AVO Multimeter survey

[Valvewhiz1]

And again,
Avo Multiminor Black Bakelite version, can't see a serial number with leather case.
P.O. MMR 14C (Avo 8 Mk V) S/n. 09505 - date 6/79 with leather case.

Any ideas on where to get the manual for the Avo Multiminor please.

AJH

[Paul Adams]

Hello AJH,
Presumably you refer to the Mk5.

Paul.

[Valvewhiz1]

No Paul,
It's case looks the same material as Avo 8's and 7's..

[Valvewhiz1]

The last of my Avo's P.O. MMR14C/2 8 MkV dated 6/80 s/n 0011808

[Heatercathodeshort]

Just noticed this interesting thread. I have a number of old AVO meters collected over the years and I will list the serial numbers as they are discovered. My current bench AVO has been in use for many years bought second hand I guess around 30 or so years ago. The serial number is 5770-15-463. I'm not exactly sure what model this is, maybe someone could enlighten me? Another rather tatty meter is a 7 Mk 2 serial number 82947-A-1259. Pictures attached as we all love pictures. Regards, John.

[M0XNA Neil]

John,
Your AVO 9SX looks just like my type 12889 discussed in the thread 'Is there an AVO 9 Mk4?':

https://www.vintage-radio.net/forum/...ad.php?t=83516

Neil

[Leon Crampin]

Another Avometer and resistance range extension unit - irresistible at the price:

Avo Model 7 Mk 1 ser. 71145 - 24432.
Model 7 Avometer resistance range extension unit (no serial number).

The meter is in fair condition (dirty) and has been opened but not molested inside. It has the horseshoe magnet and the bimetallic slidewire temperature compensator which works beautifully. The 1 microfarad block capacitor for the capacitance range is is shunted by a waxed paper 50nF 500V component dated Feb 44. The scale plate of the meter has the letters KF hand marked in black ink on the rear and a paper disk is fitted in a recess in the back of the front panel near the block capacitor with 55 marked on it. The battery compartment is clean.

This instrument clearly dates from 11/45 but has a few problems - annoying because it's basically working and accurate. The pointer balance is miles out, probably due to whiskery corrosion growths on the MC terminations and balancing arms. The movement also sticks slightly at various points.

I am quite happy to remove the meter (and reset the cutout) but would welcome PMMs advice on clearing the whiskery growths and unsticking the movement. I have a steady hand...

Leon.

Are perished rubber Avo leads worth keeping?

[pmmunro]

Leon,

Once you have the movement off the front panel, I would recommend a long and careful examination with a good light and some form of magnification - I usually use jeweler's eyeglasses of various magnifications between 2 and 9. It is quite probable that you will be able to see the cause of the obstruction which will help decide the best course of action.

It should be possibly to swing the pointer over the scale with gentle pressure applied to the pointer stem using the bristles of an artists paintbrush which is incapable of applying any harmful force. This should find the exact points at which the movement is sticking.

A close-up examination will give you a better assessment of the nature of the whiskering.

It is possible to take the moving coil with its pivots out of the magnet assembly on these movements but I would recommend placing a keeper across the pole pieces before you do this. I have no service manual information for these movements, so I don't know the official policy on retaining magnetism, but I have successfully dismantled some of these movements without having to have them remagnetised. I'm sure I don't need to tell you to unsolder the outer ends of the hairsprings, but never the inner ends if at all possible.

It will be useful to make a moving coil holder. This is a length of rectangular hardwood with the arrises radiused to fit inside the coil former. Cross drill one end with holes to slide over a length of studding anchored to a heavy base with stop nuts near the top of the studding post. The holder should be long enough to shadow the full length of the pointer, thus giving it protection. Too firm a steady hand could crush the coil former, so the holder helps prevent this.

At the age of your meter, it is quite possible that the small rectangular pivot plates which should be stuck to the fore and aft faces of the moving coil are becoming, or have become, detached; they were originally stuck by shellac. You may find that it is the moving coil wire which is holding the pivot plate in place.

If the pivot plates need to be restuck, you can use the original build up of shellac as witness marks and with luck the pivots will be straight and true, but you may need to make a jig to hold them. This could involve removing the hairsprings by carefully sliding their collets axially along the pivot, having taken careful note of their original orientation with respect to the coil axis. There is also the added complexity of the pointer being free to rotate on the pivot under the restraint of a single wire spring which is quite delicate. A repair using super glue has been reported here and this may well be sound, but my preference would be to keep to the original substance and use shellac. I don't know how the insulation of the wire might react to super glue and if it sticks and the pivots are not in line it could be difficult to realign the pivot without seriously damaging the coil. (Shellac can be bought as "Patent Knotting" from decorators merchants, although the consistency is probably not ideal.

