HP1631D fault

[GioCosco]

Hi, I have bought for few euro an HP1631D logic state analyzer, declared dead by the vendor. The unit turns on but with selftest jumper enabled the only display is a fixed pattern with some characters in top left corner. Disabling the selftest there is only the fixed pattern, so this suggest me that something is running. The keyboard doesn't work. I have removed all boards except CPU, checked for electrolytic capacitor leakage but everything seems ok. Also voltages are ok and no battery inside, so no missing nvram problems.
I have found the service manual with all schematics and I am already studying it but some suggestions are highly appreciated, expecially if somebody have already found a similar problem. Thanks

[Martian Tech]

My first guess would be bit rot in the ROM(s) that hold the CPU code.  Looks like it gets through initializing the CRT controller because the characters it displays are well-formed.