Lane,
A better method rather than measuring the shunt is to set up a bench supply
with a series limiting resistance to put a specified amount of current thru
the meter/shunt assembly and calibrate it. For most current values
encountered in amplifiers, you can put a DVM in series to measure the
actual current, and adjust the shunt for correct indication.
I have one helpful hint about shunts. I have a homebrew 8877 amplifier
built by one of the well-known amplifier guys in Dayton, OH. The plate
meter is a 0-1 mA movement shunted to read 0-1.5A. I always suspected the
meter read high, and eventually the meter started to read higher and higher
to the point that during one CQWW CW contest the needle started to
violently slam against the stop (power output and all other condx the
same).
Subsequent analaysis showed that the shunt was mounted to a small PC board
held onto the meter posts. The way the board was constructed, the leads
from the amp circuitry made contact first to the meter posts, then to the
shunt resistance. Over the years, a small amount of oxidation built up on
this connection to the shunt. The effect was to slowly raise the effective
resistance of the shunt, thus causing more current through the meter
movement and causing the calibration to slowly drift high.
The fix was simple. After cleaning the connections, I soldered the wire
leads from the amp circuitry direct to the shunt. Any oxidation build up
now only affects the path to the meter movement, the impedance of which is
many orders of magnatude higher than the shunt. This configuration has
worked fine for several years with no calibration drift.
73,
Dave/K8CC
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