Hi Eric,
Herald1360 has pretty much summed up the most likely culprits.
Check out this similar thread which deals with a similar problem
https://www.vintage-radio.net/forum/...d.php?t=147029
Regarding the Zeroing of the x100, definitely check the 15V Battery,
but don't forget to check for any corrosion on the Contacts and make
sure their clean.
This can catch people out.
Ian