[vcf-midatlantic] Apple IIe Voltmeter
Duane
DBC1964 at cox.net
Fri Nov 13 16:34:08 EST 2015
Yeah,
When I was an electronics mechanic I kept a Simpson 269 on permanent check
out, only surrendering it for calibration if they promised to give it back
to me.
100,000 ohms/volt with a 7" mirrored scale. It was like having a calibrated
eyeball with a vernier attachment.
Duane
-----Original Message-----
From: Systems Glitch via vcf-midatlantic
Sent: Friday, November 13, 2015 12:20 PM
To: vcf-midatlantic at lists.vintagecomputerfederation.org
Cc: Systems Glitch
Subject: Re: [vcf-midatlantic] Apple IIe Voltmeter
> But I found my Simpson 260 and other analog meters, put LESS load
> because they generally use 50uA movements (hence 20,000 ohms per volt,
> do the math).
Yep, one reason I don't have any non-Simpson analog meters anymore!
> As to "figuring out the accuracy".....create a 10mA
> circuit from an accurately-measured battery and an accurately-measured
> resistor. Say 5V and 500 ohms, or whatever - the DVM can measure those
> fine, other resistors give other current sources, and math gives you the
> exact current. Many batteries can generally supply 10mA stable for
> minutes and hours or more, and your DVM can monitor their voltage anyway.
Exactly my plan. I just needed something I knew was accurate on the voltage
measurement. That's the problem with two instruments you're uncertain of --
"a man with a watch always knows the time; a man with two watches is never
sure!"
> Why? Given the "digital is better" bias these days, only a few people
> call out the Simpson 260 as a valued instrument - Dave McGuire for
> instance, in that email list we used to use.
I prefer an analog meter for troubleshooting -- like you said, easier to
detect an "is it off" condition, especially with intermittent circuits. And,
the loading of the analog meter can be an advantage in many situations. My
Simpson 260 is always on the bench, and I keep the Simpson 160 in my toolkit
since it's just the tiny version of the 260.
> Jonathan, if you determine a cause for your analog meter's apparent
> errors, let me know.
I opened it up this afternoon, it looks like the shunt for the 100 mA scale
is wire (nichrome?) wrapped on a phenolic bobbin, typical of old small-value
precision resistors. It's sleeved in some sort of plastic tube which appears
to have been clear originally, but is now green over the section of wire.
I'm guessing you're right and corrosion has thrown the value off.
Thanks,
Jonathan
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