Here is the text of the mod from www.mods.dk
for your reference. I am considering it.
26-08-2003 add a comment
FT-990 Front end improvement on receive
Author: Uwe DJ9XG - firstname.lastname@example.org.MODIFICATION.NET
The front end of the FT-990 receiver has big problems if a strong radio staion is nearby (f.e. a MW Broadcating station with several 100kwatts). Looking into the circuit diagrams you can find the reason for it. The usage of cheap general purpose diodes is the reason for those problems. To improve the reception on the FT-990 do the following:
1. Remove the top cover
2. Pull the RF-Board (next to the loudspeaker) out of its socket.
3. Locate the following parts on lower right area of the board D1063 and D1057 (both are 1SS55).
4. Remove both diodes
5. Install a Pin Diode (1n4007 will be ok as well) for D1063
6. Install a 0,1µF Capacitor instead of D1057)
7. Move inductor L1084 from the anode side of D1057 to the anode side of D1063.
8. If you have problems after this mod with self oscillation on 12 and 10m add a diode 1N4007 from TX12 Line to the ATT Relay RL 1001. This will always pick the relay during transmit and will give the stage around Q1022 (2SC2407A) a 50ohms impedance.
9. Install the RF-board into its socket again.
10. Install top cover.
One more remarks to this stage...
on later production levels Yaesu has installed a non documented fix into the same area. This fix included 2 diodes and a rf-choke (D1069, D1068 and L1092). If you have this fix installed (obvious additional parts on the foil side of the board) the receiver is more worse then with the original version (like circuit diagram).
To bring it to my level too, you should remove the D1068 and D1069 diode and the L1092 choke... the L1084 choke and the 0,1µF capacitor is already, with this factory change, in the right location. Then replace only D1063 with a pin diode (1N4007) and add if self oscillation occurs on 10 and 12m another diode 1N4007 from TX12 line to the Att relay.
With this mod you will have an excellent receiver.
73 and good luck Uwe DJ9XG