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Old 18th Nov 2020, 10:00 pm   #1
The Philpott
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Default Magnet calibration in Avominor- experiment

The magnet shunt in this early DC (HR) Avominor was falling short of FSD by only 1% - having the original tamper seals on the rear i was quite impressed, but whilst the case was open to add a resistor to bypass a failed bobbin i thought to have a try at achieving an improvement.

I increased the angle of the shunt to decrease it's effect on the magnet, but (as sometimes happens) the rounded end then entered the path of the needle/pointer and wouldn't allow it to move past 40% FSD!

I know that removing one of these shunts altogether is hardly ever an answer- it usually results in an increase in flux such that the pointer then exceeds FSD by 1 to 2%

The shunt is around 26mm long, and as I have a spare I cut 4mm off the pointy end of said spare to assess whether this would modify its effect on the flux- it did, but not quite enough. Cutting an additional 2.5mm off the rounded end made a further improvement on its suppressive effect, and also allowed unrestricted movement of the pointer. I filed the sharp bits from the cut edges, a bit concerned that it would hold some magnetism- but it is clearly made of the 'right stuff' as what little attraction it held faded away. Full calibration achieved at around 50 degrees of swivel. Success!

Other things had to be done to this meter (covered under the Avo survey thread) but i was really quite pleased with this experiment

Dave
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Old 19th Nov 2020, 6:59 pm   #2
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Default Re: Magnet calibration in Avominor- experiment

I wouldn't be at all surprised to find out that a similar technique to yours would have been used by ACWEECO technicians for meters sent in for repair.

I call what you have done good thinking.

PMM
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Old 31st Mar 2021, 6:33 pm   #3
The Philpott
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Default Re: Magnet calibration in Avominor- experiment

Update-

I have now repeated this procedure on a Type E testmeter with similar success. This time lopping 4mm off the pointy end of the magnet shunt was sufficient- the fat end was left alone.

An unusual problem with the 2v bobbin reading 2% too high....had to be remedied with a piggy-back resistor, after which testing showed the 2v,4v and 20v ranges to be very accurate, and in synchrony with each other. A bit of head scratching before i realised what was wrong...you cannot always rely on the bobbins being spot-on.

Dave
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Old 31st Mar 2021, 7:13 pm   #4
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Default Re: Magnet calibration in Avominor- experiment

You could also try raising an uncut one by a few mm on a non-ferrous washer as a spacer.

David
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Old 31st Mar 2021, 10:31 pm   #5
The Philpott
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Default Re: Magnet calibration in Avominor- experiment

-Didn't think of that, sounds like it would work. Would require a longer BA screw in this instance.

The awkward bit when replacing the screw is that as the shunt sticks to the magnet it's virtually impossible to see if the holes are lined up, and the shunt (when your back is turned) has a tendency to pull to a position in which it's comfortable, which never lines up exactly with the tapped hole. Blu-tak on a cocktail stick helped run the screw in, and I think a cranked piece of copper rod would be useful for locating (A boon would be bronze miniature BA spanners- the hunt is still on for them..)

Dave
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