GE 578 crackling

[superdeez]

Hi, all, I hope this is the right forum to post my question:

GE 578 clock radio. Some years ago it used to work well for a time, then it would begin to crackle and produce unintelligible audio. I replaced the tubes today (I have more free time on my hands lately) and that brought it back to life, but it is still after a time popping and giving me garbled audio. It sounds good and loud when it works, but then it again begins to make nose.

Could anyone give me a hunch where to begin troubleshooting? I have a photofact, but I'd rather not replace every component in the set if I could avoid it. Audio output transformer failure? Hard to tell from afar, I know, but I'd like to bring it back again. Clock still keeps perfect time.