Found a Commodore SX-64 but with a strange problem.

OK - So it might not be strange to those experts in Commodore hardware, so I hope one of them reads this! This was one of the few missing things in my CBM collection until yesterday. The system first appeared dead, showing just a blank screen. But then it worked fine with any of my assorted game cartridges in the cartridge slot. To test the disk drive, I pulled out a cartridge-based spreadsheet which had load and save commands and the disk drive worked OK. The big surprise was when I put in the Simons Basic cartridge and this heavy old "portable" C64 came up with its normal opening screen "SX-64 Basic" etc and it loaded and ran a variety of programs from disk without a problem. But without a cartridge, or with a C64 Super Expander cartridge, just a blank screen. Could it be a RAM chip that is faulty, with Simons Basic causing some relocation of memory that allowed the system to work? Unfortunately, the insides are not nearly as easy to get at compared to a standard C64.