Repairing AVO's.

[martinhall]

Hi there, my first post so here goes.

I used to work in a very busy repair and calibration lab and worked on loads of Avo's. Some came in in an absolutely attrocious state but we still managed to repair them.

Main faults were broken movements and shunts caused by measuring mains on the ohms range, broken glass, and non standard repairs.

Movements can be very carefully cleaned with sellotape and the the meter balanced by gentle heating of the weight and very carefully moving it. Under reading is usually caused by non standard parts, or faulty resistors. Check the meter movement by directly connecting to a current calibrator. If it still under reads it could be a weak magnet almost always caused by someone taking the movement apart. The magnets will immediately loose their strength. Avo used to be able to remagnetise them but stopped doing this years ago. Some dubious repairs were carried out by using an adjustable pot on the movement. The real cause was a burnt or overheated shunt.

A meter glass can be fairly easily cut from a piece of 3 or 5mm window glass and glued into place. To do the curved bit, scribe the semicircle, and them scribe loads of criss crosses into the bit you want to remove. Gently remove the glass with a pair of parallel pliers.

Non existent serial numbers on the movement are probably because the movement has been replaced.

Cases can be repaired with araldite and small missing pieces can be replaced with a mixture of araldite and powdered carbon from the centre of a battery. They still passed flash testing. The case can be cleaned with foam cleaner, and finished with thin oil.

The 15v battery can be replaced with a stack of button cells (thats what was inside the BLR battery)

I may have some new black and red caps hidden away somewhere. If there is any interest in them.

Best regards
Martin

[Pamphonica]

Martin,
Welcome to the forum.
Thanks for a very useful post. Personal experience from someone who repairs equipment professionally is always valuable to capture, especially your comments about rebalancing and adjusting movements, a perennial problem.
thanks again
Jeremy

[Kevin Hoyland]

Hello Martin.
and welcome to the forum you will find us a friendly lot.

Regards Kev.

[Dekatron]

Welcome to the forum!

The kind of information you have written about here is invaluable! Especially since there are so few papers written about how repairs were actually performed, and also why some things should not be done - like the information you shared about the magnets loosing their magnetic flux if removed!

Do you have any written information about how the repairs were performed? If so I am very interrested in obtaining copies of those.

Kind regards,

[martinhall]

Hi there,

Forgive me for not being in touch, very remiss of me!

Sorry I dont have any written information or service manuals, but if you can find the service manual for an Avo 8 Mk5, I think from memory (and its almost ten years since I had anything to do with these instruments) that covered most of the techniques. However these were only issued to authorised service centres and may be a bit difficult to track down. I will however check my old filing cabinet and see if I have anything lurking about.

Best regards

Martin

[WillieQ]

Thnaks for your guidance Martin,great info!

PS: I sent an PM but it seems it does not reached to you. I just cleaned my PM storage.

[WME_bill]

I have the Service Manual for the Avo Mark 8 & 9. It is helpful, and quite detailed. I am sure I posted a copy last year, so I suggest a search might be the quickest way for you to get up to date. If martinhall can't find his copy, and you search isn't successful, then I have mine available. Bill m0wpn

[stuarth]

Originally Posted by martinhall

Main faults were broken movements and shunts caused by measuring mains on the ohms range, broken glass, and non standard repairs.

Martin

Very useful information, this. I've often wondered whether the AVO ballistic cut-out, which relies on the needle hitting the zero or full-scale stop, would work if AC was applied to DC or ohms range where the rectifier is not in circuit. Theoretically you would get no deflection, but I wondered if the first half cycle of a severe overload would cause enough deflection to trip the cut-out. I guess Martin's post giver the answer - it doesn't.

Does anyone know why AVO stuck with the ballistic cut-out rather than fitting something more effective (they made other far more extensive changes over the years from the Mk1 to the Mk7)?

I have multimeters with rather better cut-outs, eg Simpson in "P" versions of their 260 models use a relay circuit powered by the PP3 ohms range battery, and Unigor use a very sensitive passive cut-out based on a solenoid normally held "on" by a weak magnet and released by a small current in the coil which reverses the magnetic field. The current comes from the circuit under test (even on 100kohm/V versions) through various resistors etc on a separate pole on the range switch, and always through a rectifier (the silicon diodes prevent any current in the cut-out at normal meter deflections).

I have tried these meters on AC, and both Simpsons and Unigors trip quickly when measuring the resistance of live 6.3V heaters. The AVO didn't trip, but at 6.3v, the energy was too low to cause damage. I have a "spare" Unigor 3n (bought for 99p, rather scruffy, but fully functional) which I deliberately risked connecting to mains on an ohms range. The cut-out tripped - no other damage. I'm gald I didn't try this with an AVO.

Stuart

[Brian R Pateman]

The effectiveness of the AVO cut out is an interesting subject. The basic design is old and AVO obviously saw no need to update it during the long life ov the Avometer through all of its many iterations.

Of all the meters I've used over the years the best cut out is that on the Unigor (in my opinion) and they are also among the most rugged instruments I've used. One of mine was dropped on repeated occasions without suffering fatal damage. I wish I'd kept one as I prefer them to the AVO 8.

[pmmunro]

The original AVO cut-out was introduced on the Model 7 in August 1936. This was not only an end-stop cut-out but also incorporated an dynamic mechanism which used the difference in inertia between the long pointer and the compact moving coil frame to initiate a trip on heavy overload. It was claimed that the mechanism would trip at less than one third deflection if the acceleration was sufficient.

