CPC 464 blue sceen problem

[Morn]

So in other words the crystal is bad?

[Bryce]

As stated above, if the crystal was bad or the signal wasn't correct at pin 8 of IC125, he would not get the correct measurement at the 4Mhz signal path, which he has already measured as correct.

I suspect that he is either measuring it wrong or his scope isn't set up correctly. Eitherway, it's highly unlikely that the crystal is the problem. We should concentrate on where the real problem is.

My next suggestion would be to check each data / address line coming from the CPU. Are they toggling at all?

Also as mentioned earlier, a picture of the screen would be helpful.

Bryce.

Edit: Alexandre, I've been trying to find more information about your scope online, but I can't find a thing. Is it really just called PM3008? Do you have a picture of it?

[Alexandre Rouma]

so, while I take picture
My address lines are toggling
My data lines are also toggling

[Bryce]

Ok, well that's a start and it confirms that both the 16Mhz and 4Mhz clock ar eworking fine.

Bryce.

[Morn]

so, while I take picture

Take pictures of your scope and the PCB too while you're at it.

[Alexandre Rouma]

here are already picture of the PCB:

http://redalex18.olympe.in/img/DSCN1261.JPG
http://redalex18.olympe.in/img/DSCN1262.JPG
http://redalex18.olympe.in/img/DSCN1263.JPG
http://redalex18.olympe.in/img/DSCN1264.JPG
http://redalex18.olympe.in/img/DSCN1265.JPG
http://redalex18.olympe.in/img/DSCN1266.JPG

[Morn]

Wow, that CPC is dusty and dirty inside. 

[Alexandre Rouma]

Here are some picture of my oscilloscope and video cable:

http://redalex18.olympe.in/img/DSCN1267.JPG
http://redalex18.olympe.in/img/DSCN1270.JPG
http://redalex18.olympe.in/img/DSCN1271.JPG

[Morn]

Alright, here's the manual for the scope, Bryce: http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Philips/pm3208.pdf

[Morn]

But seriously, you should really use some wipes, compressed air, etc. to get rid of all that dust. How are you supposed to measure anything on the board when it looks like it was stored in a sawmill?

[Alexandre Rouma]

So I made a little video so you can realy see what's happening. I'm uploading it on youtube so while it's uploading, I wanted to show you the schematic of my cable so you can tell me if it is correct:

[Bryce]

Yes, it's a PM3208. Alexandre's been saying PM3008 up to now. Eitherway, it's more than enough to fix a CPC, a lot more than I had when I fixed my first CPCs!

Definitely dirt will be a problem. If that what the PCB looks like, then I can imagine what the terminals in the monitor port must look like. Alexandre, clean the PCB with a dry paint brush. Clean the monitor port socket with some form of contact cleaner.

Bryce.

[Bryce]

Oops, wait! You've got that cable connected to a TV? That possibly gives a blue screen when nothing recognisable is connected?? So it's not a CPC Blue screen, it's a TV "no input signal" blue screen?

Bryce.

[Alexandre Rouma]

no, The blue screen is from the CPC you will see on the video that the no signal screen of my TV is black with No signal written on it

Also, should I clean the contacts with some alcohol

[Alexandre Rouma]

Here is the video :

[Morn]

So it looks like the system is at least trying to display Mode 1 characters as it should when it boots up. Each vertical stripe appears to be four Mode 1 characters wide. The characters itself look garbled though.

We need more Bryce advice here. 

[Morn]

Also, in the video the text colour looks like pen 2 (light blue-green) when it should be pen 1 (yellow).

[Alexandre Rouma]

no, It's my camera, It's blue with white lines

[Bryce]

Ah ok. that's not a blue screen as I had imagined it, that's why a picture is always good. One of your RAM ICs is most likely dead.

Bryce.

[Alexandre Rouma]

How could I know which one ?

[Bryce]

You might be able to check by monitoring their Dout pin. As the CPC is only pulling 500mA the chip won't be shorted, so it won't be getting hot.

Bryce.

[Alexandre Rouma]

Is it possible to replace it by making a circuit with a 24LC64 ?

[Bryce]

?? A 24LC64 is an EEPROM, you need RAM.

Bryce.

[Morn]

One of your RAM ICs is most likely dead.

So I was correct with my initial wild guess. Yay, remote diagnosis! I demand the golden CPC award for repair excellence for that. 

[Bryce]

So I was correct with my initial wild guess. Yay, remote diagnosis! I demand the golden CPC award for repair excellence for that. 

Consider yourself awarded. Please come to the front of the class to get a star on your copy book.

Bryce.