Not a project yet, but it might be. (Note: See update at the bottom)
Due to the fact that my Enhanced CAT project decodes the data lines that communicate the MCU to the display driver TMS2370, this can be used to solve a few common problems for this transceiver.
I don't have the datasheet for the TMS2370, but it seems to be a simple 13 chars, 7 segments VFD display driver. You can deduce this from the FT-757GX Tech manual.
- If the TMS2370 is faulty but the VFD is ok, it can be modified to drive the original VFD.
- If the TMS2370 is faulty and the VFD is faulty as well, it can be modified to drive a LCD
- If the TMS2370 is ok but the VFD is faulty, then you have to replace both, so it is the same as above.
With the spare pins you can even keep some of the USB CAT functionality, unthinkable for such an oldie!
If there is real interest in this, I can do it, please email me.
Testing the VFD
The most usual problem of a VFD seems to be the loss of vaccuum. You can esily check that because the getter material (silvery metal deposition on
some parts of the glass) turns white when in contact with air.
Also, something youn can test, are the voltages. According to a HAM I know, the voltage on the filament should be arround 2 volts. And there is a resistance of 10 ohms in series with the filaments (R01 at pin 30 of VFD in my circuit), that might be open.
You should desolder that resistance, check it with a multimeter, and also test for continuity both the filament and transformer, and possibly replace the resistance with a new one.
According to this friend, it is as well possible to replace the VFD with one from an old calculator, of similar characteristics. I guess it is easier to say than to actually get one. If you do this, you will loose the fancy characters (VFO A-B, CLAR, LOCK, etc), which will be represented by segments of the first 2 digits. I guess you can figure that out easily and get used to it anyway.
Here you have a link which gives a nice explanation on VFDs: http://hem.passagen.se/communication/vfd.html
It is well known that there is a transistor, Q01 (or Q4001) 2SC496Y in the display unit, which is a NPN Silicon and its application is as oscillator for the DC to DC converter. This is used to obtain -8V for the whole radio, and -13V (or was it -16? I don't remember now) for the VFD display.
If that transistor is faulty, then your display certainly does not work, and your radio might or might not work. Before even thinking of replacing the chip, test this transistor, and verify you have the correct negative voltages. Refer o the Tech manual. (It can be replaced with the very common BD135)
More info in this page http://www.kb2ljj.com/data/yaesu/ft-757.htm
Second important thing. If the transistor is fine, the next step is to try to verify if the problem is the chip or the VFD, or both. This can be easily done with an oscilloscope. If you don't have an oscilloscope then you'll need a tester with Freq meter. If you don't have that.... you can try reading an AC voltage at the VFD or disconnecting the VFD and aplying the right voltages directly to the VFD, but it gets a lot more complicated.
IMPORTANT UPDATE! TMS2370 Replacement
I have good news! An OM from Germany, DB6RK Rudolf Kauls, has done a small board with a PIC and SMT transistors to fit as a replacement for the impossible to find TMS2370. I have some photos of it, and if you need one, you may contact him directly to (To avoid SPAM, you'll have to manually copy the address).
If you need help
If you plan to email me about any problem you may have, I'll be happy to help you, just...
- Please describe the problem you have in the best and more detailed way you can.
- Can you transmit? Do the top right buttons work? Do you have regular pulses on the display lines K1, K2, K4, K8, INT? Do you have -8V where you should?
- Make sure you have the tech manual with the schematics, if you don't just ask me for it.
- Tell me if you have an oscilloscope or not.
- Consider that diagnosing and solving any problem requires you to have the guts to disassemble everything, are you confident with that?
- I cannot repair it because sending your equipment to Argentina is certainly not an option.
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