Avo 8 mk3 cutout button stays out..

[stevehertz]

I was taking some readings on a piece of kit with my AVO 8 mk3, and I can't remember exactly what happened, but the result was that the cutout button popped up. The problem is, it won't stay down again now. Having used AVOs all my life including 12 years in a test department, I don't recall this being the case, it normally 'locates' when pressed down after popping up. What's going on?

[pmmunro]

Steve,

If you don't already have a service manual for your meter, send me a private message with an email address and I will send you one. The Model 8 has some long hexagonal contact posts on the back of the front panel and once the meter is out of its case, care is needed not to stress these posts as their mounting plates are easily broken.

Your meter is over 50 years old so it is quite likely that the brass components of the cut-out could benefit from some cleaning. The cut-out latches by means of a steel blade which pivots on the brass plate known as the cut-out table. This is a sensitive mechanism which needs to be handled carefully. AVO recommended that an exchange cutout assembly was bought from them but that is no longer an option.

It is probably best to remove the movement from the front panel and care must be taken only to undo the correct two screws using a technique which avoids a steel screwdriver touching the magnet. A brass screwdriver or a thick sheet plastic shield can be used. This will also low you to clean the ohms divided by 100 pot.

Following the procedure in the manual, the cut-out should be dismantled, cleaned and polished. The assembly includes some shouldered cheese head screws and these seem not to be entirely interchangeable due to small variations in the height of the shoulder. The brass parts can be cleaned and polished with Brasso then thourghly cleaned with isopropyl alcohol or methylated spirit. The reset button stem should also be cleaned. The tubular part of the table can be cleaned with a cotton bud, perhaps with some thinning of the cotton head. The cut-out is operated by a wire pushrod which passes through a right-angle bracket near the movement. This bracket has two holes so it is important to check which is the pushrod guide hole before the pushrod is removed.

It is very important to make sure that the pallet stone on the bell crank is in place, it can become detached and may be found in the dust holding grease smeared inside the casing. If the pallet stone is detached, it can be refixed with shellac, as it was originally, but possibly a modern adhesive could also be used.

Reassembly of the cut out is quite demanding and the screws are easily lost so suitable working conditions are essential.

Once the cut-out is reassembled, it will need to be adjusted to trip at the correct overcurrent.

The work needs a methodical and patient approach but is worth the care to restore such a useful instrument for several,more decades of work.

PMM

[stevehertz]

Peter, many thanks for replying so quickly. I am in possession of a pdf document entitled 'Avometer Servicing Information'. It seems to be a generic Avometer document as a quick glance through it does not seem to differentiate or even mention different versions of AVOs. I also have an M8 spares list. I have a brass screwdriver that I can use. I hear what you say about servicing the brass components of the cut-out assembly, so that is what I will try to do. However as far as I can see there are no instructions for removing the cut-out assembly in the data that I have, it merely talks about replacement, so I will PM you re the proper info, thanks.

[Silicon]

I bought a grubby AVO 7 at a boot sale with a cutout button that refused to stay in.

I cleaned the area around the button and then I repeatedly pushed the button in every second for about a minute until it stayed in.

It may not be the correct method but it worked for me.

[The Philpott]

Little to add to what pmmunro says, but i agree that if the meter movement is out of the instrument for ANY reason the 'ohms divided by 100' adjuster should be serviced, ie cleaned. The dullness which causes noisy operation is usually on the shaft, which is polished up with the knob removed from the instrument. The bronze spring leaf should be a good springy fit against the shaft, if not it can be tweaked to ensure continuity. The above operations can certainly be done WITHOUT removal of the movement, it's just a bit more fiddly.

Should the glass be loose this is the opportunity to reglue it, as the clips alone are insufficient to hold it securely and the original glue has not maintained good adhesion to the glass in many cases.

I do have a meter which is suffering the malady you mention, in my case the bellcrank is stiff in operation against the cut-out table. Cause yet to be established but contamination possible.

