FT 757 Filter modification ========================== During RTTY/AMTOR operation it is useful to have a 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. 1.) 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 than LOT 4. 2.) Take a new diode (1N4148 or similar) and solder the cathode to the open end of the 150 Ohm resistor. Connect the anode to the middle contact of a double pole toggle switch. 3.) Connect the OFF contact to pin 5 of J10 (SSB 8V) and ON contact to pin 8 of J10 (TX 8V). 4.) Take another diode of the same type and solder it's cathode to pin 1 of J09 and the anode to the ON contact of the second pole of the toggle switch. 5.) Connect the switching (or middle) contact of second pole to pin 7 of J10 (RX 8V). The OFF contact will be left free. For the SPDT switch I am using the MOX switch on the front panel. It is a 2 pole toggle switch and you have to cut the PCB between the contacts. Further you have to remove all wires to the MOX switch. That's all. Attention : There are some errors in the Service Manual. Eg. The filter connection is shown wrong. If you want to have a corrected circuit diagram or complete modification diagram please write to Roland Robic OE6RCD, Augasse 15, A-8430 Leibnitz, Austria. Please enclose an A4 SASE ( 2 IRC or 4 IRC outside Europe Airmail) This modification should work also on FT757GX/II. 73 Roland OE6RCD @ OE6UBG or @ DK0MAV or to G3PLX AMTOR Mailbox. FT757 - FP757GX faults ====================== STIFF VFO CONTROL : This is caused by a polythene washer tearing in the photo-interrupter unit. This can sometimes be repaired but it is better to replace the unit, or replace the washer if it is available. Later units had an improved washer. NO TX : Check linear switch at rear is not depressed. Check Q08 (TX 13.5v. switching) in the PA unit. DEAD : On a couple of occasions I have found burnt out print on the local/control unit - I'm sure this has been caused by users drawing too much current from the 13.8v output socket at the rear. MOX ERRATIC, LOW O/P, NO FM RX : An assortment of seemingly unrelated faults can be caused by problems with the TX/RX 8v switching circuitry. When in RX the TX8 line should be at 0v. (or slightly negative), and vice versa. If this is not the case then check Q56 and Q57 on the RF unit. If these are OK and the voltages are still incorrect then suspect leaky diodes in the TX/RX switching, especially the two diodes mounted underneath the RF board near J01 (on later versions of the PCB these are mounted on the board). There are several diodes mounted around J01 and these should all be checked if the first two do not cure the problem. NOISY RX : Diodes as above. AUDIO AMP STILL WORKING ON TX : Diodes as above. LOW O/P ON SOME BANDS : Suspect a leaky diode in the signal frequency filter switching circuitry (D07 through to D20). One of these being faulty causes two band-pass filters to be selected simultaneously. KEYING FAULTS : The keyer IC has failed on a number of occasions. POOR QUALITY AUDIO ON TX : There is a miniature slide switch on the corner of the PCB next to the mic amp. This gives full or reduced bass response on TX. Note the speech processor is an audio clipper. NOISEBLANKER INEFFECTIVE : Realign noiseblanker IF stages. FP757GX ======= *DEAD* : This is a switched mode PSU and therefore chain reactions are not unusual. The following parts should all be checked :- R9 - R10 - D83 - D84 - D10 - Q1 - Q2 - D81 - D82. ************************ PORTED AND REALIGNED FROM PC TO AMIGA FORMAT BY BOB G0LBQ - N6XDI.