Q: Make Mac Classic display full white screen

Hello everyone once again.

I have acquired yet another broken Mac Classic recently.

When powered up, it shows random lines on screen and random sounds in the speaker. It seems to be a bad ROM.

As I am not gonna bother fixing it anyway, I want to convert it into a sort of an oscilloscope.

I connected the horizontal deflection coil on the CRT to an amplifier (made out of an old guitar amp) and now it sorta works, except that because of the random lines, I cannot see the whole waveform. So, what voltage should I supply to the analog board's VIDEO pin to have a completely white screen, and what I need to supply as well to omit the logic board from the device completely?

Also, won't running it without a horizontal deflection coil connected to the analog board make something blow up on the inside after some time?

So basically i need it to be running and showing a white screen without a logic board, is this possible?

Also, how do I turn brightness down on the board itself, without the software?

Thanks!