Garbled screen? Help!

[greg007]

Hello, new to both Amstrads and cpcwiki. I have just picked up a 6128. But unfortunately it has an issue. Starts up and can load software(after band change) but the background on the startup screen is not solid (see pic) and then when you get into a game the picture is garbled. My feeling is a faulty chip. But which one??

[Bryce]

Hi and welcome,
         it looks like you probably have an issue with the CRTC (if all games show jumbled screens) or possibly the lower ROM (if only some games have jumbled screens). It's definitely not a RAM problem.

Bryce.

[arnoldemu]

Hi and welcome,
         it looks like you probably have an issue with the CRTC (if all games show jumbled screens) or possibly the lower ROM (if only some games have jumbled screens). It's definitely not a RAM problem.

Bryce.

I doubt it's the CRTC because CRTC doesn't fetch pixels and the pixels look good mostly. The screen is a good size too.
Strangely the text on the screen is good too so that rules out the ram mostly.

Could be the gate-array, but colours are good, mode is good and rom paging seems ok.

It almost looks like one of the buffer chips or ROMDIS or something like that is not right, maybe something up with memory paging.

Could be the PAL16L8 chip which you can bypass.

[Bryce]

How could the PAL cause something like that?

RAM is completely ruled out because he says he can load a game, which rules out the buffer ICs too. But you could be right with the Gate Array. As it's socketed, I'd try replacing that first.

Bryce.

[greg007]

Thanks for all your help! Great site! I had the opportunity to remove both the pal 40031 chip and the gate array 40010, and replace, one at a time, then both together, from a doner working 6128. Unfortunately it made no difference and the chips ran in the doner 6128 fine. So looks like its something else, any more suggestions? And do you know anywhere I may be able to get replacement parts??
cheers Greg

[Bryce]

Every other part in the CPC is readily available online, so no matter what has broken, it will be easy to find. My bets are back on the CRTC. The only other thing it could be is that one of the multiplexers has failed (the 4x 74LS153).

Bryce.

[arnoldemu]

The blue lines are from bit 3 being set. (value 08)

Please try this in basic:

mode 1
PEN 1
PEN 2
PEN 3

and also:

10 a=&21:FOR y=1 to 24:for x=1 to 40:locate x,y:print chr$(a);:a=a+1:if a>&7f then a=&20
20 next:next

that change in pixels at the bottom is near &C645. Not sure if that will be helpful or not.

It *could* be the crtc changing the address output, but I would expect more of a mess on the screen than that. I still think it's ram or one of the multiplexers. Possibly some data bits stuck on.

[Bryce]

If it was RAM then the games and OS would never load. That's why I suspect that one of the address bits of the CRTC is stuck (or the associated MUX that handles that address bit).

Bryce.

[BluesBrothers]

If it was RAM then the games and OS would never load. That's why I suspect that one of the address bits of the CRTC is stuck (or the associated MUX that handles that address bit).

Bryce.

I concur

[arnoldemu]

The reasons I don't think it is the CRTC:
* CRTC generates addresses only and the screen there is well formed, the text is in the right place and says the right thing, where the text has been written it's clear without corruption in the background.
* The gate-array generates the pixels.

I understand why you say it's ok:
* Programs load.

I don't mind being proved wrong.

[Bryce]

We'll all know soon enough when it has been fixed

Bryce.

[VincentGR]

Damn this post is getting more and more interesting.