Commodore B128 with I/O issue

I have a Commodore 600 series B128 (low profile version similar to the European 610) and I have been trying to diagnose a problem and fix. I do not have diagnositcs for one of these guys to check the chips automatically. When I power on the machine, it reports to be READY with BASIC version 4. I can type one line of code (for example: 10 PRINT"TEST"), but when I hit enter, the cursor disappears and no more screen IO can be generated. In other words, the cursor does not jump down to the next line so that I may proceed with another line of code, it just disappears. If I reset the computer and type "LOAD" at the prompt and then hit enter, I am not asked to start the tape drive. There is also a return key associated with the numeric keypad but this does not generate different results than the Enter key. I am guessing that I need to replace a chip or two someplace. I would like to know if any of you have an opinion/suggestions. I have found one interesting web page, http://www.funet.fi/pub/cbm/firmware/computers/b/ that links to schematics, etc. I am curious if anyone has had a similar experience with any Commodore of this era. I am sure the cause of the problem is not unique to the 600 series. Once I have a better idea what the problem is, I can start replacing the appropriate chips. If I can find any (!) Thanks Bill Wilmington, Delaware