I've been having a recurring problem with my Zenith 8S563, and it's not getting any better. The volume will randomly go up a little bit, then it will go back to normal. It will repeat this indefinitely. I replaced the tuning belt with a NOS one, but it's not working. I'll just buy a new one from Adams.

If it's an AM or Shortwave set say 30s and up the signal almost constantly drifts in and out I have found unless you use a proper antenna. The old 20s radios won't hardly pull in a signal without a good long length of antenna and a ground.

Does the B+ voltage change when the volume changes? Do you have a schematic? If not, this might be helpful:
https://images.campyent.com/sites/default/files/schematics/zenith_radio_corp_8s563_pg12-1.png

jr