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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 RetroPassion

Re: A500 with no video output - Help wanted with trouble shooting
« Reply #1 on: February 02, 2023, 12:42:53 PM »
does the floppy drive click when its switched on? have you tried a modulator?
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Offline nomikeTopic starter

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Re: A500 with no video output - Help wanted with trouble shooting
« Reply #2 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.
 

Offline Castellen

Re: A500 with no video output - Help wanted with trouble shooting
« Reply #3 on: February 10, 2023, 07:46:08 AM »
I'm at a point now where I'm running out of ideas.

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.


You absolutely need to use an oscilloscope to diagnose this sort of thing.  You'll probably want to get someone who knows what they're doing to help you out with this.

Fortunately the U4/Denise operation is fairly simple and easy to check.  For starters:

1. Verify you've got a valid AC waveform on all 4 clock/timing inputs on pins 32, 45, 35, 36 - expect 15kHz, 7MHz, 3.57MHz
2. Check there's constant bus activity on all 8 address inputs
3. Check there's constant bus activity on all 16 data I/O lines
4. For each colour group (4 bits each of red, green, blue), expect to see 5V waveform activity on all of those outputs when the machine should be displaying the 'insert disk' screen, see the schematic for details
5. Check there's ~15kHz on the horizontal sync out (RGB connector pin 11)
6. Check there's 50/60Hz on the vertical sync output (RGB connector pin 12)
7. Check the 5V video supply is there at U40/U41 pin 20
8. Check for about 4.5V on pin 21 of the video hybrid, HY1

Another easy way to do it when you're not familiar with the circuit operation is compare various signals against a working A500.

You might like to check for video signal on the composite output as well as the RGB output.

And don't rule out intermittent IC sockets causing problems.  Visually check each contact with the IC removed.  If the IC is overly easy to extract from the socket, then replace it.
« Last Edit: February 10, 2023, 07:48:53 AM by Castellen »
 

Offline hcoder

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Re: A500 with no video output - Help wanted with trouble shooting
« Reply #4 on: February 16, 2023, 02:57:32 PM »
I've having a similar problem that keeps my head spinning.
I'm now in the proces by following every connection on the board to see if the components are actually connected to the onces that it should be.

You can use amigapcb.org for a visual of the traces
It seems like a huge proces but you can start at the end and work your way backwards. I've heared sooo many beeps already.  ;D
« Last Edit: February 16, 2023, 02:58:47 PM by hcoder »
 

Offline Castellen

Re: A500 with no video output - Help wanted with trouble shooting
« Reply #5 on: February 16, 2023, 06:26:41 PM »
I'm now in the proces by following every connection on the board to see if the components are actually connected to the onces that it should be.


Ouch, that's the equivalent of trying to diagnose why your kitchen light won't switch on by examining the entire house brick by brick, where you could simply measure if there's power at the lightbulb, or measure if the lightbulb is open circuit.

The other issue with that approach is that PCB problems aren't common, except when people use screwdrivers as crowbars to remove ICs from sockets and gouge the PCB surface in the process, or when there's visibly obvious corosion damage.  In addition, measuring PCB track continuity won't identify connectivity issues with IC sockets, which is very common.

If you've not got the right tools and skillset to repair it, I'm happy to sort it out.  Return freight for an A500 main board to NZ isn't as expensive as spending days measuring PCB tracks.

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