FT 2000 Modification

FT 2000  Modification - Added SMD diplexer and reducing C1334

It is surprising that results of Rob Sherwood’s (NC0B) tests show that FT950 has 16dB better 2 kHZ Narrow Spaced Third Order Dynamic Range than its “bigger brother” FT2000.The problem has been identified by VE3GNO/YO3GJC and US6IQ some time ago. The first reason is poor impedance matching between first mixer SPM5001 and roofing filter. The second  reason is improper  first mixer loading  with a  10pF capacitor C1334 for a DMU Bandscope unit.  See  Picture 1.

Picture 1: FT 2000 Shematics



In the newer receiver FT 991 with the same first mixer SPM 5001Yaesu has improved this impedance mismatch and added a diplexer to properly terminate the roofing filter. Diplexer terminates the impedance of the roofing filter to the mixer in a wide frequency range. The Yaesu FT 991 has also the same first IF frequency of 69,450 MHz as FT 2000. See Picture 2

Picture 2:  FT 991 Diplexer

As an enthusiastic contester, I wanted a better receiver performance so I decided to insert a SMD diplexer at the output of SPM5001 mixer and T1021 transformer circuit. The added SMD diplexer was built into a housing of the first mixer. See Pictures 3 and 4.

Picture 3: Diplexer shematcs f= 69,450MHz

Picture  4: Location of  the  SMD diplexer 

Replacing the coupling capacitor for the bandscope.

As the original 10pF capacitor C 1334 inappropriately loads the first mixer, I reduced it to 1pF, which is enough to connect the RTL SDR  receiver for a panoramic adapter. When the SDR panoramic adapter or original Yaesu DMU Bandscope unit is not used, remove the capacitor C 1334.

Picture 5:   C 1334 location and  T1021 transformer output pin.

Picture 6:   C 1334  location and  T1021 output pin.

To reach the elements of the mixer,the mixer housing at the bottom side of the PC board must be unsoldered.At first unsolder mixer shield on bottom PC board side and take it out. see Picture 7

Picture 7:   Unsolder mixer shield

Preparing the diplexer for installation

1. Locate the work area on PC board. (see Picture 8)
2. Unsolder and take out mixer shield on bottom PC board.  (see Picture 7)
3. Unsolder SMD capacitor C 1334, 10 pF. (see picture 5 and 6)
4. On its place solder new SMD capacitor 1 pF.
5. At the existing original transformer   T1021 carefully unsolder the output (right) wire.  (see Picture 5 and 6 )
6. Solder the transformer T1021 output   wire  to the diplexer input.
7. Connect the diplexer output back to the place on the board where the transformer T1021 has been unsoldered.  
8. Solder mixer shield back.( see Picture 8) 

Picture 8: Final installation

Diplexer has an attenuation of 0.7 dB, but I did not detect any difference in signal strength after the diplexer has been built in. But for improving receiver,s dynamic range  is recommended to redistribute the RF gain reducing it ahead of the overloaded  stage ( 2nd mixer) and building it up again after the narrow IF filter. However we can change IF gain for 1dB in FT 2000  hidden  menu setting  B11 to B20  if  we want. But be very careful - this is a hidden service menu accessible for maintenance by a qualified technician only!

More about the build in procedure can be seen from the pictures.

After this modification and already some years ago built-in AC0C 2 kHz NS roofing filter, adding 4kHz Ceramic filter in second IF 450kHz ,   I measured IMD dynamic range at offsets of 2kHz , 5kHz and 20kHz  with   2 x -6dBm  DC4KU   HF- Zweitongenerator FA-2-HF.  Two-tone third order dynamic range (IMD DR) is the diference between MDS and the levels of two interfering signals causing IMD products 3dB over the noise floor.RMDR is Reciprocial mixing dinamic range,measured as a 3dB increase in noise floor.. Picture 9

                                 

Picture 9: Two tone RF generator 2x -6dBm

Results of two tone IMD testing - 1.st preamp Off; 500Hz Bandwith; Roofing AC0C 2,7KHz; Test band is 7MHz ;
MDS = - 138dBm, IMD2 Dinamic range second order on (14102kHz)=87dB , Blocking above
noise floor @ 10kHz ,measured as 3dB increase in noise floor =107dB

Spacing offset:            IMD DR3:           RMDR

2  kHz                           86dB                  90dB

According to the experts’ opinion (e.g. Rob NC0B and Tom W8JI), the IMD DR3 of 85 dB is enough for CW. Thus my 2 kHz IMD DR3 86dB and RMDR 90dB results are very good.. 

In the future I will still try  to improve  the receiver,s dynamic range  with option to increase the mixer Oscillator injection levels. The  simplest option is to increase R 1319 at first mixer SPM5001. We,ll see.....

Disclaimer

Warning!!! I am not responsible for any hardware failure. You make this mods at your own risk! This modification requires a high level of soldering skill, possibly beyond that normally possessed by an average HAM. Professional assistance is advised, if you are not confident that you have this ability.