A1200 graphics glitches

[Rixa]

I posted about this into another thread some time back, but it lost popularity two post later and I'm still wondering, so I'm trying it again as a new thread.

My A1200 has a 2B motherboard. The worst problem I have with it seems to be strange graphics glitches with some screenmodes, at least while the computer is warming up. I've only used PAL and NTSC for a long time so I haven't ran into it, but the document

http://aminet.net/package/docs/hard/flickerfix

sounds very much like it, flickering stripes appearing in 28mhz screenmodes. The document suggests shorting a resistor (E127R) to fix it, but also says the problem affects Escom Amigas and I thought mine was older. Does anyone have knowledge of this issue? Would shorting the resistor (if one exists on the board) be a good idea?

I'm also thinking that the issue may be irrelevant once I get my brand new Indivision, but I'd like to know nevertheless.

[TheCorfiot]

@Rixa

Rev 2B is usually found in ESCOM (AT) A1200's & the fix is recomended, You will still need to carry out this modification to remove the glitches even if fitting the Indivision.


TC

[Rixa]

That's curious, because according to a warranty sticker on the original hard drive I bought this a1200 in March 1994. It has the old kind of a floppy drive, was shipped with 3.0 roms, and all the stickers and text on the board says Commodore. Theres nothing anywhere in it that says Escom.

So is Rev 2B an older board that predated Escom but was re-used later? Or are there perhaps two kinds of Rev 2B? This makes me nervous because shorting random resistors on the board could obviously kill it. I wish I could be certain that E127R on this board needs shorting. The problem at least is just as described, and changing palette can stop it from happening.

Have you performed this hack?

[TheCorfiot]

@Rixa

Rev 2B boards were also used in the last of the Commodore A1200's too but all ESCOM (AT) A1200's use this motherboard. I believe the floppy mod / hack was only used by ESCOM.

If your symptoms are exactly as described by the "Hack" documentation then it should stand a good chance of curing it for you. (I think the hack is even bundled with Classic Workbench)

I have not carried the mod out myself due to not owning a Rev 2B Mobo ( I have 1B, 1D2 & 1D4 ).

I very much doubt that trying the hack would damage your A1200 if carried out competently,

Maybe some one who has carried out this mod can confirm this for you

Good Luck
TC

[DJBase]

Originally Posted by TheCorfiot

@Rixa

Rev 2B boards were also used in the last of the Commodore A1200's too but all ESCOM (AT) A1200's use this motherboard. I believe the floppy mod / hack was only used by ESCOM.

ESCOM used 1D.4 too, not only 2B.

[StevenJGore]

ESCOM used 1D.4 too, not only 2B.

Yep, my Amiga Technologies A1200 has a 1D4 motherboard. Must have been using up some old Commodore stock or something like that.

Steve.

[DJBase]

Yes, ESCOM used old Commodore stock too but also produced new boards of both revisions.

[illy5603]

I have a 1D4 in my A1200 and I may or may not have this issue. I just get random flickeryness until the system warms up. I had this when using multiscan productivity from the RGB port with a VGA adapter and I still have it with the indivision board. Once the system warms up everything looks fine, well at least on my multiscan monitor, the LCD monitors have their own issues.

[Rixa]

So theres at least two boards that exhibit the same behavior. I wish there was some more information on it to know if it's safe to apply the mod. I also wonder what the mod does. The problem can be worked around by avoiding certain bits in the palette colors, so will the mod somehow do the same in hardware? Wouldn't that permanently degrade color resolution?

Another option would be to tweak the palette where possible. The document mentions there is some software to do just that for system screens, but doesn't mention it by name. It sounds dirty but electrically safe, and most non-system things that I care about are games that work in PAL/NTSC and thus won't be affected by the issue.

I'd prefer a real solution though, if it was good and safe.

@Illy5603,
It's on the underside of the motherboard, so you have to remove the board from the casing to view it. The picture in aminet is poor, but does point to the area where to look for it. The names of the components are written next to them on the board.

[illy5603]

@Rixa,
Thanks I didn't realize it was underneath the board. I shouldn't bitch about it I guess, I am just frustrated at the moment.

[DJBase]

If I understand the mod you have to connect both ends of resistor E127R together so you will bypass the resistor. You will find the resistor on the back side of the mainboard. I uploaded a better picture.

[illy5603]

Thanks so much DJBase!

@Rixa, well yet again more news. It turns out my system had OTHER issues that were more likely the cause of at least some of my problems. My power lead was loose and depending on what angle it was sitting at, the system would behave in various stupid ways. Sometimes my network card wouldn't start, sometimes I would hear weird sounds through my monitor and all of this while having intermittent problems with the indivision board.

I took a pair of pliers to the power connector for the Amiga and applied a bit of pressure to make the connector grip the Amiga more. I also put a little balloon filled with sand (a stress ball, for your hand) under the power lead to keep the lead pointing in to the Amiga at the best angle.

It seems now that everything is working at once. The indivision board, sound and network card. Make sure you have good power!