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Old 13th Mar 2019, 9:29 pm   #1
DigitalNoMore
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Default Avo 8 Mk5 Broken Pointer

Just wondering if anyone could offer any advice on this:

A couple of years ago I acquired an Avo 8 Mk5 which I've finally got round to having a look at. Everything appears to work okay apart from the movement zero screw which has no effect on the pointer when it is turned.

On dismantling and removing the movement I noticed a large splash of solder towards the bottom of the dial and also the pointer was slightly bent and appears to have been broken and then crudely soldered together at a place where it has been broken. This repair looks very delicate (I suspect it's flux rather than solder holding it together) and I'm reluctant to put the whole meter back together again with it still in place. (The zeroing issue just seems to be caused by the peg on the back of the screw not having been located properly in the prongs of the fork on the movement).

Is it worth my while (or possible) to try and repair the broken needle by some means eg. a blob of epoxy or does it really need a new movement?
Any advice would be welcome please.

Cheers,

DNM
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Old 14th Mar 2019, 3:32 pm   #2
See_Mos
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Default Re: Avo 8 Mk5 Broken Pointer

I think the pointers are made of aluminium so cannot be soldered, well not easily anyway.

I have three that I don't need, awaiting attention. I will dig them out and see what there is.
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Old 15th Mar 2019, 12:07 pm   #3
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Default Re: Avo 8 Mk5 Broken Pointer

Thanks See_Mos.

Upon closer inspection one of the movement hairsprings is distorted. Even if I did manage to glue the pointer back together I suspect that the extra mass of the glue along with the altered tension of the hairspring will make the movement inaccurate.
So, it seems that the only solution would be to replace the movement.
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Old 15th Mar 2019, 4:58 pm   #4
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Default Re: Avo 8 Mk5 Broken Pointer

I have one MK5 that is labelled 'to test' I haven't checked it yet but the movement looks OK and the balance is quite good IE the zero varies little from upright to laid flat. Yes, I know they are only supposed to be used horizontal!

The others are two MK7 and one that I think may be a MK6

The one I keep on my bench is also a MK5
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Old 15th Mar 2019, 5:14 pm   #5
WME_bill
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Default Re: Avo 8 Mk5 Broken Pointer

Repair broken pointer
I suggest a scrap of dried grass seed stem. Comes as an amazingly light tube. Slip it over the broken sections to join together. Secure with a scrap of glue or shellac.
Now go and select the dried stem. Spring is not the best time, so you may have to wait for the autumn for a nice dry one. Keep looking during your summer walks.
You can even use one to replace a complete pointer. Amazingly light.
This suggestion came from Wireless World about 1955 for repairing military surplus meters.
Bill.
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Old 15th Mar 2019, 9:28 pm   #6
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Default Re: Avo 8 Mk5 Broken Pointer

DNM,

WME_Bill's suggestion is good advice. It will certainly upset the balance of the movement but that is true of many repairs and rebalancing is not too difficult with patience.

Hairsprings often look much worse than they really are. Avo's repaire manual advises that tangled hairsprings should be carefully traced outwards from the centre with a small needle as this can sometimes free them. Other distortions can be carefully straightened one part at a time using non-magnetic tweezers.

PMM
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Old 19th Mar 2019, 1:01 pm   #7
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Default Re: Avo 8 Mk5 Broken Pointer

Firstly, thanks for all your suggestions.

I've managed to untangle the top hairspring using a needle and have also managed to glue the pointer back together using a very small blob of epoxy.

With the movement back in the case however and switched to the 10V DC scale, a PP3 battery which measures 8.88V and 8.85V on my two DMMs is only measuring 6V. This is an error of over 30%.

Anyone have any ideas what the problem may be?
I wouldn't have thought that the weight of the small blob of epoxy could have that much of an effect.
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Old 19th Mar 2019, 1:07 pm   #8
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Default Re: Avo 8 Mk5 Broken Pointer

Quick update:

The 30% error seems to be present on both the voltage and current ranges so I suspect it could well be an issue with the movement or possibly the shunt?
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Old 19th Mar 2019, 1:51 pm   #9
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Default Re: Avo 8 Mk5 Broken Pointer

In these meters, there should be a chain of shunt resistors known as a 'universal shunt' connected permanently across the movement. If in any part of this shunt is open circuit, the apparent sensitivity of the meter will be increased as you describe. This will be confirmed by very little damping of the pointer which will swing quite wildly. Normally these meters are quite heavily damped.

If you don't have a service manual, please send me a private message with an email address and I will get it to you.

PMM
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Old 19th Mar 2019, 2:21 pm   #10
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Default Re: Avo 8 Mk5 Broken Pointer

I have got a copy of the service manual thanks.

In this case the meter is reading 30+% low so the sensitivity of the meter is reduced. If there is an issue with part of the 'universal shunt' across the movement then it must be the case that part of it has gone out of spec low rather than open circuit.
I'll check this out later.

Having measured the resistance of the movement on the 50uA range it is measuring 1996 Ohms rather than 2667 Ohms which I've read on here is the correct value. This does tend to suggest a shorted or too low resistance for part of the shunt and would account for the 30% error.
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Old 19th Mar 2019, 10:16 pm   #11
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Default Re: Avo 8 Mk5 Broken Pointer

DNM,

Sorry I didn't read your previous post correctly.

Is the value of 1996 ohms the moving coil plus swamp, or just the moving coil?

In any case, the low reading is worrying because it does tend to suggest that there are shorted turns as otherwise the proportion of the current in the moving coil would be higher than normal giving a high reading.

The three full scale deflection figures for the movement plus swamp have to be right 37.5 microamps, 2667 ohms and 100mV. The swamp in these meters is an almost square ceramic "tile" but a multi-turn cermet trimmer can be used and has the advantage that it can be easily adjusted to give the correct 2667 ohms.

PMM
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Old 20th Mar 2019, 2:27 pm   #12
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Default Re: Avo 8 Mk5 Broken Pointer

PMM,

No problem and thanks for your help.

The 1996 Ohms is the movement plus swamp plus shunt and measured across the meter terminals with the meter on the 50 uA DC range.

I haven't measured the meter + swamp on its own yet.

Do you happen to know whereabouts the swamp is mounted?

I'm starting to wonder, like you, if the problem might be shorted turns in the moving coil.

DNM
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