What you describe, if you mean successive four-note chords, is indicative of hard- or floppy-disk trouble, which you can isolate by trying to boot with each drive's data cable disconnected in turn. If neither drive will boot the PB, it is probably an MLB fault. However, the partial return-to-life more probably indicates poor data cable connections than defective drives. All or many of the connections to the MLB may be in poor health, however, and may be in need of breaking and remaking. If your video is now also playing-up:
it may be because brightness and contrast controls are both on the video inverter board, which is directly underneath those sliders. The slide pots, or the inverter board itself, may be failing, or the connecter between the inverter board and the interconnect board may in turn be poor or defective. You are going to have to climb inside the PB to do anything about any of these potential connection faults. You will find some clues about opening the PB here. While you are inside it will be a Good Thing to break and remake every connecter so as to forestall other problems.
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