My old faithful SB-220 has been (and continues to be ) running just
fine. Today, however, I noticed that the plate meter was sitting about 40
ma above the 0 line. It seems to be a mechanical change, because it sits
there with the amplifier completely off. But here's the rub -- the resting
current, which had read 120-130 ma, is now (with the meter readjusted to
put the needle back on zero) only reading about 70 ma. At 110 watts from
the exciter, grid current is ~135 ma and plate current 750 ma, showing
about 1250 watts out in SSB position -- in other words, apparently healthy.
It seems unlikely that the diode string (I have the Harbach metering board
which replaces the bias zener with a string of diodes) is having problems,
so is it more likely that the metering has changed? Is there any
connection with the change in the mechanical zeroing of the plate meter?
It occurs to me that a couple of years ago the multimeter stopped working
in the relative power and HV positions -- it still works for grid
current. Could the switch be progressively failing, since that's about the
only component in common between the two metering functions, it
appears? If so, does anyone know of a replacement switch with the proper
indexing to match the panel?
73, Pete N4ZR
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