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Old 27th Sep 2021, 10:39 am   #21
pmmunro
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Default Re: avometer 8 reads high

These windings can be replaced with any suitable resistor if you are not concerned about keeping the meter original. The criteria are accuracy, temperature stability and power dissipation. Ten turn pots. are perhaps an expensive way of getting the correct resistance, unless you have some already.

AVO used quite low specification carbon resistors to trim wire wound shunts and multipliers, presumably reckoning that their contribution to the overall value was small enough to retain the accuracy of the meter concerned.

PMM
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Old 27th Sep 2021, 11:27 am   #22
The Philpott
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Default Re: avometer 8 reads high

Agree. A selection of 1% or 2% tol. resistors is what's needed, you can then hand-pick the closest ones, often two in series or parallel is suitable since as you're aware the values you need are hardly ever available (as a single component). Sengpielaudio has a useful series/parallel resistance calculator tool incidentally.

Leave the o/c board windings in place, just snip them away from their terminations to prevent them from ever coming back to life and skewing the shunt values. Removing these old windings just creates a shower of debris; best left in situ.

Dave
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Old 27th Sep 2021, 1:06 pm   #23
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Default Re: avometer 8 reads high

Quote:
Originally Posted by Superscope View Post
So basically you must replace the entire Movement/Scaleplate/Shunt and all from the Donor Meter.
Be careful to avoid any contact between the magnet and your screwdriver. Don't rely on a steady hand, the magnet is powerful and will grab the screwdriver instantly and be degraded. Packing of some sort is a good idea. The time honoured expression is "guess how I know".
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Old 27th Sep 2021, 5:31 pm   #24
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Default Re: avometer 8 reads high

Also, don't assume the Windings are all un-repairable.

They can on occasions go High Z
where they connect to the Terminations and might
just need disconnecting, cleaning and resoldering.

Sometimes it is also possible to find a break and
resolder the ends back together.
It is very difficult to solder Manganin wire though.

They can also be re-wound, but that's not for the faint hearted!
especially the long ones.

I have had success on occasions with all three of the above methods.





Ian
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Old 27th Sep 2021, 6:14 pm   #25
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Default Re: avometer 8 reads high

Post No.23- Yes, also insulating the screwdriver from metal-to-metal contact and just leaving a short length bare at the end improves your chances. Silicon sleeving, heatshrink sleeving, insulating tape, whatever is to hand.
dave
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Old 28th Sep 2021, 9:38 am   #26
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Default Re: avometer 8 reads high

do the resistors 2-3 and 4,come into play on voltage ranges or just for the amps ranges?
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Old 28th Sep 2021, 3:48 pm   #27
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Default Re: avometer 8 reads high

-Two of them appear to be devoted to AC LOW VOLTAGE ranges and from the circuit diagram it looks as though they can be o/c without affecting any other ranges. Consumption of the meter at full scale is quite low even on these ranges,so there is not a huge amount of heat dissipation going on.

i am still looking for the 1200 ohm shunt on the circuit diagram!

Dave

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Old 28th Sep 2021, 4:02 pm   #28
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Default Re: avometer 8 reads high

Additional- The circuit diagram i have in front of me is for a later MkII so there could be detail differences....but so far i'm guessing your AC ranges of 2.5v and 10v should be dead- and possibly also the 25v...?

Dave
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Old 28th Sep 2021, 6:25 pm   #29
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Default Re: avometer 8 reads high

does this help?
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