What would cause a radio – after it’s been re-capped and the tubes checked out – to work for several minutes then fade out, sometimes crackling and the volume changing on it’s own. I’ve got a couple of radios doing this now: CGE 601C and Musicaire MD249. I’m not sure where to start looking for the issue.
My first guess would be one of the large value resistors in the AVC circuit, including the volume control itself. Perhaps you can supply an alternate resistive divider by removing the center lead of the control ( the wiper ) and using a couple of fixed resistors to give you an appropriate audio level.