I can't find a service manual for a 996. The 8888 comes from a power on reset cycle that is driven from the main board. The kbd is input to the main board. Without a schematic, it is hard to say what on the main board is bad. If the common things, switches or HV diode were bad, you would still be able to use the kbd to set time, etc. Since nothing works, something on the main board is malfunctioning.
I would remove the main board and first do a careful visual for any overheated resistors or expanding capacitors. Then try to test first all the diodes then the transistors on the board with a DVM. I say "try" because sometimes it is not possible to test them in circuit. If you find a questionable one you can remove it and retest it. However, if they are SMD parts, it will be difficult to remove/replace.
Otherwise, it is something else on the main board and you need to replace the board. I have a 997 schematic and there are a couple of PCBs. One has a power supply. If yours has more than one, do the above on it also.
PLEASE BE AWARE, THAT YOU MUST INSURE THE HV CAPACITOR IS COMPLETELY DISCHARGED AFTER OPENING IT UP.
Otherwise, you may become the problem. Refer to vidios on Youtube, if you don't know how to do that.