Hello Top-o-hill Banders!
I did some more goofing around with 870 mods. I did the intermod mod, it
helped (fun with "invisible" SMT resistors!), but I must have neglected gain
adjustment by pot ahead of IF amps, rx seems to be less "hot" which I can
tolerate for now on 160, using preamp anyway. I will try to use SQUELCH pot
(useless for contester) for separate RF gain control.
I put in IR 2.1 kHz 8.8MHz 2nd IF filter and it seems to improve out of
passband strong signal problems. When I plotted the curve for old original
Kenwood filter and IR, there seems to be exactly the same slope/angle on both
filters. I figure that this is probably thanks to DSP stuff, because I know
from 930 that there is a big difference between those filters. So conclusion
is that on SSB there should be some significant improvement from overloading
and brute force narrower filtering, but skirts/slope is about the same
(excellent). I tried also 1.8 kHz bw filter, had the same results, narrower bp
curve, but slope/shape factor remains the same.
When I tried to put 400 Hz filter at 8.8MHz IF, CW passband at normal
settings went screwy, almost no signals getting through, Appears that this is
due to the frequency shifting arrangements and DSP "cooperation". Appears
that Kenwood uses SSB filters for CW by shifting the passbands and oscillators
(done by microprocessors). This needs some more digging around. There is a
room for two filters and switching, so still possibility.
The 3rd IF 455kHz has small ceramic filter abt 3kHz bw, I will try to
replace that one with IR 2.1kHz, but it might not be necessary due to DSP
function and brick wall 2nd IF filter ahead.
2nd IF filter mod is not too difficult to do. The RF board has to be
removed, original filter unsoldered and removed, then run two pieces of coax
to the new filter mounted on the side of the board.
Now the TS870 is a lot more contester friendly radio! DSP is even better
now. I would make it now No1 contestpedition radio. I will put it through test
of fire in WPX SSB from VE1 on 160.
Hope it helps 870 fans.
73 de Yuri Blanarovich, K3BU, VE3BMV
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