Amiga 500 black screen

[Kepler]

Hi all,

I recently acquired an Amiga 500 that looks to be in pristine condition...unfortunately, it only produces a black screen. I have done the usual basic checks of replacing all custom ICs, power supply, cleaned the sockets...still just a black screen.

Now, what is odd is the behavior of _RST when testing with my logic probe: it stays low when I first turn the machine on and then after a second, the green "low" light of my logic probe goes out. HLT stays low. If I restart the machine, the same behavior will occur. According to my meter, the voltage on pin 18 is 4.9V.

I have never seen this behavior before and am not sure what to do next.

Any ideas?

[BennehBoy]

Is there a clock?

[Kepler]

Yes, the clock is stable. I have also discovered the cause of the disappearing RST signal, it was my logic probe. Or more specifically, the positive being connected to a 12V source as opposed to 5V.
RST is actually low > high on power up.

So, I have the not uncommon black screen but where all custom ICs are good...

[BennehBoy]

Any access to diagrom?

Probably worth probing around Gary to see if anything is floating...

[Kepler]

Diagrom doesn't work...when you say "floating", what do you mean exactly?

[BennehBoy]

Diagrom doesn't work...when you say "floating", what do you mean exactly?

No LEDs light on your logic probe, so there's no path to ground or 5v.

If Diagrom isn't working then the CPU aint even running code. If the chips are good then I'd be suspecting bad sockets.

[BennehBoy]

Have you made any progress?

[Kepler]

Unfortunately not, I have been side tracked by yet another A2000 green screen conundrum! :mrgreen: