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Author Topic: A500 with no video output - Help wanted with trouble shooting  (Read 1088 times)

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Offline nomikeTopic starter

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A500 with no video output - Help wanted with trouble shooting
« on: February 02, 2023, 12:56:41 AM »
Hi!

I have an Amiga 500 board revision 5, which doesn't have video output. If I connect it to a modern flat TV via a scart cable, I get a "No Signal" message and I'm trying to figure out what's wrong.

I have a second A500 with board revision 8a and that one works fine.

So this proves there is nothing wrong with the PSU, the TV or the video cable.

I also have an old Amiga CRT screen and if I hook up the rev. 5 computer to it and power it up, the following happens:

The floppy drive starts clicking, waiting for a disk to be inserted.
Once I insert a game disk, it gets loaded and after a few seconds the cracktro could be heard on the speakers.
I could press the left mouse button to skip the cracktro and the system is continuing to load data from the disk.
The screen however stays blank the whole time. The caps lock key lights up the caps lock LED.

So this proofs at least basic functionality of: CPU, clock crystal, RAM, Paula, Fat Agnus

I've tried the same CRT with the rev. 8a board and that one works. So any weird issues with the screen are ruled out as well.

AFAIK the CIA chips don't deal with video at all, but as they are easy to swap I did to rule them out as well.

Now my main attention fell on DENISE, which is doing all the video stuff. However, I swapped it with her counterpart from the rev. 8a board (which despite being a A500+ had the same main version number printed on it) and the rev. 8a board still works and the rev. 5 board still has the same symptoms. So that's ruled out as well.

It could be an issue with the SCART connector, but I tried the mono out and that doesn't work either. So the SCART connector could be ruled out as well.

On the board, I wasn't able to visually find any leaked caps (besides AFAIK that is more an issue with the A600 than the A500).

I measured the surface temperature of all big chips on both boards, plugged them in and turned them on, waited 10 minutes and measured it again. None of the chips got unusually hot. GARY stayed quite cold, but that happens on the rev. 8a board as well, the others warmed up slightly (except the ROM chip of course).
So nothing noteworthy here either.

I also tried the floppy drive in the rev. 8a board and it works.

I'm at a point now where I'm running out of ideas.

Any suggestions on what's left to possibly be the root cause and how to nail it down to that?
Did I overlook something?
Did I rule something out which I should not have?

I could try swapping more chips between the two boards, but I've ruled out all of them as possible culprits already, so I'm not sure if it is worth risking to bend and break pins on them (yes, I do have a chip puller, but still there is a risk...).

I will try to visually inspect the board for damage. I was thinking of checking with my multimeter whether the chips all get power, but I don't think this will reveal much. The chips get warm, so they do seem to receive power alright.

I do have access to an oscilloscope in a local hacker space, but I haven't used any oscilloscope yet, so I would either require someone to help me or learn how to use it on my own. But that might be an option if everything else fails.
 

Offline nomikeTopic starter

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Re: A500 with no video output - Help wanted with trouble shooting
« Reply #1 on: February 03, 2023, 01:01:54 AM »
As I wrote, the floppy drive clicks,it detects when I put a floppy in, the floppy get's read, the cracktro runs (red sector in this case), I can here the music, I can click the left mouse button to end the cracktro and then the game get's loaded.

Literally everything works except video output. And I've ruled out DENISE already by swapping it with the one from another machine which works.