As mentioned in the similar post here. Replace the, complete, MC electrode unit. It's a known issue, I've had to do a few.

Taking into consideration that I'm in Australia, my part numbers are:

MX-50/6070N - CHLDZ2541DS52

MX-50/6070V - CHLDZ2541DS54

MX-50/6071 - CHLDZ2541DS54

This is an issue that, from my understanding, is limited to the 50's and 60's.

MX-6070N Parts Manual Section 70 (MC Electrode) - The whole unit.