What I am about to say may be nonsense because I sold my HD621 a few months ago and I forget things, but when I took it out of storage after 5 (warm, dry) years I needed to check it prior to sale. I seem to remember that I thought it was dead. It wasn’t. I had configured it when last used (don’t ask why - can’t remember) in a way that hid all inputs so there were no lights visible. I had to configure it to defaults using mconfig and RS232 to revive it. As I say, perhaps nonsense but just check via RS232 that it really is dead.