Larry,
>>You folks are all saying "carrier leakage". I thought that as well when playing with my rig. It turned out, in
my case but I expect yours as well, it is not the carrier but the unwanted sideband. <<
First as this is not the first time dealing with this, experience is from many prior
radios and the adjustment of many other commercial and kits.? ?There must?be
NO audio or other input to the modulator other than the BFO.? If that is true then
there cannot be another side band. and one can proceed to dial in the BFO.? Also any
measurement of carrier leakage requires that as a start.? Then one can add single or
two?tone audio and look at opposing sideband and carrier with modulation.
First the BFO must be set correctly and about 300hz down the edge of the filter.? More
and the audio suffers (RX and TX) from a lack of lows.
What? I measured is with audio shorted at the modulator and that cannot create another side band.?
Its the exact frequency of the BFO and its there if I remove the balanced? mod transformer and
diodes.? ? Its a board level leakage path of BFO pas the filter into the 12mhz?IF amp.? Also based
further work the balanced mod is well behaved and the carrier null should be at least 40db or better.
The first thing done was to ground the crystal cans, that improved filter leakage a few DB
especially at the edges.
The second was the filter in mine was 1670hz at the 6db points, terrible for voice work.
Change the caps to 82PF and got a 2150hz wide filter, on the tight side but much better.?
The filter shape factor is a rather soft 2.5:1 at -60db, (5300hz) that is typical for this
type of filter.
Then I set the BFO based on the filter sweep for mine that ended up being 11.99675
for about 10db/300hz down the slope.? ? Most the 11.9962 would result in the lower
700-1000hz part of the vice band being seriously attenuated both R and TX.? However
your filter could be offset lower over all, not uncommon.? FYI the upper edge is just
under 12.00000mhz as that is not the center frequency.
However the carrier due to the board issues carrier will still be there.? I've seen this on 5
units to date.? I have not seen it less than -33dbc, one CW op never noticed save for he had?
signals on the other side of zero beat, moving the bfo helped that.
An experimental fix was put a 12mhz crystal osc dialed to 11.99675mhz (for mine) and the balanced
mod in a mint tin connecting it to the filter input with a few inches of coax, same for audio.? Result
good modulator behavior and carrier at least 48db down and audio band pass starting at about
250hz to 2450hz (the filter 2150hz wide).
Allison
