Missing Right Half of Characters

[dr_slump]

Hi everyone!
New member here — long-time Amstrad user. I learned to program on a CPC-464, owned one back in the day, and recently acquired another unit that I'd like to use to teach my daughter how to program. :-)The computer is in generally good shape and basically works. It even loads and runs games without issues. However, there's a clear display problem visible on the startup screen (see the attached photo).
As you can see, the right half of each character is missing and is replaced by the background color.
Since everything else seems to work normally, I assume the CPU and memory are fine, and the issue lies somewhere in the video subsystem. The motherboard is a Z70200, MC0002D, with a Gate Array model 40010.
Before spending 40€ to replace the GA, I started checking the chips that feed its inputs. I replaced IC115 (74LS244), but that didn't change anything. My next suspect is IC114 (74LS373).
Before going further with the repair, I wanted to ask if anyone here has seen this specific issue before. I've watched several repair videos, but haven't come across this exact symptom.
Any ideas?
Thanks in advance!
—Paco

[salvogendut]

seems similar to this case 

[McArti0]

Has Games stripes too?

[dr_slump]

Yes, games also look weird with vertical stripes. Unfortunately, I didn't take a photo of the screen.

[McArti0]

373 works good. GA does not capture the second byte from memory in the video read sequence.

Try type CLG 3.

[dr_slump]

Hi!

I think you are right. Running the CLG 3 command I get vertical stripes (see photo). Does it mean the gate array is faulty?

Thank you for your help.

[McArti0]

Omg At second byte
11110000->0, pen1 to pen0
11111111->11110000 !!!! Pen3 to pen1 !
00001111->11110000 pen2 to pen1

Try type MODE 2

And try
Cpc reset
pen 2
Ready
Pen 3
Ready

[dr_slump]

There you go. With pen 2/pen 3, now the left part of the characters repeats in yellow. In mode 2, every other character is missing.

[McArti0]

Clearly, the GA is getting the same video byte twice, and with inverted color interpretation. So this error is most likely in the GA. It would be best to check this with an oscilloscope to see if the CCLK, nCAS, and CAS ADDR signals are shifted and misaligning the LS153 multiplexers.

Try replug GA

[Bread80]

That's an interesting failure mode. The system is reading each video byte twice, but the second is corrupted.

Selecting between the two video bytes is done by the CCLK signal into IC105. This is a similar failure mode to that in the video posted earlier in the thread, but on a different address line. I'd look at IC105 (possibly swap with the other multiplexer chips) and the relevant traces into and out. I could be a broken trace or a poor solder joint. Maybe rework the pins on IC105, check for any shorts between traces and, perhaps, bodge wire pin 4 to the GA (or CRTC) to bypass a bad trace.

Since the CPU side of the RAM appears to be working that suggests the signals between IC105 and RAM are good.

Note that CCLK is also the clock signal into the 6845. Since the 6845 appears to be functioning properly that would indicate that CCLK is correct (unless it is, somehow, out of phase, which is unlikely).

As to why the second video byte is corrupted, that's harder to say. It could be a symptom of the first fault, faulty RAM, or a faulty GA. My money says it's on the first of those options. I'd recommend solving that issue and see if that fixes it.

[GUNHED]

Can be an error in the ROM. Can you create a checksum of your BASIC ROM and then the FIRMWARE ROM?

[eto]

It could be a symptom of the first fault, faulty RAM, or a faulty GA. My money says it's on the first of those options. I'd recommend solving that issue and see if that fixes it.

For the byte repeat it could be the multiplexer or the track to the multiplexer. It probably can't be the signal itself as as the CRTC, which uses the same signal, works. 

What makes me doubt that it's a normal IC are the tests with the different colours. It looks more like a bit shift as the first byte is repeated but instead of PEN 2 and PEN 3 the output is in PEN 1 for the second byte. Afaik the only bit shift is done in the GateArray in the video register.

But yeah, before swapping that I would also try the multiplexer first. 

[Bryce]

Hi dr_slump,
        could you check the resistors R146 to R153 (scattered around the chips at the bottom of the board) and see if any of them are physically broken or reading open circuit? If they are all ok, I'd suspect that the multiplexers could be the problem.

Bryce.

[McArti0]

No multiplexer can swap bits.

1111 0000
To
0000 0000

And 

0000 1111
To
1111 0000

[Bryce]

No multiplexer can swap bits.

1111 0000
To
0000 0000

And

0000 1111
To
1111 0000

I fully agree, but floating bits can cause all sorts of craziness. I used to think that floating bits would always produce random results, but I've since seen systems that gave a repeatable result due to floating inputs. That's why I suggested checking the resistors first, and taking a really close look at the PCB is never a bad thing and sometimes even reveals something completely unexpected.

Bryce.