It's getting scary this close to the deadline.
Found the problem with the excessive transmit sidetone. You won't believe
this. Turns out that the MC1350 IF amp tends to want to oscillate. We
already knew that, didn't we. This one seems particularly hot. Since it is
surrounded by 12 MHz crystals, that is where it takes off. I can shunt
the input or the output with a cap and make it stop. I played around with
the circuit for a while to see if I could identify any suspect feedback
paths, but it kept coming up as excess internal gain. Now, this amp has a
gain control pin which is used by the AGC circuit. The AGC circuit has a
'threshold set' pot which actually changes the bias on a JFET in the AGC
amplifier so it moves the JFET transfer curve. One of the last things I
did yesterday was to set this pot according to the kit instructions. That
was a mistake. When the amp was on the edge of oscillating, the transmit
transient would be just enough to turn the receiver into a regenerative
receiver with phenomenal gain. Today, I readjusted it to reduce the
maximum IF gain just a bit so that the IF amp would not break into
oscillation. Result is normal clean transmit sidetone, and, I think, good
receive sensitivity.
Also played around with adjusting the transmit drive. It is clear that too
much drive causes a bit of distortion and key clicks. As others have
suggested to me, trying to get five watts or even four watts out of this
design is pushing it. I think we'll have to be happy with 3.5 watts of
clean signal.
Time to put the cover on and field test.
Mike K1MG
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