I recently got a Yaesu FT-757GX to use as a backup radio for the shack. The previous owner said it has no RX audio on any other modes except CW, but the TX was fine and the S-Meter was moving on all modes. This got me thinking there is just a problem with audio switching and by looking at the schematic I knew where to start.
When I got the radio, opened it up and started to diagnose the known issue, I discovered many more. As I came to find out these are quite typical for the Yaesu FT-757GX, I’m listing them here along with some of the troubleshooting steps I had to go through and the fix for each of them.
1. Wire cut on Local Unit connector. This didn’t cause a noticeable problem, but it’s really important to be aware of this whenever you work on this radio. There are many really thin wires near board edges and it’s really easy to cut/break/short one when you are putting the radio back together. It looked like someone opened the radio before and when closing the top lid (PA Unit) the wire got sliced. This blue wire was coming from one of the two 5-pin connectors on the real-left corner of the Local Unit and going to the RF Unit and was carrying the INH (inhibit) signal. Reconnected, insulated, tucked away to make sure it’s protected.
2. No audio on LSB, USB, AM. The audio switching for different modes is done through Q30 (MC14066B quad analog switch) on the RF board, so that’s where I started. By measuring voltages and looking and audio signals on each pin of Q30 for each mode selected I could tell two of the switches inside the IC were malfunctioning, having the outputs always at +Vdd logic level (+7.96V) no matter the mode.
After removing the board from the radio (luckily I had a new DIP-14 socket and a new 74HC4066 chip in my junk box) I realized the problem was a blob of solder that was shorting pins 1&2 to the supply trace going to pin 14. Someone tried to repair some bad contacts on the KEY socket and in doing so, accidentally shorted Q30. Removing the troublesome blob of solder made USB, LSB and AM come back to life.
3. Low output power on some bands. Turning on RF preamp decreases output power (!?). The radio was now putting out 100W on 80m, 12m and 10m, but only 1-20W on the other bands. Turning the RF preamplifier on cut the output power in half on most bands. This is a tricky one to diagnose and luckily my Google skills were helpful. Apparently the switching diodes in the frontend can be damaged quite easily and they start leaking signals everywhere; this radio was engaging multiple band pass filters at the same time. As these are also used on TX to clean up the signal before it’s amplified, a lot of the drive was lost in the filters. Replacing D7, D9, D11, D13, D15, D17 and D19 on the RF Unit with 1N4148s fixed the output power issue and now the radio puts out 100-120W on every band. Turning on the RF preamp didn’t affect output power anymore.
4. No audio on FM. FM mode RX still dead: just a very quiet hiss, like the squelch was always closed. This was caused by the +8V TX line hanging high on RX. Normally the TX line should be slightly negative on RX and go to +8V on TX, but on this radio it still had about +3.5V on RX. This made the FM demodulator chip think the radio was on TX so it would shut itself down. This was caused by other damaged diodes leaking some current to the TX line, so by replacing D112 & D113 with 1N4148s on the RF Unit the TX line was now going to -1.02V on RX and FM audio was alive and kicking.
5. +8V RX line hangs high on TX. I noticed this issue while fixing the FM audio problem: RX line was +7.96V on RX and +3.21V on TX. Not sure if this caused any real problems, but it was a sign of something not working as designed. Again, the problem was traced to damaged switching diodes in the frontend: replacing D04 and D05 on the RF Unit with 1N4148s mostly fixed the issue. Now the RX line drops to +0.39V on TX, which is not perfect but it’s too low to affect anything. Probably there still is one more diode that is leaking a bit of current somewhere.
Most of the diodes used in the Yaesu FT-757GX front end are NEC 1SS97. These are UHF Schottky barrier diodes (0.5V forward voltage drop) that are great for signal path switching (1pF capacitance) but the reverse breakdown voltage is just 10V (!). Any high static charge coming from the antenna would damage these easily. As the 1SS97 are no longer manufactured, the equivalent part is NTE584 (not very easy to find) and the recommended replacement by Yaesu is the 1SS83, which supports 300V reverse voltage but is not Schottky (1V forward voltage drop). The best replacement from my junk box were the generic 1N4148; reverse voltage is 100V (still a large improvement over the 1SS97) but the capacity is 4pF, which is 4 times higher but still low enough for HF (about 1.3kohm of reactance at 30MHz). 1N4148 also supports a peak surge forward current of 2A and a continuous current of 300mA, which makes it definitely more rugged compared to the 35mA of the 1SS97.