Help needed with a video fault on an A600

[Merlin]

Hi,

I need help from a Miggy surgeon please. One of my A600s has suddenly developed a video fault as per the attached pictures, I suspect a video sync fault but I can't pin it down.

The pictures are the output from the Composite Video socket, i haven't tested the tuner yet.

Can anyone help me bring this ailing A600 back to full life again?

Thx

Merlin

[Zetr0]

@Merlin

still pondering m8y

[Merlin]

Thanks Zet, once I have figured it out I will post the solution so that others can fix theirs.

We need a Haynes manual for Miggys..........

Merlin

[Zetr0]

I have the schematics

[Galahad/FLT]

Originally Posted by Merlin

Hi,

I need help from a Miggy surgeon please. One of my A600s has suddenly developed a video fault as per the attached pictures, I suspect a video sync fault but I can't pin it down.

The pictures are the output from the Composite Video socket, i haven't tested the tuner yet.

Can anyone help me bring this ailing A600 back to full life again?

Thx

Merlin

Check one of the pins on the video port hasn't been pushed in.

[elmasta]

I have a similar video problem. Although it happens only when there are moving sprites and scrolling is involved. What happens is that every frame the palette changes somewhat producing a flashing effect, and the scrolling stutters.

Maybe it's a capacitor leaking related problem. In the past my A600 wouldn't boot because of this, but now I fixed it changing some of the caps, but there are those video problems. It runs stably though. Maybe if I change all of them the problem goes away, but, because I haven't, I can't confirm the problem is related to the caps.

Is any capacitor leaking problem in the motherboard?

[Merlin]

@ Zetr0

So do I

@ elmasta

I've arrived at the same conclusion you have, I am going to change C236 and C237 on the board, these link to the Composite Video out and also the TV tuner, they are radial electrolytics and are the most likely suspects to be knackered. There isn't much else downstream of the decoder chip that could go South, as far as I can tell.

I need to take the board out and I haven't examined it closely yet, I will be giving it a good clean and examination before I start changing components.

Merlin

[s2325]

Your drive is chewing your floppy disk (look at tiny white dots on second picture).

[elmasta]

Have you tried RGB output? RGB output would allow you to see the video before the encoder (which is what converts an RGB analog signal to a PAL/NTSC standard video signal). The caps you mention are after the encoder, and the effect of of them failing would not look like this imho.

I think it happens before (up in the chain) and it's related to flipping bits somewhere. Just look at the delicious floppy "crumbles" , they are single pixels.

[Merlin]

@ s2325

Erm, that helps a lot... NOT.

@ elmasta

Thanks for that, I will take another look at the schematics upstream from the encoder and check the output from RGB to Scart.

Merlin

[elmasta]

I am no amiga expert, but looking at page 5-4 of the service manual it seems it's some problem with Denise's inputs.

Maybe there are flipping bits in RGA (register address bus). But I think that would make the machine crash, which is not happening.

The other possibility would be flipping bits in DRD (DRAM data bus) but I discard that because that would mean every chip would be reading garbage instead of reading from ram and of course we wouldn't be seeing anything and the machine would have crashed long before the floppy appears.

Other possibilities are on the other inputs CCK _CSYNC _CDAC , etc..

any ideas?

(maybe denise is dying but I don't want to think that)

[Merlin]

Maybe it's the _CSYNC_B line from the Denise chip? That may explain the patterned nature of the corruption.

Merlin

[elmasta]

CSYNC is "composite sync" which is the same that CSYNC_B except the latter is buffered.

If CSYNC_B was wrong I imagine the TV wouldn't sync to the composite signal, since it's this "clock" which is used to generate the analog composite signal by the encoder.

What I think is that this error lies in the digital domain. Denise seems to be constructing the picture erroneously. Is it because her inputs are wrong or because she doesn't do her job well, I don't really know.

[Merlin]

Ok, here's an update....

Put my ailing Miggy on a Scart lead and the picture is absolutely A1

This is telling me that Denise is alive and well and leads me down my original thought of something after the decoder chip having gone South.

At least I am making progress at last, I thought I might have to borrow an oscilloscope to track back the ripple on the signal; I am going to change C236 and C237 as I originally planned and report back.

Merlin

[elmasta]

Nice!
Definitely a good sign.

I was wrong before telling you that looking at the RGB output we are looking before the encoder. Actually it's after, because what we see in the monitor are the AR,AG,AB signals and not R,G,B as shown in page 5-4 (my mistake)

It seems I was diagnosing my video problem because I think my Denise is broken or something :S

If you have any idea, that does not involve exchanging Denises , about what is causing these sync issues, pallette changes, and flashing sprites let me know please
Video Here

[elmasta]

any news?



Denise