FT-990 Mods
 

FT-990 - 1.5 to 30 MHZ TX mod
  • Open FT990 like explained in your OPERATING MANUAL.
  • Localize JP5002 on "CONTROL UNIT" and close it with solder.
  • If no transmit between 4 to 6.5 MHZ and 8 to 10 MHZ, do the following:
  • Display 5.5 MHZ in CW mode, "meter selector" to ALC, push "MOX" button and adjust VR1003 on "RF UNIT" for normal ALC meter deflection.
  • Display 9 MHZ and do the same with VR1005.

 

Receiver IMD & front end improvement for FT-990
Both these mods greatly improve your receive. I have incorporated both mods and decided to share the experience from doing them. First of all I do not recommend that you do these mods without having the proper tools and experience. This is a tight board and a hot soldering iron can damage components quickly if you are not careful.

Starting with the IMD mod for noise reduction you must remove the shielding box in the middle of the board to get to all the diodes. I performed both the mods using the 5082-3081 PIN diodes. I procured mine from Newark at a cost of about $100. There are a few diodes that go in the reverse direction so follow the markings on the board with great care. Don’t forget to replace the shielding box when you are done.

The front-end improvement mod can use a little better explanation in regards to the factory undocumented fix. If you have this fix it is at a cost to your receive, and is worse than the original rev of the board. The fix keeps the receiver from overloading when there are strong close stations. The result of Yaesu using cheep diodes. It incorporates the use of a choke and two diodes around the Q1022 amplifier. These components should be removed. D1063 should get a diode, and D1057 should already have a cap. On the foil side the choke is there already for D1063.

Here is the part that needs clarification. Self-oscillation will occur on 10 to 12 meters unless you trip the ATT relay, which will activate the 56-ohm resister when transmitting. This is done buy placing a diode between the T12 (power on transmit) and the coil side of the ATT relay. You want the flow of the diode going towards the coil of course and this is done on the foil side of the board. T12 circuit can be found at the J1002 connector number 6. You can find the coil side of the relay by locating the D1058 diode, which attaches to both sides of the coil. The ATT relay is Matsushita HD1-M-DC12V.

The T12 circuit supplies power to Q1022, R1129 & L1082. You can draw you 12 volts from any one of these components to trip the relay on transmit. I soldered the diode between the L1082 and D1058. I have attached a picture of the foil side of the board. After you completed these mods you will notice a big difference in your receive. Weaker stations that where hard to hear now come in without a problem. Good luck.

 

FT-990 - JPS antenna
During transmitting and having selected the separate receiving antenna, 
RF is fed back into the 990 which can lead so far, that the internal 
power-supply or the tuner oscillates and the supply voltage is breaking down,
which can be seen because the S-meter lamp becomes dark and darker ...
If you look at the diagram on page 27 of the manual (in the side-bar titled "Receive-Only Antennas and Separate Receivers"), it seems to bear your observation out. The TX/RX switch is not where I would put it for connecting a receive antenna all the time.
However, there is a really nice use for the FT-990 "receive antenna" connection if you are using a JPS noise canceller (I think MFJ makes one too, and CQ Magazine published the schematics for one recently -- I have no idea how that one gets phase reversal, but the author swears it works).

If you were to connect the RX Antenna Out of the FT-990 to the Antenna connector of the JPS, and connect the RX Antenna In of the FT-990 to the Transceiver connector of the JPS, you get three things:
 
  1. the TX/RX switch of the FT-990 is used instead of the TX/RX relay in the JPS (much better switch over transients).

     
  2. RF power does not get into the JPS (I don't use a linear amp, but this should do the trick too),

     
  3. you can enable and disable the noise processing by just switching the RX ANT switch on and off, respectively.
73Kok Chen, AA6TY

 

FT-990 Receiver IMD and Noise Reduction MOD
I have had a number of inquiries regarding my FT990 Pin Diode MOD, which reduces the internally generated receiver noise somewhat, cleans up the received signals it reduces the fuzzy edges on the signals), and makes the receiver less susceptible to strong broadcast signal mixing products.

To accomplish this modification, replace the following 31 diodes on the RF Unit with 31 HP 5082-3081 PIN Diodes. This diode has been carefully selected for its low IMD characteristics, so I DO NOT RECOMMEND using any other kind of pin diode for this mod. These diodes can be procured from Allied Electronics and Newark Electronics.

D1015
D1016
D1019
D1020
D1021
D1022
D1023
D1024
D1025
D1026
D1027
D1028
D1029
D1030
D1031
D1032
D1033
D1034
D1035
D1036
D1037
D1038
D1039
D1040
D1041
D1042
D1043
D1044
D1045
D1057
D1063

Enjoy your greatly improved receiver!

Kirby, K7EC

 

FT-990 Front end improvement on receive
The front end of the FT-990 receiver has big problems if a strong radio station is nearby (f.e. a MW Broadcasting 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

 

ATTENTION
The KB2LJJ takes no responsibility for any damage during the modification or for any wrong information made on this modification.