In message , raden
writes
There are a couple of things which could be causing this

an electrolytic having dried out or a zener diode going down
Electrolytic going down would be my guess as well but high value
resistors have a habit of going higher as well

the ST 6400 is mainly surface mount inside

you're probably as well off just replacing it as messing around with it

My thoughts exactly.
--
Clint Sharp