avatar_Rygo

CPC464 MC0044A Blue Screen, I'm new to all this!

Started by Rygo, 11:33, 22 November 22

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Rygo

Hey guys. I have a CPC464 on my bench and have been doing quite a bit of reading on the forum but would like to jot it all down first hand. When I turn it on i only have a blue screen, tape deck doesnt play but will rw and ff.

So far and after reading on this forum (what a wealth of information!) I have pulled the 40009, socketed it and dumpped the ROM, I have then checked this against a ROM downloaded form here and they are identical so I would assume that the 40009 is ok. 

With that back on I have a feeling I should be looking at the RAM?

I have visually inspected the board, removed and cleaned the pins on the Z80 and the 40010 but nothing has changed. 


I know there are some RAM resting roms floating around but would any work if written to the 40009? I dont want to waste time pulling RAM modules if i dont need too. Even if it owuld be fun to socket them all! :laugh:

Please bare with me on this one. I am a board level technician but I generally work with phones and laptops but writing roms and testing ram modules like this is a little over my head at the moment. 

Thanks in advance!

d_kef

Hi Rygo and welcome!

The Amstrad Diagnostics ROM is what you need.
There is an image that can be burned in an eprom and replace the 40009.
The diagnostics will start automatically on power up and the RAM will be tested.

d_kef 

Rygo

Ahh ok thanks, I have ordered an M27C256 from eBay and will post my findings when this arrives and has been tested. 

I thought this would be the way to go but didnt know if the 40009 was writable so I didnt need to order anything in. 


Bryce

Hi Rygo,
     if the entire screen is always blue on start, then it may not be the RAM (but still could be). Dodgy RAM will usually give a square of another colour in the centre of the screen. The ROM was a good place to start though.

Bryce.

SerErris

Can you please post a picture of your screen?

For now without seeing it, it also could be the CRTC not putting the address lines to the memory (very unlikely all of the CRTC address lines failing), so the multiplexer in front of the RAM might be wrong (all 4 chips unlikely) or the GA not outputting the enable bits or CAS/RAS bits correctly. That all would lead to the vsync and hsync still getting outputted correctly but nothing to show for th GA (all characters actually 00). So that would even mean that the GA could still output colour information correctly. 

Another possibility would be that the CRTC does not send out the DISP_EN signal anymore or the GA not interpreting it (broken). 

But again, all of that could be completely wrong with only having "blue screen". 
Proud owner of 2 Schneider CPC 464, 1 Schneider CPC 6128, GT65 and lots of books
Still learning all the details on how things work.

eto

Quote from: Bryce on 16:30, 22 November 22if the entire screen is always blue on start, then it may not be the RAM (but still could be).

Wouldn't a short on DataIn on any of the RAM ICs have the same effect as it has on the data lines of the  ROM? Or is it more unlikely to with RAM?

Rygo

Give me a couple of hours and Ill post up an image of the screen. I have ran a rough continuity test across the ram pins and dont see any shorts, Im waiting for the eprom to arrive so i can flash the ram test. If this doesnt display on start up then I can probably say the RAM isnt the issue and look elsewhere. 

Im hoping the eprom will be here today as its bloody quiet this week and it gives me something fun to do. Better than replacing FPC connectors on a switch lite anyway lol

Rabs

Don't know if it helps, but Noel's Retro Lab has a few good videos on CPC screen fault diagnostics.

Bryce

Quote from: eto on 11:22, 23 November 22
Quote from: Bryce on 16:30, 22 November 22if the entire screen is always blue on start, then it may not be the RAM (but still could be).

Wouldn't a short on DataIn on any of the RAM ICs have the same effect as it has on the data lines of the  ROM? Or is it more unlikely to with RAM?

Not exactly, the RAM data lines are behind a buffer, the ROM data lines are connected directly to the CPU.

