8568B YTO ERROR

[Gianni]

Hi,
I bought a lovable 8568B declared as not working. The first problem it had was easy: just a stuck key. Fixed it, all the instrument seems to work correctly, but ...the trace is always zero, whatever I connect to the input.
After some time, an "YTO ERROR" appears on the screen. Please note: not "YTO UNLOCK", but "YTO ERROR".

This message is not listed among the possible error messages. Could please someone help me or give general advice to afford the beast?

Thanks

Gianni