AVO 7 sluggish

[ManxDave]

My AVO 7 is now working on all ranges tested. Whilst standing vertically it works well but when lying horizontally the pointer is sticking slightly near mid scale and needs a gentle tap. What is the best way to solve this?

[pmmunro]

Dave,

The only simple answer is by very careful visual inspection as there could be many causes of this problem.

You will need to remove the movement from the front panel; this has been described several times in previous threads. You will have to follow the procedure carefully to avoid the risk of serious damage to the movement. Afterwards it will be necessary to adjust the sensitivity of the cut- out.

Holding the movement under a good light and looking with your preferred form of magnification, move the pointer carefully with the tip of a small artist's paint brush. It may be possible to see the point at which the pointer sticks and the cause.

The pointer may be touching the scale plate or something may be fouling the moving coil. This may be a particle of contamination a distorted part or the moving coil pivots may be out of adjustment so that the coil is not correctly centred. Correction is likely to be quite delicate and the movement balance may need to be adjusted.

Have a look at previous threads on Avometer repairs and you should find most of what you need to know. It would take quite some considerable time to cover it all again and it should be a useful learning experience to read what is available on this forum already.


If you need help with specific questions, please ask rather than taking any risks with what will be a very useful and rewarding instrument when it's restored to full working condition.

PMM

[ManxDave]

Thanks
I will search relevant threads and proceed very carefully. It is interesting you mention the cut-out. So far it hasn't triggered even though the needle has hit the end stop ( 1.4v on the 1v range, accidentally ). Also you may have read my comment on your AVO survey thread, initially this meter was under reading on some ranges. Further investigation found it was erratic depending on slight pressure on the cut-out, cleaning the contacts with servisol made it more reliable.

[The Philpott]

pmmunro on here and also the websites of richard's radios and peter vis are the best source of troubleshooting information.

Before you even start dismantling note whether there is any sort of a clink or ting when the meter is tapped or gently shaken up and down. I mention this as it can signify a fault which is simple to fix and that is not often mentioned. There are a couple of screws which, if loose, can allow the magnet and curved concentrator assembly to move relative to the (moving) coil thereby jamming the latter. This situation is orientation sensitive, ie the weight of the magnet causes it to flop around. Needless to say these errant screws cannot be tightened until the movement is removed from the meter. The big priority is to avoid touching the magnet with anything ferrous eg screwdrivers or tweezers.

This is a 100-1 shot but is the easiest of many to diagnose.

The glass is prone to coming loose so when and if the movement is out it wants checking and reglueing if needed.

[ManxDave]

I haven't heard any clinks or tings but I shall listen very carefully before dismantling. Thanks for all the tips, appreciated.

[Phil G4SPZ]

I can add very little to PMM's comprehensive advice, other than to mention that I have cured several Avo movements with this type of fault, and in all cases the cause was debris in the air gap. Sometimes the foreign body is so small as to be virtually invisible. When holding the movement on its back, try blowing VERY gently on the pointer from the left-hand side to get it to move across the scale. With practice, this can be done smoothly and has the advantage of not physically touching the mechanism, and thus mimics the actual action of the movement.

[The Philpott]

Yes, and often the debris is magnetic so does not want to shift. Microsurgery under a x10 jewellers loupe (eyeglass) will be needed if it's foreign object trouble. A cocktail stick and blu-tack are useful for removal.

[Phil G4SPZ]

Originally Posted by ManxDave

...the cut-out... hasn't triggered even though the needle has hit the end stop (1.4v on the 1v range, accidentally)...

The cut-out is not meant to be sensitive enough to detect a mere 40% overload! PMM will confirm the actual figures, but the cut-out should trip at between an 8 times and 12 times overload, e.g. 8 volts applied to the 1 volt range. The moving coil is very robust and can tolerate short-duration electrical overloads below this level.

The Model 7 movement also incorporates an ingenious 'acceleration trip' which can operate the cut-out long before the pointer hits full-scale. It is quite remarkable to watch this in action; apply a heavy overload, the pointer starts from rest but the cut-out trips by the time the pointer has reached mid-scale. The moving coil drives the pointer not directly but via a tiny pawl-and-ratchet mechanism. There were two types, depending on the age of your meter; the earlier mechanisms were phosphor-bronze and the later ones aluminium. Avoid touching it, as the mechanism can part company with the moving coil, necessitating delicate microsurgery with Araldite. Possible, but not to be recommended (guess how I know this...) Avo dispensed with the acceleration trip in the Model 8, but you'll also find it in the Model 40.