Bryce.

eto

Quote from: Bryce on 09:48, 24 November 22Not exactly, the RAM data lines are behind a buffer, the ROM data lines are connected directly to the CPU.
the data OUT are behind a buffer, but afaik the data IN lines are directly connected to the bus.

WacKEDmaN

#10
Quote from: eto on 10:04, 24 November 22
Quote from: Bryce on 09:48, 24 November 22Not exactly, the RAM data lines are behind a buffer, the ROM data lines are connected directly to the CPU.
the data OUT are behind a buffer, but afaik the data IN lines are directly connected to the bus.
yup the way i see it..the 244 is only there for the GA to read the data bus... the 373 is for ram out and GA ram read.. the GA can disable the 373 to read from ram itself..or enable it to allow the ram to be read by the rest of the system...
the data bus is directly connected to the individual ram chips 'in' pin.. only the 'out's go through the 373...

edit: the 244 can be enabled/disabled aswell depending if the GA wants to read from the bus..or from ram
eg both disabled, the GA is reading directly from the ram outs..
..244 enabled..373 disabled.. GA is reading from data bus...getting input from the rest of the system..
373 enabled..244 disabled.. rest of the system can read the ram outs..

Bryce

Quote from: eto on 10:04, 24 November 22
Quote from: Bryce on 09:48, 24 November 22Not exactly, the RAM data lines are behind a buffer, the ROM data lines are connected directly to the CPU.
the data OUT are behind a buffer, but afaik the data IN lines are directly connected to the bus.

Yes, but that is enough to cause different failures if a Dout shorted to GND isn't it?

Bryce.

eto

Quote from: Bryce on 13:01, 24 November 22Yes, but that is enough to cause different failures if a Dout shorted to GND isn't it?
sure, if Dout is shorted to ground, it would be different.

My question was, if a short of Din could have the same effect like a short on a ROM data line. Or if that would still be different (or just unlikely).

Bryce

Quote from: eto on 14:47, 24 November 22
Quote from: Bryce on 13:01, 24 November 22Yes, but that is enough to cause different failures if a Dout shorted to GND isn't it?
sure, if Dout is shorted to ground, it would be different.

My question was, if a short of Din could have the same effect like a short on a ROM data line. Or if that would still be different (or just unlikely).


Yes, generally it would result in the same failure, assuming it's a dead short. If it was just a low resistance to ground the CPC may make it some way through the boot sequence before things start looking wrong. This is why I was questioning whether it's always blue, because that may point at the CPC booting to a certain stage before it fails.

I've had 6128's where ROM7 was damaged (not shorted to ground, but not working properly). This resulted in the CPC making it all the way to displaying the first bootscreen text, but stopping before the Ready prompt was displayed.

Bryce.

eto

Your experience is SO valuable. Thanks a lot for the explanation.

Rygo

Quick update. I have had the M27C256 arrive and have written the lower diagnostics rom onto it. now just need to wait until the boss is out for a few hours so i can tinker as hes already starting to pass me funny looks... 

should run it on saturday and ill take some pics of my findings :) i hope is something as simple as a ram failure as all the stuff in the thread above is a little over my head at the moment lol! You guys really know your stuff! 

Rygo

Installed the eprom and hit the powerbutton (finally...)

I now have diagonal red lines and I now get 1 low and 2 high beeps on power on. Time to scroll the forum to see what this means!

SerErris

Still without images it would be very difficult to help out. 

Diagonal red lines does not sound good. but without a proper screenshot it is very hard to tell what exactly could go wrong.

As this already some days old, let me know if you are still working on it and we can provide more help.
Proud owner of 2 Schneider CPC 464, 1 Schneider CPC 6128, GT65 and lots of books
Still learning all the details on how things work.

Anthony Flack

I'm not the guy to ask, and you've probably already tried this, but my first thought was to wonder if it's just a video output problem (bad CRTC?) or if the whole machine is locked up. Does it beep if you push delete?

The tape won't play by default, but if the machine does beep when you push delete, hold CTRL and push the small enter key, followed by any key, and see if the tape plays then. 

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