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Oric Atmos blue garbage on screen

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Message par WiZkiD le Mer 30 Mar 2016 - 8:17

Hello everyone, could you please halp me a bit with my defective Oric? I recently bought a Oric Atmos and it is in very good condition but unfortunately when I turn on the computer the screen shows only blue garbage. So I've searched the net to find some informations about the problem and it seems that this issue is due to defective ram chip, often but not always. So I opened the computer and realized that it has already been repaired by the replacing of two ram chips. What I wanted to ask is how to procede, can I start by replacing the ram chips or there is some important test to do first?

many thanks in advance Smile
WiZkiD
WiZkiD

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Date d'inscription : 30/03/2016

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Message par Godzil le Mer 30 Mar 2016 - 10:20

The first thing you should do, is to powerup the computer and leave it powered for a few minutes, if the problem is in the DRAM, one of the chip should be hot. If one is hot, there is a high chance it's the culprit.

Check also all the big ICs (you can also the 74LS chips, but they are likely to fail, but who knows)

Can you take a picture of what you have on screen?
Godzil
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Message par WiZkiD le Mer 30 Mar 2016 - 16:17

First of all many thanks for the reply. I left the computer powered for five minutes and none of the chip was hot. I've checked the ULA and it was cold as the cumputer had not be turned on at all, the other chip was only slightly overheated as they should be.

This is a picture of the screen garbage.
Oric Atmos blue garbage on screen Dscn1411
WiZkiD
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Message par Godzil le Mer 30 Mar 2016 - 16:37

Ok so you have a coherent display, are you connected with RF or RGB? because the background should be black and not that blueish, if it's RGB, your TV may have a setting problem, or something is wrong in the SCART plug.

Anyway, that's not the main problem here.

What chips are on a socket?

There are, I think, no faulty DRAM as far as I know because there are no vertical lines on the display, it's not a 100% confident test of course.
It seems the ULA display the random memory content before the memory is initialised.

One possibility, the ROM is badly socketed, you will have to remove and put it back in the socket
The 6522 may also have a problem and prevent the 6502 to finish the boot process
Or the culprit is the 6502, that could happen that it died because someone does something wrong with the expansion port...

What you can try is to remove the ROM chip, power the computer and check the difference in pattern
Then is resocket it does not change anything, if the 6522 is on a socket, remove it.

One test if the 6522 is not socketed, it to short I/Ocontrol with ground on the expansion port that will force the VIA to not be online.
Godzil
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Message par WiZkiD le Jeu 31 Mar 2016 - 4:09

Thanks for the tips, I've ordered a buch of dip socket on ebay to so I can replace every chip without having to desolder every time.
As soon as I get the sockets I'll do some test.
By the way, I'm connecting the computer via RF for now, but I want to make an RGB cable by myself Smile
WiZkiD
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