>"I've not looked at the Alpha 70 circuit, but from the discussion it
appears that the HV is measured by putting a 1 mA meter in series with a
high resistance voltage divider, with a shunt across the meter to give the
correct calibration. "
In the case of the Alpha 70, changing the 15K resistor to another value
won't calibrate the meter unless it reads high -- and even then, there's
almost a 500:1 ratio between the meter's 32 ohm internal resistance and the
shunt. In effect, the 15K resistor is bridging the meter with a high Z, and
therefore has no effect on calibration unless the resistor is dropped to a
very low value, something less than approx. 10x the meter's resistance.
That means less than 500 ohms. So, in the current 70A design, the 15K
resistor only functions as an HV pull-down on the switch terminal. Hey, I'm
glad it's there! The HV meter is calibrated only to the extent that the
divider resistors and meter are within tolerance, and no other high
resistance paths are in the way.
Apart from a better design like the one you developed, to make the shunt
work as a calibration device - in addition to a pull down HV on the switch
terminal, Alpha could have designed the divider with a lower resistance --
say 4 meg instead of 5 meg. Then a 1K pot as a rheostat could have been
placed across the multimeter terminals when the multimeter switch is in the
HV position. Who knows, maybe Alpha thought that within the tolerance of
the meter and 1% resistors, the resulting accuracy would be "good enough."
Paul, W9AC
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