[ionburn]

I had a problem with the cutout button on my AVO 8. It would work if pressed in a time or two but was irritating so I dismantled. I found that the button internals were broken - I think on one side of the fixing so that is why it was possible to 'catch it' if pressed a certain way. I did repair it, but it was tricky as (if I remember correctly) there was not any great substance to the switch. Probably why they are so un-reliable. It still plays up occasionally so may be going again, but I only use the AVO for current measurement when testing valves so not my workhorse instrument.

[stevehertz]

In the light of what has been said wrt the cut-out button shaft going 'sticky', and as a fair amount of it is in view sticking up from the top panel, I gave it a quick burst of Servisol contact cleaner. Immediately the feel of the button as it was pushed up and down seemed much freer. After a few goes at getting it to stay down I succeeded. FWIW, it seemed to me that pressing on the front edge of the button did the trick (?). It may be a tenuous hold, but anyway, the AVO works fine now and if the cut-out is over-sensitive, that's much better than it not working or being under-sensitive. Oh, I swear the ohms calibration knobs all (!) work better now. And no, I didn't give it a massive blast of Servisol!

[jez_145]

That's great that the Servisol did the trick BUT, assuming you sprayed it rather than using a cotton bud, as a general rule it's best to avoid using any sprays anywhere near an AVO movement. The main risk is that droplets can reach the moving coil contact springs and the gap between the moving coil and the magnet assembly. This can cause further problems down the line with the movement sticking.

PMM'l tell yer !


Jerry

[stevehertz]

I know, it did cross my mind that I was committing a great faux pas, but in the spur of the moment my thoughts got the better of me. Anyway time will tell, but having taken AVOs apart in the past and seen their construction, layout etc I doubt if any damage is done.

[Phil G4SPZ]

I try to leave cut-outs alone if at all possible. I've sometimes spent more time repairing the cut-out alone than the entire Avometer! I once dropped a cut-out hairspring on the floor... that's an hour I'll never get back

[The Philpott]

I must admit the only repair i have ever done is to replace a broken knob and spindle assembly for a good one, or replace an incorrect diameter knob/spindle for the correct one. I don't at this point even know if any of the cut-outs are set correctly; i treat the meters as if there is no protection whatsoever. (Which on some there probably isn't!)

[Phil G4SPZ]

I follow PMM's advice on cut-out adjustment, which (from memory) was that the cut-out should trip somewhere between an 800% and 1,200% overload. This is counter-intuitive to some extent; it seems rather severe. However, set it too sensitive and the cut-out trips as soon as you look at it! It is, however, worth persevering, making tiny adjustments by rotating the cut-out table and checking its sensitivity each time.

The Model 8 does not have the acceleration trip mechanism that the Model 7 and Model 40 had. This means that the pointer on the Model 8 does indeed have to hit the end-stop with some energy before the cut-out should operate. Avo designed it that way.

[The Philpott]

Cut-out platform on my Model 40/II is less than convincing- there's use of die-cast components and a couple of coil springs (one of which had understandably come adrift as there wasn't anything holding it onto it's peg-not even gravity) I slid a piece of silicon sleeving onto the peg which has held it ever since. The Model 8/9 assy. looks like the art of a watchmaker by comparison.

[pmmunro]

The die cast cut-out was used on the Model 8 and Model 9 Mark IV dating from 1971 and on Models 7 & 40 from about then until the end of production. It was probably a Thorn innovation to reduce costs as Mr. Thorn considered Avometers to be uneconomical to produce and, according to some sources, was seriously considering closing down production altogether.

I haven't found the die-cast cut-out to be too bad but it does have three points to watch:

1. The wire push rod is a simple length of piano wire and is held in the hollow end of the adjuster screw "2" in the diagram. The push rod is very easily lost.

2. The detent on the reset button rod is a very small steel ball which is also very easily lost. It's important to remember it's there when dismantling. For re-assembly, it can be held in place with a short length of rod or tube which is then pushed out by the reset button rod as the rod is put back in place.

3. The tension springs are easily distorted and this can make them difficult to fit. The method of attachment probably depends on their being the correct shape.

The attached pages are from the Model 8/9 Service manual. Note that the cut-out "must-trip" overload is an even more brutal 2000% figure.

PMM