CPC 464 start screen failure

[DFender]

I'm refurbishing an old CPC 464. It seems to work but the startup screen looks anything but ok. What would be the likely cause of this? Failing Gate Array? Perhaps some oxidizing in its connections?

[DFender]

Just found a good answer from another thread:

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/



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.

[DFender]

Looking at the schema and calculating the repetition on the screen, it is inevitable that data is repeated after 128 bytes. First 128 bytes on screen is the same as 129-256 bytes that are shown correctly, and so on.

This means that for some odd reason, the A7 address line (MA6 on CRTC) is always high. I ordered myself an oscilloscope but as it hasn't arrived yet, I did some simple tests and trials. All multiplexer chips seem to be powered up as they should and there seems to be nothing wrong between pins in CRTC and IC109. I even desoldered IC109 and soldered a socket instead for trying out a fresh 74LS153. This did not help.

I'm inclined to think there is something wrong with the CRTC. Desoldering a 40-pin IC is a bit larger task so I think I will wait for the oscilloscope to arrive before starting with it.

Meanwhile I have plenty of time to complete my Scart cable that currently relys on 1,5v battery to switch RGB mode on.