AVO Multimeter survey

[The Philpott]

John, depending on what info you are after- user instructions and service manual for the 8 and the 8 Mk.2 freely available as a pdf doc. Most of the data also relates to the mk3.

[pmmunro]

John,

The last three figures of your meter's serial number relate to April 1964. The 'D' suffix is very likely to indicate that it was made at the Dover plant which opened at just about this time and ran in parallel with the Vauxhall Bridge Road plant in London for some time.

If you need a user or repair manual, let me know.

PMM

[ex 2 Base]

I have an AVO 8 Mk 1V serial number 1589 1V 9.69, so September 1969 and have recently noticed the 3 volt DC doesn't work, any ideas what could be amiss. 10 Volt range is OK, so not urgent as I have another meter. Ted

[Phil G4SPZ]

Originally Posted by ex 2 Base

...the 3 volt DC doesn't work, any ideas what could be amiss. 10 Volt range is OK...

Ted, according to the circuit diagram I have, the lowest DC range on the 8 Mk IV is 2.5 volts. Could your Model 8 actually be a Model 9? The Model 9 had ranges in the sequence 3, 10, 30, 100 etc whereas the Model 8 stuck to 2.5, 10, 25, 100 etc.

This fault is usually due to failure open-circuit of R28, 47.5 k ohms in the 8 Mk IV. You can replace it with a standard 47k resistor, selected on test on a DMM. I could let you have a suitable resistor if you need one, drop me a PM.

[The Philpott]

As regards the 100/30 scale of the Model 9 and 100/25 scale of the Mod 8, If i remember right certain military issue ones of that period had a different scale from that which you would expect..? Is it in a steel case?

[ex 2 Base]

The meter is a mark 9 and have sent you a PM Phil. The case is quality plastic type, not sure of the material but its not steel. Ted

[Phil G4SPZ]

Ted, the equivalent resistor to R28 that you will need for the Model 9 is 57.5 kilohms, which can be made up from a series combination of 56k plus 1.5k, as long as there is space to fit them. I've not yet worked on a Model 9 Mk IV so I can't advise on that.

It would be best to continue this discussion by PM; your PM has not yet arrived.

[The Philpott]

As data on ww2 period meters is patchy, here are mine for info. (general request from pmmunro)

E - 14410-1944 (parts donor)
E- 22381-1945

D- 18272-245
D- 14551- 544
D- 17428- 1244 (parts donor)

47A- 13374-1243
47AS- 17543-1144 'R'
48A- 8022-342

Now then; D- 17428-1244 differs from the other two D types insofar as the scale plate is the arrow/phaeon type. The scale plate is the later printed type, with a discrete date mark of 1955- the 195 is printed and the 5 is hand painted in. The movement and scale plate are clearly a replacement as there is a reconditioning mark of WOP/R/4/55 on the scale plate and a WOP stamp on the instruction plate on the back. The main casing mould is a MODEL 40 UNIVERSAL typeset on the front, which, again, is different from the other two units i have. It is also different insofar as it has an aluminium back-box. I have coaxed the movement into life however as a whole the unit is a parts donor as the rotary switches are obstructive as if they have carborundum/grit in them. It has other major problems inside.

I hope this will assist.

[The Philpott]

Universal Avominors-

78085-645 ^10S/46
94529-946
161-550 Model 2
112884-551
121316-752 ^10S/46
122268-952 BBC No.1138 (parts donor)
127497-1253
3312-155 Model 2

This might seem too many, but 4 of these have the noisy pot problem so the resistance range is either u/s or of use as a continuity tester only. The other 4 are in use fairly regularly, 2 of them in cars. I rely on them more than moderns simply because most of the possible failure modes are immediately obvious-somewhat unlike a DMM. I try to keep them boxed when not in use simply cos of the amazing amount of fluff that builds up in the sockets otherwise. The glass had been missing from 121316 for some time, grot inside and the needle properly bent,which makes it pleasing that i was able to resurrect it. One Model 2 metamorphosed into a Model 1 by the time it had arrived at my house, i don't believe it to have been deliberate and it was cheap so i let it go- and i needed a 2.5mA movement anyway.

The Model 2's seem surprisingly robust, one has a balance issue with it's needle, but unless it starts to talk nonsense it will be left alone- a lot of faffing is involved in getting inside. The earlier Mod 2 does not have trimmer resistors for the 100v and 25v ranges, it relies on the accuracy of the main resistors alone. It seems to do ok despite this. I'm finding the earlier spidery scale plates rather hard to read without readers these days-shame!

