A500 disk drive stopped!

[Zac67]

If there's a memory expansion in the trapdoor slot remove it and see if it changes anything. Usually, they've got a battery inside which leaks and eats up the board. If the fuzzy stuff has already spread to the board it'll need a good cleaning at best.

[Zac67]

The 500 doesn't output RF, it's only got (monochrome) composite and - much preferable - RGB. (Plus digital RGBI, but you wouldn't want that either.)

For RF you'll need an A520 modulator but as long as your TV has any way of directly inputting a video signal you won't really want that.

Try connecting the RCA jack on the left edge to your TV's AV input (any RCA cable will do). There's no color but you should be able to see something.

[Zac67]

PSU = Power Supply Unit - the power brick
If you're using a multimeter, measure the voltages loaded, i.e. while the PSU is plugged in and the machine is running (or trying to). Just clip the black cord to ground somewhere (e.g. RF shielding), easy access to +5V & +12V is on the floppy power connector. DO NOT short circuit any power pins with their neighbors!

The "Vidiot" hybrid is that black card right next to the Denise chip (upper left corner). It's converting the digital video signals to analog ones for RGB output. Have never seen one die but you never know.

Apart from trying the very easy CIA swap my guess is on one the RAMs or the Agnus socket.
Is the floppy spinning up?
Is the head moving?
How long does it take from inserting the disk to the flashing LED? Anything on the screen then?
Have you tried removing the trapdoor expansion to see if it makes any difference?

[Zac67]

There's something wrong with your ground connection... 10+V all over the board would fry just about everything.

E.g., you can use the board's screw holes for ground reference, they're definitely grounded. Then, check the two outer contacts of floppy disk power - they should read 5V and 12V.

You're not by any chance using an analog multimeter that's severely decalibrated? The needle needs to be trimmed to 0 when it's idle.