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CPC464 no display :(

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NorthDevonRetroArchive:
Hi All,


I have just been gifted a CPC464 with green screen monitor and a box of sofrware but it's not working.


I purchased a Scart cabe for it with the 5V powr feed and connecting this up to my OSSC I get no dsiplay and no sync.


Connected to the green screen monitor I just get a bright green screen when I turn on the CPC. I tried pressing the dlete key but I get no beep.


Opening up the CPC I have a long board version with part number Z70200 written on the board. One of the RM chips was getting hot so I purchased 8 new chips and socketed them on the board, they no longer get hot but I still get no display.


The Z80 has a stable 4 Mz clock and the Reset and M1 pins show activity. I don't see any activity on the Data pins though. I do see activity on the Address lines. I swapped this out with a known working Z80 but I am going to try this again because I can't remember if I did that before or after replaing the memory.


I checked the clock on the CRTC chip and that is stable at 1Mhz. I see activity on the Horizontal Sync but the Vsyc appears to be stuck high. Could this signify a dead CRTC chip? or would there be another cause for there to not be a vsync.


What should be my next course of action to try and bring this back from the dead? Because I am impatient and hae waiting for deliveries I have already ordered a replacement Z0, CRTC chip and a blank EPROM to replace the ROM chip if needed.


I have an oscilliscope, logic probe and multimeter so is there anything else I should be testing now?


Thanks in advance for any help. Would love to get this working as I have never used a CPC464 before  :-[


Thanks, Matt

Bryce:
I think there should always be some sort of signal on the vsync, even if everything else is not working, so I'd guess it has an issue. However, that wouldn't explain the lack of activity on the data pins. Try the other Z80 first.

Bryce.

NorthDevonRetroArchive:
So I have just powered up my spectrum +2 and measure the Data lines to make sure I could see a signal, which I could. I then transferred this Z80 over into the CPC and still nothing. The scope shows a flat line at 1.28V across all the Data pins.


Clock signal is fine. Address lines show activity on all apart from A14 and A15 (Is this right?).


Memory request shows activity, Read pin shows activity but write pin does not (Is this right?)


Would be such much eaier to have a working CPC alongside so I can compare stuff :P


Thanks, Matt.


EDIT*


Just done some more testing comparing the Z80 on the +2 to the 464 and also doing some reading up. I document I have read says a number of pins need to be high for the Z80 to work, one of these is the Wait pin (24). On my +2 this pin is high but on the 464 I am getting a 1Mhz clock signal. This doesn't seem correct. Where should the wait signal come from?

llopis:

--- Quote from: NorthDevonRetroArchive on 22:46, 25 April 21 ---The Z80 has a stable 4 Mz clock and the Reset and M1 pins show activity. I don't see any activity on the Data pins though. I do see activity on the Address lines. I swapped this out with a known working Z80 but I am going to try this again because I can't remember if I did that before or after replaing the memory.

--- End quote ---
There is your problem. The /RESET pin should be constantly high. If not, check out the series of logic gates leading up to it. It's possible that they're faulty and preventing the Z80 from starting to work correctly.
Although it's really weird that M1 shows activity, that indicates the Z80 is fetching instructions from memory.

NorthDevonRetroArchive:
Looking at the circuit diagram the Ready/Wait signal comes from Pin 2 on the Gate array (40010). I have mesured this pin and can see the 1Mhz signal still. Could this still be caused by one of the logic chips? I am worried that the gate array is at fault. But if one of the logic chips (IC114 if I am reading correctly?) is pulling the line low then I guess that could still be the fault?


Is there any sure way to tell if a gate array is working properly without replacing it?


I am new to troubleshooting at this level so please correct me if I am speaking nonsense lol.


Thanks, Matt.

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