Cleaning the Thread Tension Regulator on a Toyota 5000?

This machine, a Toyota 5000, is 40 years old. I haven't used it that much and it usually works well. It was serviced 18 months ago. The lower thread underneath the fabric is bunching up a lot. I understand this is to do with the threading and tension of the upper thread. I have noticed for a little while that the thread tension regulator is behaving inconsistently. I have cleaned and oiled the bobbin area and underneath the needle plate as instructed in my manual. However, I do not have any instructions for cleaning the thread tension regulator. Could you advise please?