TV shuts off after 3 seconds

SOURCE: I just bought a Panasonic SA-PT480, set it up,

I can't find a manual for this thing, which I believe is part of the SC-PT480 package.

I'd plug it straight into a wall outlet for further fault isolation and treat it like it has never worked, no assumptions.

Surge protectors only address outside power surges. Overheating, speaker shorts yada yada can still happen. Effective self-protection circuitry activation need not be preceded by smoke and light shows.

When the audio cut off did the unit also die, or did it look otherwise alive? If the former...

Generally speaking, an amp protects itself from heat, shorts and overloads by refusing to turn on or stay on.

Overloads can be from excessive periods of high output or marginally low impedance loading by the speakers; and shorts would be wiring issues or a speaker blowing up.

You should be able to feel if it's hot. WHY is it overheating? Make sure it has sufficient ventilation on all sides and that vent holes are not blocked by dust balls. Ensure the fan (if equipped) is running as designed (some only operate on demand). Clean dust and debris from it.

If the amp comes back on after cooling, you're lucky. They only have so many self-protection cycles in their lives so continuously resetting or cycling their power without addressing the cause can do more harm than good.

If it protects immediately on a cool power up you should disconnect the speaker connections and try it 'naked'. If it comes up then diagnose which lead(s) are shorted. If it does not come up the problem is internal and should be left to an experienced and competent hands-on tech.

Check for loose speaker connections as a root cause for intermittent shutdown.

Maybe a sleep timer clicked it off. Or if it seems to power on but remains silent some other setting like muting, tape monitor got activated. Or the source audio died. Lots of things.