PTT is handled by A3 on the Arduino Nano. I would check that your PTT wiring is driving the A3 pad/pin low with respect to the GND on the Nano when your PTT switch is pressed.
(this may be what you mean by "Amin is also 0 V by default."
If your wiring is correct and that pad/pin does go LO (logic low) when PTT is pressed, it is indicating a faulty Nano that needs to be replaced. Easy to do and many here will help you if you have concerns/issues.
It is fairly common to have an Electrical Over Stress / ElectroStatic Discharge event that damages a CMOS Input/Output buffer.
You don't want to wire a switch that shorts T/R to ground.
By the way your written English is very good. "Sorry, my English!" was the only clue you are not a native speaker.
I would also have a quick question. I built an ubitx V5. My problem with it next is when I want to switch to trasmititng with the small micro switch nothing happens. If, on the other hand, I press PTT on a USB cable with Pocket RXTX software over USB, it switches to transmiting immediately.
My question is , is there any other place where the PTT switch is available besides the pin 5 of the contorls connector (ORANGE wire) and the GND (BLUE wire).
I would like to ask if I put a switch between the spindle pin 16 (T / R) and GND, would it work as a PTT switch?
That is, if you look at the PTT switch, the orange wire of the VFO panel and the 8 pin audio connector (blue wire) of GND. However,
Tnx and 73's Tamas