ASR 33 repair tips sought.

After shelving the silly idea of converting my machine to 220V ( you guys were right, my "native 220V" machine has a big transformer on the righthand side of the typing unit, and a 115 V motor... ) I still needed to get them running. My latest ASR 33 comes out quite nicely after a removal of dried grease and reoiling and seems to be fully functional, just drops a bit from time to time when using the keyboard. (i.e. LSB missing )I hope a cleaning of the keyboard contacts wil take care of that . My second, older machine however has more severe malfuntioning : it drops bits every second character, and often generates extra characters, @ being a favorite. I assume the error is inside the keyboard, as both units type ascii text But what failure mode would cause extra characters from the keyboard ? Jos