Phil, there are bandpass filters on the LO Mixer board.
If the LO signal falls low enough, the DBM losses will increase. The post
mixer amp provides a proper impedance terminal for the mixer, and makes up
for some of the conversion loss.
As I recall, those LO mixer filters covered more than one band, did you
check all bands to see if the problem is on more than one band? It
appears that 40 and 30 meters share the filter on the 81596 board.
I would at this point assume that the JFET amps are working, and measure
the LO by removing the coax cable going to J26, and measuring the LO
voltage directly on that cable, using a 50 ohm terminating resistor. That
will show you the approximate drive levels from the LO mixer board.
Pete
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