You’ve been given a powerful device, probably for serious work. I used to work for a taxi service about ten years ago, and we had the same ones. My radio wouldn’t turn on at all, and it turned out that the power supply capacitors had dried out. Then I found the user manual for Yaesu FTL-2001 on Manualmachine, which really helped me figure it out. In your case, it’s most likely a faulty PTT button on the tangent or the cable. The manual has some basic troubleshooting steps. It’s better to look at them before taking it in for repair. To be honest, I didn’t think people still used these, they’re good in themselves, but the electrolytes and soldering after 30+ years… Better to choose something newer in my opinion.