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Cbm pet 8296-d black screen

daver2

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>>> If i turn on with pettester, screen it's always ok!

Excellent, so that is good news.

I would test the keyboard and get that working next.

My "gut feeling" is that the 6522 VIA is dead as well - either that or the socket is also faulty.

Dave
 

daver2

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It should do.

You should get the initial Commodore BASIC 'welcome' screen - but no beep, no cursor flashing and no keyboard working.

Dave
 

daver2

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The trouble is there is a signal that comes out of the VIA (GRAPHIC) and goes to the character generator. If the VIA is removed, I am not sure what effect this will have on the display itself.

I think last time you did this the CPU SYNC pin (UB14 pin 7) stopped oscillating didn't it? This would indicate that the CPU has stopped executing instructions - and nothing to do with the video side of things. Can you confirm what the state of UB14 pin 7 is with your logic probe with the EDIT ROM in circuit please.

Dave
 

Dave_C78

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The trouble is there is a signal that comes out of the VIA (GRAPHIC) and goes to the character generator. If the VIA is removed, I am not sure what effect this will have on the display itself.

I think last time you did this the CPU SYNC pin (UB14 pin 7) stopped oscillating didn't it? This would indicate that the CPU has stopped executing instructions - and nothing to do with the video side of things. Can you confirm what the state of UB14 pin 7 is with your logic probe with the EDIT ROM in circuit please.

Dave
UB14 PIN7= low
 

daver2

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That is a possibility. But didn't you verify it previously against the EDIT ROM contents obtained from Zimmers?

Just out of interest - exactly what type of EPROM is the EDIT ROM - manufacturer and make? Can you photograph and post the markings on the top of it please? Is this an 'original' part or something you have programmed?

Dave
 

Dave_C78

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That is a possibility. But didn't you verify it previously against the EDIT ROM contents obtained from Zimmers?
Yes i verifyied and result was ok....

I burned this eprom:
 

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daver2

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Another quick question.

Can you send me a photograph of the EPROM markings that you have programmed the PETTESTER into please?

I am just wondering about access speeds - the EDIT ROM may be programmed correctly, but I wonder if it is too slow for use within the PET?

Dave
 

Dave_C78

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Another quick question.

Can you send me a photograph of the EPROM markings that you have programmed the PETTESTER into please?

I am just wondering about access speeds - the EDIT ROM may be programmed correctly, but I wonder if it is too slow for use within the PET?

Dave
 

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Dave_C78

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Good evening Dave, before moving on to vectrex again, I would like to finish this Pet .... I tried to burn the edit rom on an eprom 2532 (added twice the .bin file). Now if I turn on the computer, the Basic screen comes out with the cursor blinking!
But I don't understand why I can't hear the startup sound :(
 

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daver2

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First of all, well done...

Even though you managed to correctly verify the original EDIT EPROM, it must either be faulty in some way (preventing it from being read by the PET correctly) or the access time is too slow (or has degraded) to permit the PET CPU to correctly read the instructions.

That was what I was coming to the conclusion it was (hence the interest in the photographs of the devices that both work and fail). I was going to try and find the corresponding data sheets tonight. So there is no need for that now :)! One thing less to do...

Good work on programming another EPROM and trying it though!

You have got all the chips in haven’t you? You need both UB12 and UB15 present to get any audio output.

You can also put your logic probe on UD5 pin 3 and reset the PET. If you observe pulses, then the problem must be either resistor R8 (10 Ohms) or the sounder itself.

I seem to remember at some point in the past you had some audio output didn’t you?

Dave
 

Dave_C78

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You have got all the chips in haven’t you? You need both UB12 and UB15 present to get any audio output.
Thanks so much Dave...
All thanks to you!

Yes i have all ics inserted on board now! in the past i had audio....
 

Dave_C78

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I tryied put logic probe on UD5 pin3 and resetted Pet but i can't see any pulses :(
Maybe bad 6522???
 

daver2

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Your guessing again aren’t you!

Follow the logic backwards on the schematic and monitor UD5 pins 1 and 2 (when you power on or reset the PET).

Dave
 
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