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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 21st Jun 2018, 3:01 pm   #81
G0HZU_JMR
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Default Re: Meter Suggestions for Basic Alignment/Calibration

My first AVO lesson came from my dad when I was a small boy when he showed me how to test batteries with an AVO 7. I was too young to fully understand 'why' the AVO 7 was a good choice here but I remember he said 'it loads the battery as it tests it'.

Today, my mum still uses the same 500R/V AVO 7 to test batteries and fuses and if I had it here I'd rather use the AVO 7 to quickly and conveniently test a PP3 than my modern DMMs here. I guess a lot of service manuals are written for the AVO 8 and how it loads circuits as it measures them so it will always be a good tool for many to have. Especially for vintage high voltage gear. Beyond this it's probably not a good idea to try and categorise who should/shouldn't have an AVO

But if you compare it against a DMM it will obviously have a few limitations. I suspect it will be quite poor over temperature and it may even vary from device to device in this respect. So that 1% FSD target will probably get muddied by temperature, even in a fairly well controlled environment like a spare room at home. It would be much worse (across all seasons) in a shed/shack though.

I think Karesz made the best suggestions for how to manage the uncertainty of setting up the 37.5uA current. The only thing I'd add is that you would have to be careful what value shunt resistor you chose and be mindful of how much the DMM loads it when you measure the voltage across it. Best to use a very high impedance DMM here. I suppose you could look at the AVO needle to see if it twitches when the DMM is connected. The whole thing could be done for under a tenner if you include the cost of a cross checking voltage reference to boost confidence that the ~4.5 digit DMM is OK. Maybe also try and use a lowish voltage reference that is close to the voltage being measured across the shunt? The reference could be used as the test source then but you would still need to do the DMM voltage measurement across the precision shunt resistor to check the current is correct. Probably quicker/easier to use a variable PSU in place of the reference and just use the reference to check the DMM on the voltage range.
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Old 21st Jun 2018, 3:22 pm   #82
David Simpson
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Default Re: Meter Suggestions for Basic Alignment/Calibration

What a coincidence - talk of Electronics & Metrology, eh? My last posting in the RAF involving in-depth Electronics was as an electronics instructor, in the early 70's, at RAF Cosford. Right next door to our classrooms & workshops was a Government Metrology Laboratory. Seemingly that part of Shropshire is the most geological stable part of the UK. Boy their test equipment was way ahead of anything I'd ever seen. Accuracy of 10 to the minus nine was nothing to them. Ultra level stainless steel tables, some even toughened glass floating on Mercury.
An earlier posting at RAF Cosford was working in their Test Equipment Workshop. AVO7's & Hartley 13A Scopes were our bread & butter, and a few AVO8's.

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Old 21st Jun 2018, 3:24 pm   #83
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Default Re: Meter Suggestions for Basic Alignment/Calibration

I'd also add that it probably isn't a good idea to apply a lot of heat to the precision test resistor. If this is unavoidable and you want to solder it then I guess you could use a heat shunt to minimise thermal stress to the resistor and measure it before and after with a DMM and look to see if the measurement changes after soldering. You obviously don't need a calibrated meter for this test because you are checking for a change in resistance that indicates a degradation.
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Old 21st Jun 2018, 6:20 pm   #84
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Default Re: Meter Suggestions for Basic Alignment/Calibration

I'm glad i'm not the only one who tests PP3's with an Avo. (An Avo40 is even better, nothing like pulling 4mA to get an honest reading.) You need earplugs if you do it while it's still connected to the smoke alarm..but that generally applies to DMM's as well.
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Old 21st Jun 2018, 7:57 pm   #85
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Default Re: Meter Suggestions for Basic Alignment/Calibration

Moving pointer meters are just so ergonomic for normal everyday uses.

Conservation of energy assures you that if the pointer moves, there must be something there.

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Old 22nd Jun 2018, 10:43 am   #86
The Philpott
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Default Re: Meter Suggestions for Basic Alignment/Calibration

My first avo lesson was at a similar age to Jeremy's,

Dad showed me how the high ohm range could measure the resistance of my skin, then quickly pointed out that i was reading it off the wrong scale. I don't imagine the designers envisaged it's use as an educational tool, but it certainly grabbed a child's attention just by being different.
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Old 22nd Jun 2018, 10:53 am   #87
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Default Re: Meter Suggestions for Basic Alignment/Calibration

Quote:
Originally Posted by The Philpott View Post
I'm glad i'm not the only one who tests PP3's with an Avo. (An Avo40 is even better, nothing like pulling 4mA to get an honest reading.) You need earplugs if you do it while it's still connected to the smoke alarm..but that generally applies to DMM's as well.
I routinely use the 10A range on an Avo 8 to test 1.5V cells (usually AA and AAA). Good ones will deliver 3A or more, dead ones will only swing the needle to about 0.5A or less. A brief test is all it takes, so the battery doesn't get unnecessarily drained. It's a quick way of sorting the wheat from the chaff.

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Old 22nd Jun 2018, 12:18 pm   #88
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Default Re: Meter Suggestions for Basic Alignment/Calibration

Quote:
I don't imagine the designers envisaged it's use as an educational tool, but it certainly grabbed a child's attention just by being different.
I can remember being thrilled just to be able to use my dad's AVO 7 and turn the controls. The sight of the needle sweeping gracefully across the detailed and exotic dial on the AVO 7 was also something I'll always remember. Also, the sense of 'knowledge' it brought was inspiring too, even if I was just a boy looking at differences between tired batteries.
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