[ManxDave]

Inspired by this mega thread, my first AVO.
Model 7. No. 38105-A-1257 ( or 125Z )
So far I have only discovered 12v DC reads 9V and that I am extremely lucky these old batteries haven't leaked! There is still 4.5v unloaded on the flat ones.
I'll be taking my time sorting it and no doubt frequently back here for help from the many experts.
Any initial information or comments appreciated.
Dave

[Zelandeth]

Thought I'd added mine here a while ago, but apparently not...

Avo 8 Mk III, number 39510-566.

I think...The numbering is very, very, very faded on this one so is really hard to read.

Everything seems to work okay, though I really should do something to restore some of the lettering to make it easier to use at some point...

[Phil G4SPZ]

Originally Posted by ManxDave

Inspired by this mega thread, my first AVO.
Model 7. No. 38105-A-1257 ( or 125Z )
Any initial information or comments appreciated.
Dave

A good meter to start with! Welcome to the hobby - or should I say obsession, Dave. Your Avo is a Model 7 "Mark 1" dating from December 1957. As a starting point for sorting out any faults, check the basic sensitivity on the 0.002A DC range. The easiest way to do this is to put the Avo in series with your best digital multimeter together with a 9 volt battery and a 22k resistor. Check that the meters read the same current. You can then go on to check the value of the multiplier resistors associated with the voltage ranges.

Circuit diagrams are available from many sources, or drop me a PM if you get stuck.

[Phil G4SPZ]

Originally Posted by Zelandeth

I really should do something to restore some of the lettering to make it easier to use at some point...

Hi Zel,

The quickest and easiest way, in my experience, to restore the lettering on the Model 8 is to use a sharp white Chinagraph pencil. A sharpened white wax crayon will fill-in the arrows on the range switches.

[The Philpott]

I use clockmakers engraving wax, although chinagraph pencil sounds more precise and durable.

The voltage under-read mentioned above (if it appears on ALL the DCV ranges) COULD be down to the leaf switches operated by the rotary switches.

[ManxDave]

Thanks Phil & The Philpott. I wasn't sure whether it was a MkI or II. I have downloaded the user manuals with circuit diagrams but thanks again.
Using the 0.002A range, 9v battery and resistors varying between 10k and 33k there is a 0.02mA difference compared to my cheap DVM, so that looks good.
Next step, probably tomorrow will be to carefully open it up.

[The Philpott]

There are now known to be 2 LRO (low resistance ohmmeters) extant.

Mine 370-441,
Now i am aware of a second one which is being carefully restored- No.375-541. It has suffered somewhat but is in the right hands.

[Barnmead]

I have been given AVO Model 3 112-685

I have straightened the bent meter needle, cleaned it up and fitted two matching terminals and had it working on DC ranges after some fiddly soldering to repair one of the wire wound resistors but no joy on AC. Eventually discovered the current transformer primary is open circuit. Interestingly this meter does not have the JK contacts shown on PMMunro's diagram which otherwise was very helpful.

[The Philpott]

There is another HR Avominor to be added to the list of extant ones; total 5.

168-108 (Oct 1938)

Others known of:
144-98 (Sep 1938)
206-108 (Oct 1938)
404-79 (July 1939)
(And one with an unknown serial no.)

Thumbnails attached, it works but reads high, (which is a bit odd!) so images of the inside will be available at some point for those who are interested.

Needle damping is virtually absent, hardly surprising given the sensitivity in such a small unit.

[Phil G4SPZ]

Very interesting, I've not seen that model. I'm keen to see the innards! For the 'reading high' fault, check for shorted turns in the movement swamp, or perhaps an open-circuit in the permanent shunt chain which would also partly contribute to the lack of damping.

[The Philpott]

HR Avominor:

Thanks Phil, i measured the V ranges with the most sensitive DMM i could find, and came up with 2.5v 50kohm
25v 487kohm
100v 1.95Meg
250v 5.3Meg
1000v - beyond the capability of the DMM.

These figures certainly correspond to roughly 20kohm/volt, but are there any other diagnostics viable before the back comes off? I would think a short within the swamp is indicated as the error occurs on different ranges.

Starting a different thread 'HR Avominor Troubleshooting' before i get carried away rambling on the Survey thread.