Yet Another Black Screen (YABS)

[namaman]

Hi all,
Been trying to repair a CPC 464 with a dead video (black screen?). 
(70200 motherboard)

What I have done:
- Replaced dead Z80
- Check the Gate Array in another system (works!)
- Desoldered the ROM and installed a socket
- Burnt a 27256 EPROM with the diagnostics binary
- Tried to read the original Amstrad ROM in my programmer, but just got FFs

...still get a black screen and no beeps from the speaker.

Here are some observations:
- No Hsync or Vsync from CRT controller.
- Address outputs from the CRT controller are not pulsing and all are sitting at about 2.5V !!!

So, my next plan was to order a CRT controller off Aliexpress, but was wondering if there is anything else I should as I wait for the IC to arrive?

Much thanks in advance.

  

[namaman]

oh...I should add that I seem to have good voltages and clock signals.

[McArti0]

On AliExpress you can buy a fake chip. Such is the risk.

[namaman]

Yes...it is a risk for sure

[McArti0]

be sure that pin40 of GA has a secure connection to pin21 of CRTC

[namaman]

Do you mean pin4 of the gate array is connected to pin21 of the CRTC?

[McArti0]

pin40 40007
pin4   40010

[Brocky]

id replace that CRTC... and check the logic/buffers for the memory are working

to me, it seems like its copped some reverse voltage...which can take out a bunch of chips....

[Bryce]

Hi all,
Been trying to repair a CPC 464 with a dead video (black screen?).
(70200 motherboard)

What I have done:
- Replaced dead Z80
- Check the Gate Array in another system (works!)
- Desoldered the ROM and installed a socket
- Burnt a 27256 EPROM with the diagnostics binary
- Tried to read the original Amstrad ROM in my programmer, but just got FFs

...still get a black screen and no beeps from the speaker.

Here are some observations:
- No Hsync or Vsync from CRT controller.
- Address outputs from the CRT controller are not pulsing and all are sitting at about 2.5V !!!

So, my next plan was to order a CRT controller off Aliexpress, but was wondering if there is anything else I should as I wait for the IC to arrive?

Much thanks in advance.

 

Hi,
just to be sure... What test equipment are you using?

Bryce.

[namaman]

pin40 40007
pin4  40010

yes, for me its a 40010, and it's connected to pin 21 of the CRTC. Thanks

[namaman]

Hi,
just to be sure... What test equipment are you using?

Bryce.

I have a multimeter, scope and a logic probe...cheers

[namaman]

... and check the logic/buffers for the memory are working

Not quite sure how to go about that...I'll poke around and see if I can see anything.
I'm assuming it's the 74ls244 and the 74ls373 I should look at?

[Brocky]

yup.. but concentrate around the CRTC for now....it, along with the GA produce the picture.. replace the CRTC or test it in another system if possible..
from there you can determine if the ram and/or logic is bad (with the diag rom).. but you need to get some sort of picture out first..

might wanna just quickly probe the clock into and out of the GA aswell.. 16mhz in.. about 4mhz out to the z80..

[namaman]

I have previously checked that those line were getting a clock, but I never actually measured the timing. I'll do that.

I just ordered an MC6845P yesterday, only to read that the 'P' variant has some potential incompatibility issues. Oh well, I'm hoping it'll be good enough to get the machine displaying something if it is indeed a dead CRTC that I'm dealing with.

[namaman]

might wanna just quickly probe the clock into and out of the GA aswell.. 16mhz in.. about 4mhz out to the z80..

Yep, 16mhz and 4mhz are both good

[eto]

Side question: How should the CRTC signals look if it doesn't get initialised? 

[McArti0]

Side question: How should the CRTC signals look if it doesn't get initialised?

Big frame ~4Hz V, ~7kHz Hz

[namaman]

Some progress!!!
Removed the CRTC, installed a socket and put in another (found it in an Apple II 80 column card).

I have a EPROM installed and programmed with the Diagnostics binary.
Mostly it looks like it's trying to work, but has some spurious randomness, and it never displays any useful information. Ends with a series os dashed lines at the top of the screen.
No sound out of the speaker either!
Relays on cassette click! (you can hear it in the video attached)
Putting in the standard ROM now shows a white screen.

Any thoughts?

Here is a video: