http://www.radiomods.co.nz/kenwood/kenwoodts870.html pisze:

Turn your TS870 on its 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).
Thats 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, its 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 its absolutly necessary to have stable conditions.