Strange A1200 display issue

Hi all

I have a 1D4 A1200 that I have started to use again and I have a strange issue with the display. Firstly, it appeared that workbench was setup to a display mode that my TV couldn't supprt so I managed to boot with a boot disk, remove the affected driver and restart and it all seemed to be OK. But this was on an RGB cable.

Now (at work messing about with it) I have the same sort of issue where the display either doesn't appear at all or if I hold the 2 mouse buttons to get the boot menu up it will appear and then start rolling/flicking? I have tried on both composite and RF and I have the same problem. I don't have my RGB cable here so I can't test that.

One of the capacitors near the RF is very slightly bulged, and I mean only very slightly, so could this be the issue? I know they can dry out. The wierd thing is that it was working fine all day yesterday but today it just no good.

Any ideas?

Cheers

[demolition]

Originally Posted by tetsu0san

Hi all
Now (at work messing about with it) I have the same sort of issue where the display either doesn't appear at all or if I hold the 2 mouse buttons to get the boot menu up it will appear and then start rolling/flicking? I have tried on both composite and RF and I have the same problem. I don't have my RGB cable here so I can't test that.

One of the capacitors near the RF is very slightly bulged, and I mean only very slightly, so could this be the issue? I know they can dry out. The wierd thing is that it was working fine all day yesterday but today it just no good.

If you have a rolling picture, it sounds like the sync signal could be missing. A bulging cap could very well be cause of the problem as this means that it most likely has leaked and the acid from it could have damaged a track or via carrying the sync signal.

The picture is not rolling all the time on the boot menu screen but I can't get a picture when it boots on workbench. However I have not tried it on RGB today so I can't be sure if it is a composite/RF issue or not.

The cap, although bulging doesn't appear to be leaking from what I can see. Is there anything else I can look at?

[demolition]

Originally Posted by tetsu0san

The picture is not rolling all the time on the boot menu screen but I can't get a picture when it boots on workbench. However I have not tried it on RGB today so I can't be sure if it is a composite/RF issue or not.

The cap, although bulging doesn't appear to be leaking from what I can see. Is there anything else I can look at?

It's often not possible to see directly if a cap has leaked.

Look on the component pins and vias around the cap. They should all be shiny, but if they look dull the cap has probably leaked acid. Rust spots on the RF modulator shield could also be an indicator.

[Amiga1992]

You are all assuming the computer is to blame, when this could probably a display issue. If the image rolls, it means the vertical sync is not supported. The TV is either not buying the 50 or 60 Hz the Amiga is throwing. It is a possibility since you were using it at home just fine and now on your office you can't.
I had problems making a capture device accept the Amiga display even though it worked fine with a PAL PS2 and an NTSC Wii.

I would try it with another television set before blaming the Amiga.

OK, this is very strange. Could it be a power problem? When I disconnect the keyboard it doesn't roll on the boot menu screen but if I connect it, it does.

I am using one of the old heavy power bricks that a mate bought in for his A600. I have one of the lighter ones at home. But I remember vaguely having the same issue at home too...

Originally Posted by demolition

It's often not possible to see directly if a cap has leaked.

Look on the component pins and vias around the cap. They should all be shiny, but if they look dull the cap has probably leaked acid. Rust spots on the RF modulator shield could also be an indicator.

The outer metal shielding is pretty dull all over to be honest. I presumed that it was due to being in the loft for a few years.

Originally Posted by Akira

You are all assuming the computer is to blame, when this could probably a display issue. If the image rolls, it means the vertical sync is not supported. The TV is either not buying the 50 or 60 Hz the Amiga is throwing. It is a possibility since you were using it at home just fine and now on your office you can't.
I had problems making a capture device accept the Amiga display even though it worked fine with a PAL PS2 and an NTSC Wii.

I would try it with another television set before blaming the Amiga.

It was doing this at home too before I used an RGB cable. I just thought it was the display resolution that was setup on the HD before I started to mess with it.

[jimbob]

Originally Posted by tetsu0san

OK, this is very strange. Could it be a power problem? When I disconnect the keyboard it doesn't roll on the boot menu screen but if I connect it, it does.

I am using one of the old heavy power bricks that a mate bought in for his A600. I have one of the lighter ones at home. But I remember vaguely having the same issue at home too...

Could be a keyboard problem then. In the early startup, pressing a key will switch from PAL to NTSC. A key stuck on or intermittently pressing itself would cause the sometimes rolling fault you describe on a screen that won't display 60Hz. Maybe just a reseat of the keyboard ribbon will fix it. Failing that, you could try opening up the keyboard and giving it a good clean/inspection.

Can you boot to WB with the keyboard disconnected?

I had actually managed to ruin my workbench it seems, which was why I couldn't boot into it. I was using the CF card hard drive in WinUAE and did something and knackered it. However, I still get the rolling screen on the boot screen and if I power off the Amiga and power it back on and let it boot into Workbench the image on the screen using the composite cable is very fuzzy and distorted. If I remove the composite cable and plug it back in without rebooting the screen seems to be OK.

Does that sound like perhaps there is a component that is not working properly and removing the compostie cable and reconnecting it is discharging it somehow?

[roy bates]

so what happens if you remove the harddrive and dont use the floppy to load up workbench?

do you then still get this problem?

what i mean is does the "insert floppy screen" roll

OK, it does seem that I do indeed have a strange issue that is not just realted to the composite and RF video outputs. Even when connected using RGB the image is a little 'fuzzy' and the colours when in workbench seem to change and flicker.
Even on RGB when I press the two mouse buttons together to get the boot menu up I can't see it as the image is out of the range that my TV (a different TV than the one at work) will work with.
I think that the first step will be to change the capacitor that is near the video chip and see what that does. The light blue one in this pic:
http://bboah.amiga-resistance.info/d...lightgreen.jpg

Any other thoughts?

[roy bates]

you can check the c/sync signal if all else fails,its on the cxa1145m on pin 11.



check all the traces on and around that area for corrosion.and broken traces etc also check the signals are going through via's as well.

Thanks for that. My knowledge of electronics doesn't go very far so how do I check that signal?

Cheers

[roy bates]

the rgb scart lead requires this pin to display the signal on the tv,so you could in theary wire the c/sync to where its needed on the rgb socket directly from underneath just to check if its a bad trace or via.

you should be able to check which pin it goes to on the rgb socket on the amiga side by simply using google(i cant remember which pin it is at the moment)
i think its on pin 10 of the 23 pin socket.

Having searched all over the internet on this I think I am going to recap my A1200 board to see if that makes any difference as there are a lot of issues people have relating to old and faulty capacitors. This A1200 is a little temporamental sometimes anyway.
If after replacing the caps it fixes it then that is a good thing, if it doesn't and it still works then it's not the end of the world.

[alexh]

I'll tell Steve to warm up the soldering iron. What are you going to bribe him with?

And he thought I was bad for asking for Retro-computing favours