconnecting the movement from the meter circuits and temporarily reconnecting in series with a known good meter, a variable power supply and a limiting resistor which will not allow more than around 50 microamps to flow with the power supply at maximum voltage.

If the movement reads correctly, look for a fault in the universal shunt chain or the cam operated switches.

PMM

[M0XNA Neil]

I have just acquired an AVO 8 Mk 3 (serial No: 91302-169). The AC voltage range is reading low (i.e. reading 10V when the input is a 25V Sine wave). On inspection, I have noticed that the bridge rectifier comprises two OA95 type diodes and the opposite diodes replaced by two 406 Ohm resistors. Is this bridge configuration correct for a mark 3 or has it been 'modified'?

thanks,

Neil.

[SPCh]

Yes, that's the correct configuration.
(Later Marks use the same configuration, but with different resistor values).

[AndrewM]

Originally Posted by pmmunro

If the fault with your meter is caused by weak magnets, I would be quite surprised and I would like to know the cause. It is possible that someone has tried to take the magnet assembly apart but other than that these magnets do not usually loose magnetism, and certainly not to this extent. (It is also possible to have them remagnetised).

To make sure I tested the movement correctly, I redid the tests again today.
With the meter movement disconnected from the rest of the circuit, 50uA current produces approx 1/3 scale deflection. The resistance of the movement coil is 2270 ohms. The pointer is also very bouncy and shows little signs of damping.

The magnets have been disturbed in the past. I found this meter on a rubbish pile during council cleanup day over 20 years ago and as I was younger (and less knowledgeable) I came to the same conclusion back then and removed the magnets for inspection. The meter was faulty when found and my disturbing the magnets made no difference to the low reading problem. The meter has been sitting under the bench all these years with little hope of ever getting it going.

I would be keen to attempt repairing the meter if someone can talk me though whats required. Should I start a new thread on this topic?

[Darren-UK]

Originally Posted by AndrewM

I would be keen to attempt repairing the meter if someone can talk me though whats required. Should I start a new thread on this topic?

Yes please, start a new thread. This particular thread is for a survey, not repairs.