The cut-out is adjusted by slackening the bolt and the shaft upon which the moving contact bar slides, then carefully rotating the brass cut-out table a tiny bit at a time. Looking from above, with the instrument lying on its front, rotate the cut-out table clockwise to increase its sensitivity, anticlockwise to reduce it. Avoid making it too sensitive or else the cut-out will start nuisance tripping at the slightest mechanical provocation. After each adjustment, lock the screws, turn the instrument right-side up and apply a ~x10 overload. Repeat, until the cut-out trips reliably. This is very much a trial-and-error process but not difficult once you've done it a few times.

[The Philpott]

As regards the cut-out in general terms, keep an open mind for what 'repairs' might have been done in a previous life, because:

The part of the bakelite knob that slides in the front panel existed in at least two different diameters. I have known nuisance tripping in an older meter to be prevented by fitting a fat one from a newer meter, into the facia which was designed for a skinny one. Hey presto, a nice interference fit...and no protection from overload.

[Phil G4SPZ]

Indeed, the cut-out shaft should be a relatively loose sliding fit through the bush, with no lubricant being necessary. Just make sure the shaft isn't bent, then clean and polish the shaft and inside the bush with a little Brasso or equivalent on a pipe cleaner, taking care to remove all traces before reassembling.

[ManxDave]

I haven't had chance to remove the movement yet but thanks for all this extra information, it is invaluable. Dave

[ManxDave]

Thanks to all your help I think I have solved this. Although visual access was limited I couldn't see any debris. I blew through the coil/magnet gap in an attempt to clear anything hidden from view. However I suspect the problem was too much movement of the pointer in the bearing. It could move significantly in any direction allowing the pointer to touch the scale in certain circumstances. I tightened the brass screw a tiny fraction and it hasn't stuck since.
I haven't adjusted the cutout yet I want to be sure the pointer remains free before I do.
A couple of questions before this is closed. If it starts sticking again can I remove the two nuts and the cross piece of the magnet to allow better visibility of the moving coil? Should I have done this in the first place?
As informed in the AVO survey thread, this looks like a MKI in every way except it has the PF sockets and associated resistors to the rectifier, which are only shown on the MKII circuit diagram. Was the MKI circuit modified before the slightly different MKII case?
Serial 38105-A-1257. Pictures hopefully attached.
Thanks Dave

[The Philpott]

I have had this happen myself. The needle can indeed jam if the clearance is either too tight or- as in your case- rather loose.

Make sure the locknut surrounding the little brass screw is tight. (It should have been loosened a tiny amount before nipping up the screw, then the screw is held from moving whilst the locknut is tightened up again)

As far as i know it is NOT an option to remove the cross piece to get better access to the screw. I believe this would result in some loss of magnetism.

[ManxDave]

Thanks. Not loosening the locknut would explain why I could hardly turn the brass screw!

[The Philpott]

Additional- there is a small amount of springiness in the movement bearings as part of the design, but when there is too much it is quite obvious, as you have seen.

The facia of your meter is interesting as it has the early type P R & Q legends, but includes the PF sockets of the MKII. There are a lot of these anomalies when you start looking around, and it has been said jokingly by some that every meter is a prototype in itself!

From what i know you seem to have an early MKII there, but i am not an expert by any means as i do not own a Model 7.

[ManxDave]

I am hoping this was just the bearings too loose as it looks near impossible to remove metallic debris under the coil without dismantling further.
I thought mine had similarities between the MKI & MKII, thanks for confirming.

[Phil G4SPZ]

I have a couple of instruments just like yours. By all accounts, the P.F. sockets were available by special order as optional extras on the Model 7, before they became standard fitments on the Mk II. I personally prefer the plain black terminal knobs of the Model 7 "Mk I" to the red and black 4mm terminals of the Mk II.

[ManxDave]

Thanks a "MKI with optional extras" will do for me.

[The Philpott]

If you ever see one of these little flat blade aluminium bodied screwdrivers (or a set of them) buy it.

I don't know where it came from, but all the dimensions meant that it suited this particular job well. For holding the locknut an open ended BA spanner is needed- quite easily available. Just need to avoid touching the magnet with it.

[TonyDuell]

Watchmakers/repairers use non-magnetic screwdrivers such as this set (but be sitting down when you see the price!) :

https://www.hswalsh.com/product/berg...ers-hs8899-a10

No, I don't have it, but boy do I want it!