There are two large filter capacitors located in a metal can, I think next to the audio transformer. They will be labeled A and B on the diagram. They are both 20 Mfd. To replace them I disconnect the leads to the can underneath, install a small terminal strip and mount the two new capacitors on the strip.
It is interesting that there is buzzing with no speaker connected. If that is the case it may be that the vibrator is in distress. To see if the problem is caused by a short somewhere, remove the 0Z4 tube which is the rectifier and see if the buzzing stops. If the buzzing continues the vibrator is bad.
The filter capacitors can fail at any time when they are old. So it would be a good idea to replace them regardless.
