Older CPLD programming

I know this is really old stuff, but I've got a Tektronix TSG-120 that uses an Altera EP610PC-35 for signal generation. That chip is acting up. I pulled the IC and read it with a Zeltek SuperPro programmer and saved the JED file. Replaced the IC socket and other stuff... still bad.

I purchased what I thought were new EP610's from a liquidator, they wern't! I sucessfully wrote all 1's via the device programmer. It won't write 0's. Is this device securtiy locked? Can it be cleared and reprogrammed?