carl / kz5ca and I have exchanged some info regarding 572B's and the
tendency of the amp to blow the 33 ohm 1 watt resistors in the grid
bias circuit of the FL- 2100 and the lack of grid metering on that
particular amp. I recalled that the Heath SB200 had a similar
circuit so I downloaded the manual from BAMA and looked at it
closely. The two circuits are almost identical. The grid circuit
resistors and caps are identical @ 33 ohms and 200p as are the rfc's
and its shunt resistor @ 3.3K ohms.
Heath meters the grid across the bottom of the RFC and a shunt
resistor of 1.5 ohms in series with the rest of the bias circuit. On
the 2100 the RFC is connected to the bias set resistor (R4 of 30
ohms and 10 watts) at its set position which is common to one end of
this resistor and the antenna relay coil. The opposite end of the
bias set resistor is switched to ground along with the other side of
the relay coil when the operate-standby switch is put in the operate
position. I inserted grid metering between the RFC and the bias
resistor set contact.
I was expecting to see a grid current of 0 ma with the operate-
standby switch in the "operate position", exciter set to "on", which
grounds the FL2100B antenna relay, but provides no drive from the
exciter. Instead the meter indicted 160 ma of current. I have
checked this current with two different meters. One a 1 ma meter of
internal resistance 86 ohms shunted with .44 ohms and the other with
a digital multi-meter. The latter showed 158.5 ma. I wonder where
this current is coming from?
Suspecting something else was wrong I recorded the following voltages:
Plate voltage: 2300 volts.
Grid RFC to ground (operate-standby switch in operate) with exciter
on standby: -18.4 volts.
Grid RFC to ground (operate-standby switch in operate) with exciter
on send, but not keyed: -2.7 volts.
Cathode to ground, measured at the centre tap on the transformer,
Zero volts with the antenna relay not grounded and 0.007 volts when
grounded. I mention this because I have installed a relay to lift
the CT ground on the heater transformer, as per W8JI, Tom's advice on
his website regarding the FL-2100B's poor bias design, and was
concerned that maybe this ended up floating the cathode above ground.
Again, my question, where is this current coming from?
VE6TI
gordon Hungerford
ve6ti@telus.net
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