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General Category => Technical Support - General => Topic started by: Curlypaul on 04:28, 19 January 22

Title: Garbled screen
Post by: Curlypaul on 04:28, 19 January 22
I came to test my new 512kb ram board and find my CPC464 has developed a fault, showing a garbled screen.


It changes, but mainly it's blue and yellow vertical rectangles or light green columns, and very occasionally, a crazy vsync. The prevelance of blue and yellow suggests some sort of initialisation is happening?


Nothing is hot, ram and crtc show occasionally flushes if warmth, gate array is warm after a minute. It's an early one with a heat sink.


In order to eliminate the ram. I tried the expansion anyway, but still the same result. It's a Revaldinho, am I right in thinking when it's working, the ram on the motherboard isn't used? I tried all compatible jumper settings.


I hope it's not the gate array :(


Title: Re: Garbled screen
Post by: Bryce on 10:49, 19 January 22
Hi Paul,
the first thing to check is the voltage, is the 464 board getting 5V or has a dodgy PSU, dirty connections or a dodgy power switch dropped the voltage to less than 4.75V?
Also, try reseating the socketed IC's in the 464.

Bryce.
Title: Re: Garbled screen
Post by: Curlypaul on 11:30, 19 January 22
Thanks Bryce, I will check the voltage level tonight. Overlooked that as the psu is new. I reseated and cleaned the legs on all but the Gate Array, didn't want to get covered in paste, but I'll give that a shot tonight :)
Title: Re: Garbled screen
Post by: Bryce on 12:43, 19 January 22
New PSU... always a suspect. What's its (claimed) ratings?

Bryce.
Title: Re: Garbled screen
Post by: Skunkfish on 15:55, 19 January 22
Hi Paul,
I'm a bit worried that might be the GX4000 PSU that was purchased off me? Hopefully not as that's 9V and 1.6A!
Thanks,
James.
Title: Re: Garbled screen
Post by: Curlypaul on 17:13, 19 January 22
Quote from: Bryce on 12:43, 19 January 22
New PSU... always a suspect. What's its (claimed) ratings?

Bryce.


5v 2amps. It's the one from the cpcstore, I think. It's not brand brand new, I've had a good few hours out of it.


Quote from: Skunkfish on 15:55, 19 January 22Hi Paul, I'm a bit worried that might be the GX4000 PSU that was purchased off me? Hopefully not as that's 9V and 1.6A! Thanks, James.



If you are the guy behind cpcstore, then yes my order had a gx4000 psu in there, but I've not plugged that into the CPC :)
Title: Re: Garbled screen
Post by: Skunkfish on 11:24, 20 January 22
Phew, I did have a moment of panic there!

I am the guy running CPCstore, but I don't think the PSU is from me.

The ones I'm selling are the TT Electronics SW3088-C2, rated 5V @3A.

From testing, they normally supply just under 5.1V, with a slight drop-off to around 5.05V after 2.5A.

Spec sheet is attached.
Title: Re: Garbled screen
Post by: Curlypaul on 11:49, 20 January 22
I don't think it is actually is it. I will double check what it's doing tonight.


With the ram card though, in a 464 is the original ram not used?
Title: Re: Garbled screen
Post by: Skunkfish on 11:58, 20 January 22
If you set the DIP switches to Full Shadow mode (Switches 1 & 2 to ON), then the external RAM is used in place of the internal for most purposes...


Title: Re: Garbled screen
Post by: Curlypaul on 00:34, 29 January 22
Thanks dude.


Finally got a minute to take a look, this gate array appears to have just 3v on both vcc1 and 2.


There's 5 from the PSU with no load, and 4.75 at the Z80, and a clock signal. There are a few spurious looking solder joints that I need to attend to on the plate caps on the ram chips, and I guess check the transistors around the gate array? I've learned something about the fault anyway
Title: Re: Garbled screen
Post by: Curlypaul on 15:43, 30 January 22
ok so I took the gate array out as it there was thermal paste all over the place and gave it all a good clean. Still seem to have 5v everywhere apart from accross the gate array. With the gate array removed the socket does have 5v across the pins, not sure if this is a red herring or not.


Would be useful if I had another machine to test with
Title: Re: Garbled screen
Post by: Joseman on 18:37, 30 January 22
Quote from: Curlypaul on 15:43, 30 January 22
ok so I took the gate array out as it there was thermal paste all over the place and gave it all a good clean. Still seem to have 5v everywhere apart from accross the gate array. With the gate array removed the socket does have 5v across the pins, not sure if this is a red herring or not.