Two small bell cranks near the front pivot deflected a leaf spring below the moving coil and the leaf spring acted against a spring-wire push rod. The lower end of the push rod was formed into a circuler eye which was coupled to another, larger, brass, bell crank
which was pivoted on the cut-out table (a circular brass disk). A synthetic sapphire palet stone set in the bell crank held a sping loaded detent which, in turn, held the cut-out plunger in place.

The same cut-out mechanism was fitted to the Model 40 when it replaced the 36-range Universal Avometer in 1939. At the same time the Admiralty Pattern 47A and 48A and the Air Ministry Model 'D' were updated in the same way.

Around 1948, Avometer movements were redesigned to use Alnico block magnets rather than the older cobalt steel horseshoe magnets. The cut mechanism was refined slightly and remained in this form until the late 1960s.

This type was of cut-out was also used on the High Resistance range of 20000 ohm-per-volt Avometers until these were superseded by the Model 8 in May 1951. The Model 8, however did not have the acceleration cut-out, possibly - and this is speculation - because the considerably more delicate 50 microampere moving coil, was incompatible with the acceleration trip. (HR Avometers are often found with open-circuit moving coils). The Model 8s did however retain a toothed aluminium disc at the front of the movement concentrator. This was an essential part of the acceleration trip.

After the move to Dover, the cut-out on the Models 7 & 40 was redesigned again and brass parts on the cut-out table were replaced by die-castings.

The cut-out was the subject of a patent taken out shortly before the introduction of the Model 7. Another patent of a similar date addressed the problem of no deflection when an alternating current was applied to a direct current range. This was done simply by putting a reverse biased semiconductor diode across the moving coil, thus shunt alternative half-waves; the point would then deflect on the other half waves, allowing the cut-out to trip on overload. In the early 1950s, some Model 7s had this diode deleted from the specification. From memory, this was to address frequency response problems.

The settings for all these cut-outs are given in various service documents and can be as high as 20 times the fsd value, implying that the mechanism is designed to withstand at least this degree of overload.

This type of cut-out becomes unreliable if any of the brass parts are significantly tarnished. Dismantling and reassembly are delicate operations, and experience would suggest that the shouldered screws on the cut-out table should not be interchanged as the shoulder height is selected to suite the thickness of the part retained. Polishing the brass parts restores the freedom of the parts to move, but any traces of the polish used (Brasso) must be carefully removed.

The cut-out mechanism on the Model 8 Marks V, 6 & 7 is entirely different and is purely and end-stop cut-out. This is a compact mechanism immediately adjacent to the moving coil and is fitted with heavy contacts designed to be easily replaceable.

PMM

[stuarth]

Wow, very comprehensive reply, PMM. Many thanks, I can't match that, but I will add that early Unigors (models 3s, 3t, 4s, 4t from the 1950s) don't have the interlock mechanism which later models had to prevent over-riding the cut-out by holding down the reset button, nor to they have a full wave rectifeir to the cut-out coil, so they will trip on positive or AC overloads, but not on negative overloads.

It seems the AVO cut-out was getting less effective as the models progressed.

For AC on DC/Ohms ranges, I connected one of my AVOs (I think it was an AVO9 Mk3) to a variac and slowly increased the voltage. I stopped at 40v or so (V^2 =1600, R=2000ohms on the AVO middle resistance range, so dissipation less than 1 watt) and there was no deflection on the movement. I must check whether the 9 Mk3 has a diode across its movement, I thought it had. The same test on a Unigor 3n and the cut-out tripped at around 2V.

The Unigor has forward reading resistance ranges, where open circuit test leads give full scale reading. You tend to notice a meter reading full scale and switch it back to a safe voltage range (just a single switch on a Unigor). I think this is a useful additional safety feature.

Like Brian, I much prefer the Unigor 3n to AVOs. I think they're designed with robustness very much in mind. As well as the cut-out, they have a HRC fuse to break huge currents (see the thread on fuses), a spark gap for high voltage transients, and sundry surge arresting neons. I've not seen one with electrical damage, but I have had a couple of A43s with dirty cut-out contacts (the mechanism doesn't have much of a wiping action). Unigor also use a taut band movement, generally recognised as better able to withstand physical shocks than jewelled movements. Triplett have a line in the handbook for their Model 60 (also taut band) which guarantees that it will continue to work after a drop of 5 feet. The Unigors also have more than twice as many current ranges than the AVO (a 1/3/10 sequence), and a wider resistance (10 ohms to 1 Meg centre scale using a DC-DC converter from a D-cell rather than a 15V battery).

Incidentally, I think the Unigor 3n (31.6kOhm/V and current transformer, so closest competition to an AVO 8) is still around as a Ganzuniv 3, built in Hungary. Up till 2007, they also did a Ganzuniv 4, identical to a Unigor 4n (100kOhm/V and no current transformer, so better for most jobs than a 3n, but with no AC current ranges so not a direct competitor to the AVO).

Stuart

[royboy70]

Hi Martin.

Can you tell me if this glass on a MK6 is the same size and shape as mk5's?

My glass is cracked on my MK6, but iI have an old MK5 in bits. It is beyond repair and I was thinking to take it to a glass company for them to make a new glass using the MK5 glass as a pattern. I prefer not to take my MK6 to bits until I have had a new piece made.

Thanking you, Roy.