Video issues on a CPC464

[Quickie]

Hi,


A friend had  non-working CPC 464 with video issues and I offered to fix it (or at least try to).
The 464 can boot with no problem, however the image on screen appears quadruplicated and a flickering horizontal refresh appears continuously.

As one image might be worth than thousand words, here is a pic:




I initially suspected of the 74LS153 muxes as there was a repetitive pattern and perhaps some lines were stuck.
Does it make sense?


If those are ok, might the problem be the CRTC?
I could do some osc readings if someone gives me a hint of what could I check.

Thank you!


[Update 25/02] - As I don't have spare 74LS153 I decided to desolder them and check if any of the problems still persisted.
Unfortunately seems that the 74LS153 may not be the cause of this problem (or at least at all) as the flicker problem persists (see pic):

[Bryce]

The main culprits are the Gate array, the CRTC and (as you suspected) the 153s. As the GA is socketed, I'd try swapping that first (assuming you have another working machine there).

Remember also, that it may be a broken track to the 153 causing that problem, not the actual chip, so give the PCB a good inspection, especially if you have removed these chips so you can check both sides of the board.

Bryce.

[Quickie]

Really valuable info Bryce (as always) 
Thanks for sharing your knowledge 


I'll do a GA swap with another working machine I have and post the results.
Hope that the problem could be a bad trace on the 153s (I will recheck all of them as you suggested [funnily yesterday I fixed another 464 which was not even powering on because of a bad trace on the power jack connector]), or the CRTC, but not the GA as this is a custom IC and difficult and expensive to found, isn't it?


Cheers

[Bryce]

There are a few limited sources for the GA still. I think TotO still has some for sale.

Bryce.