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General Category => Technical support - Hardware related => Topic started by: konc on 12:02, 22 May 22

Title: CPC 6128 garbled screen
Post by: konc on 12:02, 22 May 22
Hello again,

After failing to repair my 6128 (https://www.cpcwiki.eu/forum/hardware-related/cpc-6128-returns-bad-command-on-cat/) I was gifted by a friend a known faulty one to give it a go.
I was greeted by the classic grey box so I started socketing the RAM and trying new chips, OK that was easy.

But after it turned on, the image looks like this:

https://youtu.be/y64KhDFpUZo

What I've tried:
-I assume it's not the PC PSU/monitor/cables as I'm using the same things with my other faulty 6128 (where the floppy doesn't work but the display is fine)
-It's not any of the factory-socketed chips (HAL, Z80, gate array). I tried known good chips in their place and the inverse, these chips on another 6128.
-It's not RAM as I've socketed and replaced all 16 chips with known good ones
-It's not the CRT controller I was very much counting on, also switched the chips both ways
-The flickering/garbled rows are only those with text/characters, nothing's happening on empty rows
-Keyboard works

So what would you try next? Does this problem lead to any component in particular? I'd hate to desolder another board completely and blindly trying to find what's causing this...


Title: Re: CPC 6128 garbled screen
Post by: dragon on 12:23, 22 May 22
Look at this page of how video subsystem works:

http://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/

(http://bread80.com/wp50/wp-content/uploads/2021/05/CPC-RAM-Video-Fig4-1.jpg)

And you have the candidate ic  list.

I first try the 74ls153. The multiplexer group that select between z80 and crtc.  As it appears that the gate array is writing things that don't should be there(wrong address?).

Al thought you can start a game first that not use  firmware calls to ensure always happend.
Title: Re: CPC 6128 garbled screen
Post by: konc on 10:19, 23 May 22
Quote from: dragon on 12:23, 22 May 22Look at this page of how video subsystem works:

http://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/
...

And you have the candidate ic  list.

I first try the 74ls153. The multiplexer group that select between z80 and crtc.  As it appears that the gate array is writing things that don't should be there(wrong address?).

Al thought you can start a game first that not use  firmware calls to ensure always happend.

That was an excellent lead and exactly what I needed, thank you so much.
The 4 LS153 chips were good, but there was a broken trace from IC109 to the CRT controller. Nothing looked suspicious, just no connection. I soldered a wire at the back of the board to restore the connection and all is good now!
Title: Re: CPC 6128 garbled screen
Post by: dragon on 22:17, 23 May 22
At finish It's the same effect that if is broke. crtc tells i want draw this part of the screen and send the ram in screen memory between &coo0 and &ffff.

But with one pin disconnected one bit always was "0". And it take when is 0 o.k and fail if was 1. But if you don't draw nothing in the screen. And you draw another nothing. You don't notice that. 

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