Anyway, I have changed the preamplifier circuit somewhat and the cross
modulation now seems to have decreased a lot (not bothersome anymore) while the
gain seems to be about the same (I have no means of measuring the gain). At
least DCF has the same strength. Besides the mains noise seems to have
decreased very much, possibly due to the much higher standing current
in the FET:s.
One example is the noise generated by the diodes in the rectifiers for
the TX power supply (yes, I have capacitors, 0.1uF, across each diode).
Earlier I always had to switch off mains to the tx while listening.
This is no longer necessary. This noise has completely disappeared. The
band seems overall much quieter.
The following items have
been changed:
The 2N3819:s are replaced by high current FET:s
BF246C (the same pinout).
R3 and R4 are decreased from 2.2k to 270ohms
The
source lead in each FET is broken up and 150 ohm (see below though) in parallel
with 0.1uF are inserted.
The leads between the drains of the FET:s to
the bases of the 2N2222:s are broken up and 47k in parallell with 0.1
uF are inserted.
R5 is decreased from 1k to 180 ohms.
In my version I had to adjust one of the 150ohm
resistors in the FET sources to obtain approximately the same currents through
each FET resulting in that on one of the FET:s 150ohms had to be replaced by 120
ohms. To check this, measure the voltage on each drain. On the one with 150 ohm
the drain voltage was exactly 6V while on the other 6.6V. Decreasing to 120 ohms
resulted in 6V on both drains. No other adjustments seem to be
necessary.
The current consumption has now increased to about
72mA at 13.8V, divided into about 26mA through each FET and the
rest through the 2N2222:s. The resistance of the 1mH choke now has some
impact on the voltage to the transistors so I had to replace mine with a one
with lower resistance than the one originally used..
Hope this might help someone with the same problems
as me.
Have a nice weekend !
73 de SM5EUF / Urban
