I recently modified my FT-847 with PA1O rx and tx filter mods. I made the TX filter as the UK version of course since mine was not originally an UK version.
I then removed the 390pF capacitor from the PA circuit, not the 470pF one as suggested by PA1O since others have suggested it has better performances on 50 MHz removing the 390pF instead.
On the RX side it seem to have gained about 10 dB (MDS was about -130 dBm on 70 MHz before the mod and about -140 dBm after).
I then had an almost full realignment session on the rig to the best of my test equipments (only old equipment anyway).
Here you can see the spectrum taken at 50 MHz cw 100W:
You can see a rather broadband self-oscillation around 70-80 MHz, which is not good already.
On 70 MHz the situation is much worse:
The 70-80 MHz self-oscillation is always there, I can't explain the two side bands at about +/- 3 MHz from the 70 MHz carrier.
Does anyone have a clue? I'm not sure where to check to solve the problem.
I'm also afraid that the capacitor removal causes some broadband instability (70-80 MHz broadband noise) and maybe goes unnoticed on others having made the same mod?