AVO 9 MkII: Occasional Sticky Pointer - UK Vintage Radio Repair and Restoration Discussion Forum

russell_w_b · 19th Sep 2007, 8:20 pm

I've just acquired an AVO 9 MkII, and discovered that the pointer sometimes sticks. Now, it only does it occasionally (I have run it up and down the scale on various ranges), and it could stick anywhere. All it takes to free it is a light tap on the front panel.

I don't wish to poke about more than necessary with the movevment, and - bearing in mind the post elsewhere on this forum - is there anything I should be looking at specifically? Bearings? Off-true pivot? Gunge?

It doesn't seem to stick when a range is selected and the meter is agitated gently from side-to-side. All help appreciated!

Mike Phelan · 20th Sep 2007, 8:41 am

Hi Russell
Assuming that the pointer is free of the scale, the most likely thing is that there is some ferrous rubbish in the magnet gap and the coil is fouling it.

The pivots are jewelled with hardened pivots, so I doubt if there is a problem there unless some severe damage has occurred.

I've not heard (yet) of any problem with oil, and must admit that I don't know if the pivots were actually oiled at all (anyone?) - I suspect they were not.

It is possible that someone has oiled them, and the oil has become sticky. Clocks I have handled frequently suffer from this sort of thing.

pmmunro · 20th Sep 2007, 12:48 pm

The random nature of the sticking is the best clue to the cause, with the caveat that diagnosis is unreliable without seeing the instrument concerned. It is likely to be caused by contamination of the pivots and,if so, is best cured by dismantling and cleaning. Damage to the pivot points due to excessive forces, for example if the meter has ben dropped could also be a cause. If the meter has been dropped, the moving coil is likely to be out of balance, which is easily checked. Another posible cause of random sticking is pivots which are set too tight.

Sticking at a specific point is more likely to be due to particles in the air gap, but can also be due to pivot damage.

Repair calls for delicate work, including unsoldering the outer ends of the hairsprings, unscrewing the jewel screws and removing the moving coil assembly. The pivots and jewels can then be cleaned with isopropyl alcohol. They are not lubricated. If there is pivot damage, they can, in principle be repointed. The technique is similar to reprofiling clock pivots and needs the same skills and equipment.

After reassembly, the moving coil will almost certainly need to be rebalanced which is time consuming and best done with the movement enclosed in a draft-proofed box with a glass lid. The cut-out will also have to be re-set.

Most of this work has been covered in other threads or is described in the official repair manuals.

Amongst the causes of pivot contamination is the misguided use of contact cleaning sprays and exposure of the instrument to damp.

If you are not confident and experienced in movement repairs, or if you do not have the facilities required, it is best to leave the work to a specialist. The fine properties of the instrument depend on the movement being set up correctly.

Peter M Munro

russell_w_b · 21st Sep 2007, 3:45 pm

Mike, Peter,

many thanks for your kind and helpful replies. I'm hoping it's either gunge, or damp (through storage), or both. This is why I come to that conclusion:

I conducted a little 'experiment' to 'exercise' the pointer (I can almost hear the mass tutting 'n' sucking-of-teeth as I type

). First of all I applied AC to the DC range, with some DC bias from a series-connected supply to get the pointer mid-scale. Nothing much happened, but the pointer was visibly oscillating when observed closely.

I then connected it up to the output of an AC test-set on 'DC current', starting high and working down to achieve maximum needle-swing. I set the output of the AC test-set to +10dbm at 29Hz and gave it what-cheer for about thirty seconds.

Afterwards, I ran the pointer up and down on various DC voltage ranges, and it hasn't stuck since! When it's quiet, I've the workshop to myself and a little breathing space, I shall examine the movement as best I can without demounting it, to see if I can apply a little isopropyl alcohol to the pivot-points without incurring damage elsewhere.

I have no desire to attempt a proper repair and service, as I'm pretty sure I will make a muck of things. This sort of repair is better left to those who know what they are doing...

