FT 847 432 Mc noise transient
My FT 847 had a noise transient when switching to the 432 Mc band and each time the rig transitioned from TX to RX. Each time a strong noise burst occurred.
I tried to eliminate this transient by improving the grounding of the PC boards, but this did not completely cure the problem. (some of the PC board earth points do not make proper contact to earth)
Analyses with a spectrum analyzer showed an oscillation of the 432 Mc pre-amp during the switching transient.
The cure is as follows :
Remove the rf board completely.
On the solder side cut the connection between the gate of Q3004 and the
combination of R3015, L3010, C3018 and D3096.
Cut as close to Q3004 as possible (between R3015 and Q3004).
Take a small ferrite bead (4mm) and run 1 turn of 0,4 mm insulated
transformer wire through its center hole.
Solder one wire directly to the gate of Q3004. Solder the other wire to C3018.
Keep the wires as short as possible (and do not try to reconnect to R3015)
Press the bead almost against the PC board and ad a little drop of 10 sec. glue
between the bead and the board.
Replace the board and reconnect all connectors.
Around 432 Mc I could not measure any degradation of sensitivity. (I did not
check the rest of the frequency range)
It's quite a pleasure to QSO on 432 Mc without the constant noise bursts in your
ears..."