I bought a second-hand #Yaesu #FT780R real cheap (€30). The fuse was dead, I replaced it. It turns on. N-type antenna connector: ordered an adaptor, to be delivered today. While waiting for the connector, I prepared the radio on top of my table. Turned it on, all seemed OK. Fully responsive, it received well without an antenna and in the right frequency. Then I connected a CW key. A spark flew in it and the display turned completely erratic and irresponsive (zeros and random numbers of varying brightness). Removing power completely for half an hour does not help. I checked the manual [http://radiomanual.info/schemi/YAESU_VU/FT-780R_user.pdf], but found no way to factory-reset it. Have I bricked this beauty from the 80s? I would appreciate ideas on what to do next, thanks a million.
1$\begingroup$ It's likely that the key plug shorted the jack and transmitted briefly as you inserted it. Was it set up to transmit, and with no antenna? It's possible this would damage an 80s rig, but I'm not sure how it would upset the display. $\endgroup$– tomnexusNov 7, 2021 at 23:22
$\begingroup$ SWR probably hit the display. . $\endgroup$– tc0nnNov 8, 2021 at 14:56
The manual for this radio is quite helpful in that it has a theory of operation, block diagram and complete schematics in it. This radio doesn't have non-volatile memory, so simply removing all power should reset it.
The usual troubleshooting rules apply. First check all power supply voltages and filtering. Look for ripple. Look for bad caps. It has multiple voltage regulators - make sure each output is within spec and clean.
Since the display is scrambled, I'd look at the CPU next. Check the output of Q27 the 5V regulator for the CPU. It looks like it is supplied on pin 41. It has a 2.56MHz crystal on pins 22 and 23 - make sure it is oscillating. It likely has a reset circuit. It looks like that might be on pin 24 with a cap (C01) and a resistor (R01). Make sure it starts low at power up and rises high a bit later correctly.
The radio does have SWR protection, so briefly transmitting into an open antenna jack should not be fatal.
2$\begingroup$ Thanks a million for such a detailed response. I went to see a knowledgeable colleague, who disassembled the radio and found out that it had been modified to use the CW key as a 12 V power supply for a TNC. Undoing the modification with help from the schematics did the trick. The radio works fine now and I have used it for contesting. After PTT, every now and then, some relay gets stuck and does not return to RX. $\endgroup$ Oct 17, 2022 at 6:44