[vcf-midatlantic] Apple IIe Voltmeter

Dave McGuire mcguire at neurotica.com
Fri Nov 13 15:22:36 EST 2015


On 11/13/2015 12:20 PM, Systems Glitch via vcf-midatlantic wrote:
> 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.

  Same here, and the 260 is the king of 'em all.  I have an HP 3458A on
my bench (google it) but I end up using the 260 more, especially during
calibrations.

  Stuff like the 3458A and other ultra-precision digital meters are
great for measurement, while analog meters are much better for
*adjustments*...a distinctly different application, while both involve
measuring voltage. (or current, resistance..)  The difference of course
being that it takes less real-time brain processing to see whether a
meter needle swings left or right than to read a displayed number,
interpret it, decide if it's higher or lower than the last displayed
number, ad infinitum.  Doing that in real-time while tweaking an
adjustment is a lot less efficient.

> 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.

  The resistance wire alloy would be manganin (copper/manganese/nickel).
 Nichrome is rarely if ever used for actual resistors due to its very
high temperature coefficient.  Manganin's is negligible.

  You should be able to replace that with a modern resistor without
trouble and get the ol' girl back up and running.  Those DMMs are far
from elegant, but they work very well.

                 -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA



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