The first thing to look for is a shorted capacitor or shorted diode in the power supply. Not easy to track down.
You need to use a ohmmeter which sends less than 0.6V out its leads, so you don't cause neighboring diode junctions to conduct and give misleading readings.
Is the power supply a switching type? With miniature components? Then it will be hard for you or anyone else to work on.
A technician would probably say "Right off the bat I'll offer to test it with a new power supply. If it solves your problem the charge will be $50. If your power supply is good then I can try to diagnose the problem but my time will cost more than the unit is worth. "
If you feel like doing some hazardous experimenting on your own, try installing a variac to reduce power voltage (or add a light bulb inline). Bypass the fuse with a jumper wire.
Then:
* See how much current the unit draws.
* See if you can feel a component heating up.
* See if you can figure out if any board is drawing overmuch current.
* See if the display works.
* See if the tape drive works.
Etc.
Watch that power draw doesn't go over a few watts, lest you risk ruining the unit or causing a component to explode.
Even if you can track down the section that's faulty, to fix it will not be easy unless you have an identical spare from another Panasonic unit.