Mark 812 Timer Problem

[Antlerman]

I have a Mark 812 softener that was installed in 1993 and it's still going strong - until lately. The timer now does not appear able to keep correct time. I suspect it is losing time after it regenerates. A call to the local shop said it would be about $100 to have it replaced, IF that's the problem. Where could I order the part? If I can, I'd like to try fixing it myself...? Good idea? Bad idea?

Thanks
John