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.
View Comments (18)
Salutare Razvan, tocmai am cumparat un Ft757gx si am aceasta problema in Fm , se aude ff incet. Problema ar fi tot diodele D112&D113 ? Unde gasesc 1N4148s ? Multumesc !
Buna Razvan. F buna prezentarea problemelor FT757, utile si clare . Felicitari ptr rabdarea de a depana si de a spune si celorlalti ....73 dd YO3GH, Op Dan.
Hello razvan.
Seeing these articles, I dare to ask you an opinion about a fault I have in my 757GX. being connected to the desktop computer for digital communications, and without good connections to ground, it turned it on and at the time it remained in transmission permanently. Where should I look ?, Here I have no technical service to go. Thank you.
Carlos
Hello Carlos,
what interface did you use for digital modes ? Probably some RF gets in the PTT line and keeps the radio in TX even after you remove the transmit signal. You need good ground, interface cables with a grounded screen and one or two ferrite chokes. Try to see if setting the TX to lower power helps (so less RF gets in the PTT line).
Hi Razvan.
I have the problem you mentioned in point 4.
NO AUDIO ON FM.
I can't see diodes D112 and D113, could you mark them on an image of the RF unit part?
Thanks in advance. 73
Hi Jose,
please see the Service Manual download available at the end of the article. It includes the RF Unit component map that will help you find the diodes.
Razvan,
First off thank you for posting that service manual. I've got a 757GXII with no RF output. Strangely, there is RF at the RCA TX out used to drive a transverter. Somewhere months ago I'd read about the diode problem which you mention here as well. I've got new diodes from Yaesu and I'm trying to screw up the courage to do the replacements. From the service manual page 11 it says "For access to the solder sides of the RF and Local Units, loosen the screws (marked *) on each side and slid the front panel forward." I wish that were the case but I fear it is not that easy. You mention removing the RF board. Am I correct that the board needs to be removed to replace those diodes? Removing that board seems very difficult. Am I missing some trick?
Tnx and 73,
Mike / KA4CDN
There are quite a few screws to remove, indeed. You need to remove the board in order to solder back in the new diodes, there's no way around it unfortunately.
hello. i recently bought a ft757 gx, and it had some minor issues. some things were out of alignment. now it's working well, but it still has a similar problem like yours had. when i turn on the rf amp, power cuts out a bit, but mostly on cw. on ssb, i get around 80w, maybe fixing that will give more power output. thanks for the info. keep up the good work.
Razvan I've been reading your posts on the 757 and tried using your repairs on my ft-900. it does about 90 watts on 160 100 on 80 110 on 40 and from there on down wattage drops to the point when you get to 10 it's down to 20 watts replaced all the diodes on the bandpass board with no luck. might you have any ideas. Thank you Greg n8eio
Would be worth watching the current consumption on each band, high current and low output might mean a PA / LPF issue, while lower current & low output points to a drive issue (pre-amplifying stages, BPF etc). Without an oscilloscope or at least a RF probe to measure levels at certain stages, it's challenging to investigate.
Hello Razvan
I have a problem with the panel dismantler in FT757GXII. The VFO dial cannot be removed. Maybe you know how to do it. Thank you for your help
Tadek sp8gpq
My email sp7gpq@gmail.com
vy73
It should just pull off. Mine is friction fitted to the shaft. Its very tight. Be careful not to damage the knob.
Hello friends. I would like to tell you about my problem with my FT757 GXII. Suddently, when it was transmiting, the transceiver lost power. Only 20/25 watts in all bands. I did a checking of every diode in the RF Unit From D7 to D20. Incuding D1 D5 D6. Also I check the Q50 (2N4427). Every thing was fine. By taking meassurements I realize that there is no voltage on the Q50 collector... The problem was the L51 inductor with a very hight resistence. I replaced it an the problem was resolve. I hope that my experience can help in similar problems. 73 de LU3MAM