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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 22nd Feb 2018, 11:23 am   #1
newlite4
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Default AVO8 MK3 reading high

AVO8 MK3 reading high on AC/DC volts and resistance ranges by 30%, current ranges severe overload of movement causing the cut out to operate. This is the sixties version with the diode and thermistor in the shunt circuit. Any pointers as to where to look?
Neil
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Old 22nd Feb 2018, 11:45 am   #2
WME_bill
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Default Re: AVO8 MK3 reading high

Sounds as though the universal shunt chain in parallel with the movement has gone open circuit. So the moving coil movement has reverted from 50 ua FSD to its native 37.5ua, which is why everything is reading high.
The thermistor is across the swamp resistor, in series with the movement. The diode is across the movement. As the movement responds, neither of these can be failing. Trace out the panel with the shunt resistors and check continuity - 8k+1.2k+300+450+43+ to total 10kohms. wme_bill

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Old 22nd Feb 2018, 12:48 pm   #3
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Default Re: AVO8 MK3 reading high

Thanks Bill, that has given me a head start as to what to look for,
Neil
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Old 22nd Feb 2018, 3:26 pm   #4
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Default Re: AVO8 MK3 reading high

Brilliant, just checked over lunch, 0.45ohm O/C as if the resistance wire had snapped in half, will select 0.45ohm this afternoon using the low ohm meter. Thanks Bill,
Neil
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Old 24th Feb 2018, 6:34 pm   #5
The Philpott
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Default Re: AVO8 MK3 reading high

So that was the 1.0A (5 turns) shunt that had failed then? Rather unusual.

Dave
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Old 25th Feb 2018, 8:46 am   #6
Ed_Dinning
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Default Re: AVO8 MK3 reading high

Hi Neil, I may have some resistance wire if you need it.

Ed
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