by 5B4AIY » 17 Jul 2020 10:36
Hi, Elia,
Let's talk about how the power switch of the TRX-2 works. In the OFF state there is +12V applied to pin 2 of Q2. Q2 is off at this point. When the PWR switch is pressed, two things occur, the first is that +12V is applied via D1 to the gate of Q1, turning it ON. This pulls the gate of Q2 to ground, turning it ON, and power is applied through Q2 to the rest of the transceiver. The microprocessor powers up, and after the initial boot sequence it outputs a +5V signal, PWR-ON, from pin 59 to the gate of Q1 latching it ON, and holding Q2 ON as well. The second thing the PWR switch does is, after the completion of the boot sequence pressing the PWR switch sends a +5V signal to the PWR-SW pin 60 of the microprocessor and this is used to lock the VFO, cancel an operation, and if present for long enough, to force a shut-down.
In your case, disconnect the power supply, remove the top cover, and with a multi-meter on the OHMS range with the + lead connected to pin 2 of Q2 and the - lead connected to pin 3 (These pins are accessible via J4). If Q2 is OK this should measure a very high resistance, otherwise, if Q2 has gone short-circuit, then it will measure a low resistance, and you will have to replace Q2. It is also possible that Q1 has failed short-circuit, in which case this would also give you the same symptoms, i.e, immediate power up when applying power to the transceiver. You can check for this by again using your multi-meter on the OHMS range, and measuring the resistance between the gate of Q2 and ground. If Q1 is OK, you should measure a high resistance, if Q1 is shorted, then you will see a very low resistance, which would turn Q2 on leading to the symptoms you describe. Either of these failures will certainly result in the transceiver being unable to be turned OFF by the PWR button.
My bet is that Q2 is shorted. It is very likely that with C14 a short-circuit although this would blow the main fuse, this takes time, and Q2 could easily be damaged by the high-current surge, leading to a failure.
If you need any other assistance, let me know.
73, Adrian, 5B4AIY