If the whiskers are a metallic salt, they are likely to be from either the tin or the lead in the solder possibly reacting with a constituent of the flux, which should give a clue as to a suitable solvent. In any event, if applied carefully with an artist brush, the common solvents, isopropyl alcohol, pcb cleaner and water will not do much harm. It would be advisable to resolder the connections being careful not to heat the pivot plates too much, or they will come unstuck in any event.

Then you will need to rebalance the movement.

It's just as well you have a steady hand and with all that care you should succeed.

PMM

[Alistair D]

Any thoughts on Leon's last comment on perished leads?

Al

[pmmunro]

All I can suggest is that the metal end connections might be of some use in making up replacements, although I haven't got a scheme for doing this. It depends on whether the lead is perished throughout its length or if it is likely to.

In the past I have used elastomer spark plugs lead covers for lead ends for a Model 12; something similar might be possible with the contacts from the old leads. The Model 12 leads had a brass slug turned to fit inside the cover and the clips and probes were screwed into an axially drilled and tapped hole. (That didn't use any part of AVO leads but may inspire some other ideas).

Multiminor leads, especially the red ones, seem to perish near the ends in time. Does any chemist know if it is possible to reconstitute rubber in the affected area? Another approach might be to remove the perished rubber at the ends and mould on new covering. The black lead could be used as a pattern for the mould.

PMM.

[Leon Crampin]

Peter, many thanks for your really comprehensive advice. I'll dismantle the meter within the next few days and report back.

Although purchased for very little money, this meter is so well engineered (especially the temperature compensator) that it deserves the best possible repair.

Leon.

[Radio Wrangler]

Oh, I forgot in an earlier post, my first AVO was a CT38 valve voltmeter. Not often mentioned nowadays, but quite good. I had the 25A shunt and the 10kV divider with it. Took ages to warm up before the thermal timer cut in. Took ages to unscrew the fitted lid as well.

I remember the smell.. tropicalisation preservative. Smells really evoke strong memories/

David

[Phil G4SPZ]

Originally Posted by Leon Crampin

...this meter is so well engineered that it deserves the best possible repair...

Be careful! You've correctly identified one of the factors that turn otherwise normal people into Avo collectors...

[pmmunro]

David, Phil,

With relevance to both your comments, I have a CT38 and am attempting to acquire a second. I think of them as "Ammunition Box" Avometers. I also managed to acquire the calibration unit which goes with them - that really is a work of art!

PMM.

[threeseven]

Where does one find the serial number on the Avo 7 & 40 models ?

[pmmunro]

Up to around 1972, the serial number is on the scale plate in at the extreme lower right-hand edge. It may be necessary to tilt the meter to see it.

The earliest serial numbers are in the form 7mmyy-XXXXX or 40mmyy-XXXXX where mm is the month and yy the year. From the mid 1940s, the form does not include the model number and will be XXXXX-mm-yy.

From 1972, the serial number was on an external, self-adhesive label on the right-hand side of the back case near the centre top edge. It has silver characters on a black ground. There was a second label inside the battery compartment of Model 8s from this time, but I don't think this was done for the Model 7s and 40s.

PMM

[threeseven]

Thankyou.

Found them. Ist one is easy, a model 7 , s/no 06363, model no and date is 7/1/77

2nd 1 is a bit odd. It's a model 40 s/no 47133 model no and date is 40/11/7? The question mark is because the the last character of what I presume is the year is like a capital A with the lower right hand leg below the horizontal bar missing ? Would that be a symbol or a badly printed character ? Even if we assume it should be a A though, the year doesn't make sense ?
I should say both these meters have serial number labels as described above.

[Refugee]

That may be a slightly squiggly number 4 as my best guessing goes.

[Paul Adams]

Model 40, Serial: 405533-643.

Paul.

[threeseven]

Originally Posted by Refugee

That may be a slightly squiggly number 4 as my best guessing goes.

Just had a look under a magnifier and it's confirmed it's as I've described it. Compared with the A in Avo on the same s/no label it's identical apart from the missing leg.