I just spent another night poking around in this thing.
Sniffing around the transmitter with a receiver and very
small probe "antenna" I am reasonably certain the spur is
present at Q305 (exciter mixer) and all subsequent stages.
Using a general coverage receiver the fundamental output of
the channel element (19.195833 MHz) seems free of spurs. I
realize the third harmonic of the channel element is
selected by the filter but I have no receiver covering that
range.
Similarly, the 16.7 MHz out of the offset oscillator seems
OK. There are some very weak (more than -100 dB) spurs
above 16.7 but it is absolutely quiet at 15.790 which is
what would be required to cause my +910 kHz spur.
So I suspect a problem in either the exciter mixer (Q305) or
somewhere in the injection multipliers (Q101, 102, 103) on
the receiver RF & IF board). No amount of tuning or
detuning these stages eliminates it.
It still seems odd that I have two Micors with an identical
spur problem.
Any thoughts on what to try next?
Paul N1BUG
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