|[AMRadio] Update on BC-1T|
k4kyv at charter.net
Mon Mar 5 08:52:57 EST 2007
The idling modulator plate current on mine drifts too. From a cold start,
it usually shows below the normal 40 ma per tube. Sometimes it starts out
at a little over 40 ma total, but after a few minutes it climbs upwards to
the normal value. I suspect some of the resistors on the driver board are
shifting in value as they heat up, causing the bias voltage to drop from the
initial value. It should be a matter of simply measuring all the voltages
in the DC amplifier/audio driver chain to see where the voltage shift comes
from. It could be one or two resistors substantially changing value, or it
could be the cumulative effect of a slight drift in several resistors,
with each one individually having little effect on the bias voltage (more
likely, since per Murphy's Law, that would be the more difficult problem to
diagnose and correct).
Mine neutralised very easily the first time, and the setting holds from 1800
to 2000 kc/s. That surprised me, because from my experience, grid
neutralising does not work as well as plate neutralising, especially when
the overall circuit is as unbalanced as that of the BC-1 series
transmitters. But I doubt that neutralisation would hold from band to band
if you tried to make it cover more than just 160, unless you had some way of
moving the tap on the grid coil with each band setting.
I modified the xtal oscillator stage so that it serves as a grounded grid
buffer stage. I grounded the grid (at rf, don't recall if I grounded it
directly or through a by-pass capacitor). I disconnected the capacitor
between the cathode of the 12BY7 and the xtal feedback circuit, and inject
the vfo voltage directly onto the cathode through a coupling capacitor. My
vfo has a low impedance output through a small link coupling coil,
developing about 200-250 milliwatts into a 50-ohm load. It drives the stage
very well, through a 10' piece of RG-59 between the vfo and the transmitter.
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