1) Key that remains depressed or contact during circuit (oxidation).
2) The Buffer
3) The MCU on keyboard
4) The ACIA chip (6850) on montherboard
If you have another keyboard, to do the test, it eliminates the 3 three first point.
TT030 4/32 TOS 3.06 SCSI2SD 5.x HD, Mega STE 4Mb TOS 2.06 DD, Mega ST4 DD miniCosmosEX, Mega ST2 DD, 1040 STE DD, 2 x 1040STFM DD, 2 x 1040STF, MegaFile 30, Lynx 1