Would be useful if I had another machine to test with
Hi
Maybe test what are the ohms between the gate array ground and the other pins outside the board?
Title: Re: Garbled screen
Post by: Curlypaul on 20:12, 30 January 22
Quote from: Joseman on 18:37, 30 January 22
Hi
Maybe test what are the ohms between the gate array ground and the other pins outside the board?


You mean maybe the resistors on the 5v in?


Just snagged an aparently working motherboard, probably overpaid, but I wanted to be sure it had the 40007 the same as mine. Mine doesn't have the thick white factory bodge wires, but I'm assuming that's not an issue so long as the chip part numbers match.
Title: Re: Garbled screen
Post by: Joseman on 20:17, 30 January 22
Quote from: Curlypaul on 20:12, 30 January 22

You mean maybe the resistors on the 5v in?


Just snagged an aparently working motherboard, probably overpaid, but I wanted to be sure it had the 40007 the same as mine. Mine doesn't have the thick white factory bodge wires, but I'm assuming that's not an issue so long as the chip part numbers match.
no, i was thinking that if you check the pin ground on the gate array itself with all other pins of the gate array, well it have to show some results... now that you have 2 GA you can check it the two gate array bring the same results on all this values. I have little knowledge but maybe is a step.
Title: Re: Garbled screen
Post by: Curlypaul on 20:54, 30 January 22
Quote from: Joseman on 20:17, 30 January 22
no, i was thinking that if you check the pin ground on the gate array itself with all other pins of the gate array, well it have to show some results... now that you have 2 GA you can check it the two gate array bring the same results on all this values. I have little knowledge but maybe is a step.


Yeah I have had a look at some of the other pins, there is a clock but I don't have the equipment to see how fast.

I seem to have a unlisted variant, it's a Rev 1 (https://www.cpcwiki.eu/index.php/Mainboard_Versions#CPC464_version_1_.28original.29) but it doesn't have those white wires, which apparently have resistors in them, I've got just got resistors on the pcb. Don't seem to be be able to attach pictures here, but I'll submit to the wiki if the completeness is appreciated. The gate array a plain 40007, no revision numbers so I'm guessing this is one of the first of the production line.

Title: Re: Garbled screen
Post by: Nworc on 22:57, 31 January 22
Quote from: Curlypaul
I came to test my new 512kb ram board and find my CPC464 has developed a fault, showing a garbled screen.
It changes, but mainly it's blue and yellow vertical rectangles or light green columns, and very occasionally, a crazy vsync. The prevelance of blue and yellow suggests some sort of initialisation is happening?

Hey, sound's like you made a new demo, but in hardware, not in software. :)

Would you mind to take a picture of the monitor showing this bug and adding it here? Maybe if one could see the effect, we might have an idea where this effect comes from.
Title: Re: Garbled screen
Post by: Curlypaul on 20:30, 02 February 22
I will later, the spare board turned up today and starts up at least. I'm hoping it's not the gate array and I can get myself two working boards, but I'll document it here and try and get a decent pic for the wiki.
Title: Re: Garbled screen
Post by: Curlypaul on 23:13, 02 February 22





This is what the screen looks like


Good news is that the GA's both work, so whatever it is it is fixable. Guess I'll switch the ram over next. Should be able to get them both working and maybe find a way to mount the seond in a modern case or something


[url=https://drive.google.com/file/d/1CW4YMgpy87JoQVksFNBvWeOULlv6K8hf/view?usp=sharing]https://drive.google.com/file/d/1CW4YMgpy87JoQVksFNBvWeOULlv6K8hf/view?usp=sharing (http://[size%3d2px/)

I've no idea what's wrong
Title: Re: Garbled screen
Post by: Audronic on 23:22, 02 February 22
This is what the screen looks like.

Title: Re: Garbled screen
Post by: Curlypaul on 23:34, 02 February 22
Thanks, was getting frustrated with it, dunno why it wrapped it all in links
Title: Re: Garbled screen
Post by: Nworc on 00:02, 03 February 22
Cool, nice picture!

To me the picture looks like the display of uninitialized RAM. I saw something similar as I played around with the CPU hardware wise. Maybe the CPU is not working, maybe the RAM is not powered.
Title: Re: Garbled screen
Post by: Curlypaul on 00:36, 03 February 22
Quote from: Nworc on 00:02, 03 February 22
Cool, nice picture!

To me the picture looks like the display of uninitialized RAM. I saw something similar as I played around with the CPU hardware wise. Maybe the CPU is not working, maybe the RAM is not powered.


You weren't wrong when you said I'd created a demo, imagine it zooming in and out and some scrolling.


Thankfully these rev 1s are socketted, it appears I have two dead ram chips. I could have just took a punt on the xheapest thing to replace  :picard:  but I got new tools and a full spare motherboard.


Need to look at options on connecting a modern keyboard, if there are any?
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