[old_tv_nut]

AGC will have a particular adjustment routine in the service manual.

Generally, setting too low causes low contrast weak picture and even loss of sync, while setting it too high can cause overload and loss of sync (picture breakup). It may be possible to set it high for super contrast without breakup, but then have the picture break up when you change channels.

For my RCA CTC-5, they tell you to adjust upward until breakup, then back off to get a picture, then switch to a blank channel and back; if it breaks up again, reduce the setting some more.

The laboratory procedure for any set would be to adjust for a specified known video detector output level with a scope on a test pattern.

Don't know the recommended procedure for your set, but if you don't have it you can try the breakup-restore procedure. You may very well find that setting it wrong increases the buzz too.