When I got the TS870S first I was really not amazed about the selectivity, also not about the quality of the build-in front end. But the filter-mod and also Kenwood$B!-(Bs front-end-mod helped to solve these problems. But there was one additional possibility to improve something. When I reduced the output power to drive a bigger linear, I found, that the ratio is changing
between SSB in comparison to modulate with a single-tone. If the percentage
was 50% between voice and CW using 100W output before, the ratio is
dropping down if you are running only 10W (measured with a peak-meter).
That is not very helpful, because it$B!-(Bs not easy to drive the linear
correctly in that case. And you can$B!-(Bt improve that using higher microphone
gain or processor. The reason is Kenwwods ALC-curcuit (it$B!-(Bs simmilar to
the TS50S) which can be found on the RF-unit.
In the curcuit diagram it$B!-(Bs named "Reserve Current Prevention". This part
is directly controlling the TX-amplifier Q39. And this part has normally
nothing to do with the power adjustment or regulation. But using lower
output it$B!-(Bs more sensitive and therefor more effective.
Modification:
Turn your TS870 on it$B!-(Bs head (foots up) and remove the cover. If the
display is looking into your direction, the big curcuit board on the
left-hand-side (underneath an additional metallic shielding) is the
RF-board. Remove all connections carefully and afterwards the complete
board. There are 3 electrolyte capacitors named: C286, C287, C289. Place a
small resistor (I used a small chip-resistor) with 1 KOhm in parallel to
C289, which is in the middle. Best is to do that on the soldering side
(not component side).
That$B!-(Bs it ! Bring the TS870 back to live and afterwards you will see the
improvment in SSB-mode and you can easily measure it. Everyone told me
afterwards, pumping effects are gone and my voice sounds fresh and clear
(is that possible with an age of 40 ?)
If you are only running your rig with full output, it$B!-(Bs not neccesary to
realize this mod. In my case I am going down to 5 Watts - afterwards I$B!-(Bm
driving an attenuator going down to 20 mW (-24dB) and this is driving a
commercial russian linear with the possibilty of 2KW out (+50dB). Therefor
it$B!-(Bs absolutly necessary to have stable conditions.
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