Yet Another Black Screen (YABS)

[McArti0]

Even a person familiar with electronics should send their damaged Amstrad CPC to one of the many Amstrad authorized service centers to take advantage of the lifetime warranty.

[namaman]

Just a comment before I get into it.
Fistly I actually do have a basic understanding of what could happen through shorting certain signals in that the IC delivering the original signal could get damaged. So I guess thats my own risk, but I also agree that before any mention of shorting pins, then maybe it would be wise to mention that there is some risk involved.

Anyway, I've checked all the connections from IC112 and they all look fine.

[namaman]

I've just checked the connectivity from all my new sockets and I can't see any issues at all.
I've tried a few other varieties of 74LS32s in IC112, but nothing works! I only have one 74LS32 that seems to do anything. I guess my biggest question at the moment is why? Hopefully the answer to that may lead to the underlying issue.

[namaman]

...I did try swapping in some other Z80s, 2 didn't work (possibly dead), two NEC Z80's were as per the screen image I showed before with text corruption...and then there was this one (see video). At one point it actually displayed clean text with a crazy flashing boarder...not sure if it's useful info, but thought I'd share.

[McArti0]

Let's get back to the stripes. ROMDIS Hi.

I suspect a lack of contact on one of the address lines and maybe the GA data line.

a lot of characters appearing after Ready means that the 8255 has started reading 00000000 from the keyboard, i.e. all keys pressed.

[namaman]

Here is the halt ROMDIS hi test:
Lots of...noise?

[McArti0]

And add RAMDIS Hi?

[namaman]

And add RAMDIS Hi?

this is ROMDIS HI and then a quick LO on the RESET line.
...do I just do the same but do RAMDIS Hi instead?
if thats not right, can you explain the sequence?

[McArti0]

CPC on, next ROMDIS Hi, next RAMDIS Hi.

[namaman]

No reset?

[McArti0]

no

[namaman]

ROMDIS HI (screen glitches), then RAMDIS HI doesn't change anything. 
Then doing a RESET the screen is just as the video before...noisy lines.

[McArti0]

Ok check on the oscilloscope if this noise at ROMDIS is visible on Lines D0-D7 GA (i.e. Pin14 RAMs)

[Bryce]

Bryce, yes, that is indeed quite possible. I think I'll stop that for now.

So after some more playing around, this is where I am at.
I am fairly sure the CRTC, Z80, GA and RAM are all ok.
I started looking around IC112 again. I removed the new 74LS32, and put back the original that I desoldered...I saw more signs of live, and on some restarts I even got a white screen and black boarder!!!...I then I looked in my parts bin, mostly I have one kind/maker (which proves not to work at all) and then I found a third variety of 74LS32 and tried that...this time I was able to consistently get a boot screen, but with garbled or inverted colour text (see image).

Soooo....not sure what is going on and why the various 74LS32 have different outcomes, but I feel I'm starting to focus in on the problem a little bit, or at least I'm in the right area.
Anyway, it's real late here now, so I'll pick this up again in the morning.
Any help or insights would be greatly appreciated.

Thanks again

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That could just be a bad connection on the ROM IC. I'd recheck the solder joints and pins of the ROM.

Bryce.

[namaman]

That could just be a bad connection on the ROM IC. I'd recheck the solder joints and pins of the ROM.

Bryce.

Thanks Bryce. Just rechecked the ROM pins, and all have continuity as expected. This is actually the 3rd time I've checked these...lol
Cheers

[namaman]

Ok check on the oscilloscope if this noise at ROMDIS is visible on Lines D0-D7 GA (i.e. Pin14 RAMs)

Checked on pin 14 of the RAM.
Looks random, but maybe there is a repeat pattern here.
All pins basically look the same, but with variations in the repeated patterns.

[namaman]

this is Pin 2 of the RAM:

[namaman]

And here is RAM pin 14 from my working 6128:

[namaman]

And here is RAM pin 2 from my working 6128:

[namaman]

I'm not adverse to doing a bit of educated shotgun repairs as I'm pretty sure I can remove any IC with no damage insured...It's just about patience.
As I mentioned before, the GA, Z80, CRTC, EPROM have been tested and I an 99.9% sure are all functional. So what does that leave as culprits?
I have spare Buffers and Mux ICs.

[Audronic]

@namaman 

Hi

Ok Its been along Journey (Still is)

My Thoughts :- for a CPC 464

After Power up Press the "DELETE" Key several time — Do you Get a Beep From the Speaker
Assuming the Cassette is connected. ??

Put back the Eprom with the Diagnostic on it.
Carefully look at what happens at Power Up.
Photo Included of the Diagnostic Stopping ??

Have a look around IC102 AY-2-8912 (Sound Generator) Check Pin 16 to see if it is /LOW

And IC101 74Ls145 (Decoder)

Ok Try That

Keep Safe (and Patient)

Ray

[namaman]

Audronic, thanks for the advice about Sound and Keyboard input.
I'm will get back to looking at that shortly...but now I'm trying to figure out why my screen has become corrupt :-(
See post #72 in this tread

[Audronic]

Hi 

Before you go an look for the problem ?

Try the Diag and see what is on the screen It may help ??

Keep safe

Ray

[namaman]

Unfortunately the Diagnostic test now does this...
No colour redrawing of the screen at the beginning. Briefly shows results up to 'CRTC' then hangs with garbage on screen.

[Audronic]

Ok — Youch

Could you check Ic 108 6845 

Pin 40 Vertical sync (50 Hz)
Pin 39 Horizontal sync (15.625 KHz)

As it looks like the frequencies are Incorrect ?

Keep Safe

Ray