AM Filter Modification
The original AM filter is too wide for shortwave listening (6 kHz). There is a very simple way to decrease the bandwidth on AM mode to about 3,5 kHz.
Just cut the wire bridge in front of JP 1020 (left of CF01, looking from front of transceiver) on RF UNIT board.
Now take a new 1N914 (or similar) diode and solder it's anode to cathode of D61 and the cathode of this new diode to cathode of D58 or D59. Now the smaller SSB filter on 455khz IF side is selected during AM. The function of WIDTH system is also improved on AM now (sideband selection is possible in AM with WIDTH knob). SHIFT is out of function as before modification.
SSB Filter Modification (not mk2)
During RTTY/AMTOR operation it is useful to have an narrow filter to reduce QRM from other stations. It is possible to use the narrow CW filter of the FT 757 for SSB, because of the shift/width system.
Cut the connection between cathode of diode D 102 and a 150 OHM resistor on RF UNIT. This resistor is not shown in service manual,but it exists on all rigs later LOT 4.
Take a new diode (1N914 or similar) and solder the cathode to the open end of the 150 Ohm resistor. Connect the anode to middle contact of a double pole toggle switch.
Connect the OFF contact to pin 5 of J10 (SSB 8V) and ON contact to pin 8 of J10 (TX 8V).
Take another Diode of the same type and solder it's cathode to pin 1 of J09 and the anode to ON contact of the second pole of toggle switch.
Connect the switching (middle) contact of second pole to pin 7 of J10 (RX 8V). The OFF contact will be let free
Mk2 SSB Filter Modification
During RTTY/AMTOR/PACTOR operation it is useful to have an narrow filter to reduce QRM from other stations. It is possible to use the narrow CW filter of the FT 757 II for SSB, because of the filter shift system.
= Cut the connection between cathode of diode D 101 and D102 on RF UNIT.
Take a new diode (1N914 or similar) and solder it's cathode to the the cathode of D101. Connect the anode to middle contact of a double pole toggle switch.
Connect the OFF contact to anode of D102 and ON contact to collector of Q56 (TX 8V).
Take another diode of the same type and solder it's cathode to pin 1 of J09 and the anode to ON contact of the second pole of toggle switch.
Connect the switching (middle) contact of second pole to collector of Q57 (RX 8V).