The FT757 lives again

During the weekend I spent a bit of time with the cover off the FT757GX, making some repairs and adjustments. This was brought about by the display showing all 8s and the front panel buttons not working. You can read about that here if you wish.

By Sunday night I had attended to two major known problems;

  • All 8s display. This problem is a continuation of my earlier display problems. The display board at the front of the radio has two header connectors where 11 and 4 wires terminate from the local unit. These wires bring a variety of signals to and from the two boards and, when these connections fail many strange things start to happen. What I had found back in June 2010 was that the pins on the header connectors had problems making contact with the copper traces on the component side of the board. What normally occurs with double sided boards is that connections that pass through the board are plated between one side and the other. It’s called through hole plating. Where through hole plating is not used the assembler needs to solder both sides of the board to ensure a good connection to both traces. The display board in the FT757GX is through hole plated so a a good solder connection on one side should mean good connection to both sides. As I have found this is no longer the case.

Back in June 2010 I added two wires to the board to make contacts between the two sides so things would work. By the end of this repair session I had added three more wires to the board and redirected two of the incoming wires and soldered them directly to components. One if the wires affected by this problem was the +5 volt power supply line for the integrated circuit on the display board. Without the +5 volts supply nothing on the board works, as seen by the all 8s display. For interest, the board pins that have now been bypassed are from header JP01 pins 2, 6, 7, 8 and 10 and header JP02 pin 1.

As I was reassembling I notice the purple wire going to pin 7 of JP01 had broken out of the header, probably because I had been pushing and pulling it too hard, trying to isolate the intermittent connection. Rather than return the wire to the connector I simply soldered it to the cathode of D21. I had done a similar thing with the green wire to JP02 pin 1. I was unable to get to the underside of JP02 as it sits under the display, so I cut the green wire from the connector and soldered it to the copper trace behind the 500kHz step switch.

  • 100 Hz off frequency. The 100 Hz problem is two fold. One relates to the steps the VFO takes when moving between frequencies and the second is if the VFO is actually on the correct frequency. What I have found is that when the dial is moved up, or down, 1 kHz the VFO takes 100 steps. When correctly aligned each step is 10 Hz. When not correctly aligned each step is more or less than 10 Hz but the total of the 100 steps must still be 1 kHz. The table to the right shows the received audio frequency as I stepped up from 14.076.01 to 14.077.00 in 10 step increments. I got the 10 step increments by using the up button on the microphone. What I found was the 10 step increments started at 73 Hz, instead of 100 Hz, and then got progressively bigger until the last two steps of 101 Hz and 220 Hz. This explained why the radio always sounds like a big jump right on the kHz point.

The solution to this problem came from Alejandro, LU1FCR via the FT757GXII Yahoo group. He posted the following advice, for the FT757GX MK1 only;

1)- Disconnect the antenna.
2)- Push the MARKER switch on.
3)- Select MODE SSB
4)- With the VFO knob and the microphone Up and Down buttons select 7101.00 kHz.
5)- While moving between 7100.99 kHz and 7101.00 kHz with the microphone buttons, adjust VR2014 in the LOCAL Unit until you determine by ear a difference of 10 Hz.
6)- Push the MARKER switch off.
7)- Input a known RF frequency into the antenna and adjust TC2006 in the LOCAL Unit until zero beat. This is easily done with a good pair of earphones, not with the included speaker.

8)- Continue using your FT-757 GX for the next 25 years.

Using Alejandro’s method, and with the aid of Hamscope, I got the 100 steps to be 10 Hz each and right on frequency.
As for step 8 I’ll have to wait a while to see if that comes about.

After fixing the two major problems I put the radio back together, put the covers on and, as I always do, gave the radio a good shake to see if there was anything loose. To my surprise I heard something rattle inside. It sounded like a screw but I had screwed the unit back together with the same number of screws as I had taken out. There has always been two screws missing from the bottom cover and they were the only ones missing. All other screws were correct and accounted for.

So, not wishing to power up with the known rattle inside, I took the top cover off again and shook the unit. Nothing fell out but it did sound like the noise was coming from inside the RF Unit. I took the screws out of the RF unit cover and had a good look around. I found a black cover screw in the RF unit. This must have been dropped there by a previous repairer as I have never been inside the RF unit.

While looking around in there I saw a discoloured toroid in the Low Pass Filter section along with a couple of burnt wires. Looking at the circuit I found that this was L11 in the 24 – 29 MHz section. The Low Pass Filter section is arranged before the SWR protection circuits so any fault in this area would remain undetected and the rig would continue to transmit full power. My guess is that the wires were touching the toroid and melted causing a little bit of smoke and a fair amount of heat as the toroid would have been de-tuned. I taped over the exposed wires and positioned them between the toroids so hopefully all will be well again. I have not yet used this radio on 12 or 10 metres and I don’t have my power meter here at present to check but next time I have the meter or a 10m antenna in the air I will try it out and see if it needs more repair. It probably will.

73

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