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Author Topic: P8245 Graphics Chip fault.  (Read 2449 times)
davegsm82
Trying to get the cartridge in...
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« on: June 11, 2016, 04:06:53 PM »

I have just decided to look at a G7000 I was given a while ago and have found that the graphics IC appears to be defective.

When you power on the machine there is a blank screen and no sound coming from the TV. I have checked the processor and the Video IC for clock pulses with a scope and they appear to be present and correct (17-ish MHz at the proc and 3.x MHz at the Video IC). Looking at the data busses there appears to be activity but still nothing on screen.

I have the older type machine with the 2x 5V Regulators on board, they get warm to the touch normally so the system is drawing some power. I noticed that when I accidentally shorted one of them to ground (the inner most one) there was suddenly a flash of colour on screen (I will add a photo of this later) which stayed on until I reset and it went blank again.

Looking at the circuit diagrams I found online again I noticed the P8245 has an interrupt output which must poke the processor every so often to tell it to serve up the next line of video. I connected a signal generator up to this line and set it to various frequencies which causes the vertical lines to change, at certain frequencies you can almost see what looks like characters but garbled and stretched vertically. Pulsing the line at a few Hz allows you to see each line change. When you run the Video chip like this, the regulators start to heat up so looks like the system isn't running properly at all.

So, it looks like the P8245 is partially fried. Or is there a failure of something else which causes this kind of problem? It at least partly works, since it is able to generate some of the graphics on screen when you prod the processor and tell it to serve up more data.

Is there anywhere I can get another P8245 in the UK? I'd really like to get this machine running as I have a few games and would love to do some homebrew stuff in the future as I'm accustomed to programming already.

Dave.
« Last Edit: June 11, 2016, 07:23:25 PM by davegsm82 » Logged
davegsm82
Trying to get the cartridge in...
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Posts: 5


« Reply #1 on: June 11, 2016, 04:24:03 PM »

Heres the photo.
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Rene_G7400
Pedit5
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« Reply #2 on: June 17, 2016, 09:50:22 PM »

An interrupt pulse is generated each frame (50 Hz), not each line.
The only way to get a 8245 chip is to take one out of a Videopac console.
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davegsm82
Trying to get the cartridge in...
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Posts: 5


« Reply #3 on: July 04, 2016, 10:00:38 AM »

An interrupt pulse is generated each frame (50 Hz), not each line.
The only way to get a 8245 chip is to take one out of a Videopac console.


Ok, thanks for that Rene.

Another forum member has sent me a P8245 which I will be installing and testing sometime this week, with any luck this will cure the problems, if not then at least I have a starting block to push against.

Dave.
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Sweersa
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« Reply #4 on: July 06, 2016, 12:32:03 AM »

An interrupt pulse is generated each frame (50 Hz), not each line.
The only way to get a 8245 chip is to take one out of a Videopac console.


Out of curiosity, are these chips interchangeable between the Videopac and Odyssey 2?
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Rene_G7400
Pedit5
I take the Videopac and leave the Canoli!
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« Reply #5 on: July 06, 2016, 09:37:22 AM »

Out of curiosity, are these chips interchangeable between the Videopac and Odyssey 2?

No they are not. Odyssey2 consoles have a 8244 chip, Videopac consoles have a 8245 chip.
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Sweersa
Give this man a Jopac!
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Posts: 1060


« Reply #6 on: July 17, 2016, 02:51:12 PM »

Out of curiosity, are these chips interchangeable between the Videopac and Odyssey 2?

No they are not. Odyssey2 consoles have a 8244 chip, Videopac consoles have a 8245 chip.


Interesting to note, thanks!
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