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Until someone better qualified forwards an answer, I'll give it a shot.
I checked my troubleshooting book for causes of loud buzzing and discovered a note that suggested many older tube-type radios and power amps use a "high voltage B+" power supply that consists of a power transformer, a rectifier and filter capacitors. I think this refers to the large, shielded "multi-section" capacitor (labled "C1" in your SAMS) thats rest on top of the chassis. Anyway, this converts the 110v from your outlet to the appropriate DC voltage for various circuits of the radio. Did you check / change this? Because of how it’s made and where it is, it is difficult to see if it’s leaking or leaving ‘white powder’. I know that it is nearly impossible to find NOS can-style stacks (you probably wouldn’t want them anyway) with most opting to fit a proper value electrolytic under the chassis. I did this (don't remember the value! Sorry) and left the can in place with leads clipped. If you want, I'll put it apart and tell you what it is. Let me know.
More good advice suggests mounting the new capacitor (the one I used was as big as a roll of dimes) as far away from the transformer as possible, as not to cook it. Zenith mounted it pretty close -the transformer is far right, the cap-stack almost dead center- but again, it is shielded for protection.
Hope this helps! I really love the C730; it’s my favorite Zenith radio.
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