19th Sep 2007, 8:20 pm	#1
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	AVO 9 MkII: Occasional Sticky Pointer I've just acquired an AVO 9 MkII, and discovered that the pointer sometimes sticks. Now, it only does it occasionally (I have run it up and down the scale on various ranges), and it could stick anywhere. All it takes to free it is a light tap on the front panel. I don't wish to poke about more than necessary with the movevment, and - bearing in mind the post elsewhere on this forum - is there anything I should be looking at specifically? Bearings? Off-true pivot? Gunge? It doesn't seem to stick when a range is selected and the meter is agitated gently from side-to-side. All help appreciated! __________________ Regds, Russell W. B. G4YLI.

20th Sep 2007, 8:41 am	#2
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: AVO 9 MkII: Occasional Sticky Pointer Hi Russell Assuming that the pointer is free of the scale, the most likely thing is that there is some ferrous rubbish in the magnet gap and the coil is fouling it. The pivots are jewelled with hardened pivots, so I doubt if there is a problem there unless some severe damage has occurred. I've not heard (yet) of any problem with oil, and must admit that I don't know if the pivots were actually oiled at all (anyone?) - I suspect they were not. It is possible that someone has oiled them, and the oil has become sticky. Clocks I have handled frequently suffer from this sort of thing. __________________ Mike.

20th Sep 2007, 12:48 pm	#3
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: AVO 9 MkII: Occasional Sticky Pointer The random nature of the sticking is the best clue to the cause, with the caveat that diagnosis is unreliable without seeing the instrument concerned. It is likely to be caused by contamination of the pivots and,if so, is best cured by dismantling and cleaning. Damage to the pivot points due to excessive forces, for example if the meter has ben dropped could also be a cause. If the meter has been dropped, the moving coil is likely to be out of balance, which is easily checked. Another posible cause of random sticking is pivots which are set too tight. Sticking at a specific point is more likely to be due to particles in the air gap, but can also be due to pivot damage. Repair calls for delicate work, including unsoldering the outer ends of the hairsprings, unscrewing the jewel screws and removing the moving coil assembly. The pivots and jewels can then be cleaned with isopropyl alcohol. They are not lubricated. If there is pivot damage, they can, in principle be repointed. The technique is similar to reprofiling clock pivots and needs the same skills and equipment. After reassembly, the moving coil will almost certainly need to be rebalanced which is time consuming and best done with the movement enclosed in a draft-proofed box with a glass lid. The cut-out will also have to be re-set. Most of this work has been covered in other threads or is described in the official repair manuals. Amongst the causes of pivot contamination is the misguided use of contact cleaning sprays and exposure of the instrument to damp. If you are not confident and experienced in movement repairs, or if you do not have the facilities required, it is best to leave the work to a specialist. The fine properties of the instrument depend on the movement being set up correctly. Peter M Munro

21st Sep 2007, 3:45 pm	#4
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: AVO 9 MkII: Occasional Sticky Pointer Mike, Peter, many thanks for your kind and helpful replies. I'm hoping it's either gunge, or damp (through storage), or both. This is why I come to that conclusion: I conducted a little 'experiment' to 'exercise' the pointer (I can almost hear the mass tutting 'n' sucking-of-teeth as I type ). First of all I applied AC to the DC range, with some DC bias from a series-connected supply to get the pointer mid-scale. Nothing much happened, but the pointer was visibly oscillating when observed closely. I then connected it up to the output of an AC test-set on 'DC current', starting high and working down to achieve maximum needle-swing. I set the output of the AC test-set to +10dbm at 29Hz and gave it what-cheer for about thirty seconds. Afterwards, I ran the pointer up and down on various DC voltage ranges, and it hasn't stuck since! When it's quiet, I've the workshop to myself and a little breathing space, I shall examine the movement as best I can without demounting it, to see if I can apply a little isopropyl alcohol to the pivot-points without incurring damage elsewhere. I have no desire to attempt a proper repair and service, as I'm pretty sure I will make a muck of things. This sort of repair is better left to those who know what they are doing... __________________ Regds, Russell W. B. G4YLI.