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Commodore pet 2001-8 no video from logic board

There are two (2) different issues here - and you need to eliminate one of them before you can start to see the wood for the trees.

The two separate issues are:

1. The character generator ROM and/or circuitry.
2. The video RAM itself.

In order to make any headway with item 2 you really need to bottom item 1 first.

You can do this by removing the video RAM and 'forcing' logic 0 and 1 signals onto the RAM outputs via low-value resistors. This will permit various characters to be generated that would occupy the whole screen image.

The character screen presented in post #440 is still not 'random' though. For example, there are no inverse video characters (assuming this is a 'power-up' screen display).

Dave
 
daver2 said:
There are two (2) different issues here - and you need to eliminate one of them before you can start to see the wood for the trees.

The two separate issues are:

1. The character generator ROM and/or circuitry.
2. The video RAM itself.

In order to make any headway with item 2 you really need to bottom item 1 first.

You can do this by removing the video RAM and 'forcing' logic 0 and 1 signals onto the RAM outputs via low-value resistors. This will permit various characters to be generated that would occupy the whole screen image.

The character screen presented in post #440 is still not 'random' though. For example, there are no inverse video characters (assuming this is a 'power-up' screen display).

Dave
Click to expand...
Swapping the video ram around I am getting lots of different results.
When I get a working set I guess I should be getting at least a flashing curser is that correct?
Pic showing some gfx characters
 

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LOL You are some way off the flashing cursor stage!

If the CPU isn't running the correct code, the video memory is not accessible or the video memory is faulty then the screen cannot be cleared and then rendered correctly... you may be at that stage.
 
Nivag Swerdna said:
LOL You are some way off the flashing cursor stage!

If the CPU isn't running the correct code, the video memory is not accessible or the video memory is faulty then the screen cannot be cleared and then rendered correctly... you may be at that stage.
Click to expand...
What would you do next. Some of the characters do flicker occasionally.

Is it time to start a new thread.
 
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RetroGadgetMan said:
The person I got the eprom from said it is 64kb and printed the starting section on it of 0xF800 for my information.
Click to expand...
OK, then it is a 27512 EPROM that has the upper five address lines strapped high to point to $F800. However how does it match the pinouts of the 6540? Is the EPROM in surface mount with a PCB? Pretty nice if it is.
 
dave_m said:
OK, then it is a 27512 EPROM that has the upper five address lines strapped high to point to $F800. However how does it match the pinouts of the 6540? Is the EPROM in surface mount with a PCB? Pretty nice if it is.
Click to expand...
Yes it is that model. Winbond. There is an ic mounted on the Pcb below the socket that takes care of business.
 
Check the 6502 CPU pin 7 (SYNC) for signs of activity. This line should pulse HIGH for every machine instruction that is fetched and executed. If this signal is permanently LOW then the CPU is not executing and you can forget your flashing cursor I am afraid. The ROM/RAM and CPU support logic has to be working to get this to work...

Just out of interest, where are you in England? Anywhere near the M5/M6 from Gloucester to Lancaster? Send me a DM if you wish. I regularly travel this route (although I am less inclined to do this until the new year now) so I could stop off and help out if you wish (if you are near).

Dave
 
daver2 said:
Check the 6502 CPU pin 7 (SYNC) for signs of activity. This line should pulse HIGH for every machine instruction that is fetched and executed. If this signal is permanently LOW then the CPU is not executing and you can forget your flashing cursor I am afraid. The ROM/RAM and CPU support logic has to be working to get this to work...

Just out of interest, where are you in England? Anywhere near the M5/M6 from Gloucester to Lancaster? Send me a DM if you wish. I regularly travel this route (although I am less inclined to do this until the new year now) so I could stop off and help out if you wish (if you are near).

Dave
Click to expand...
Thanks for the offer, very kind of you but I am in the East Midlands.
I think it's probably time to start a new thread seeing as we are now heading into new territory.
I will do this tomorrow with the result of pin 7.
 
especially if the original IC now appears to require negligible force to insert it into the socket, I would suggest fitting a new socket.
Click to expand...

As a stop gap measure prior to the eventual replacement of the socket, what you can do is insert a turned-pin socket (which has thick pins) into the suspect faulty socket and then place the device which was originally in the socket into the turned pin socket. The thick pins of the turned-pin socket will make better contact with the contacts in the dodgy socket than the thin pins of the IC would but they will also make the dodgy socket an even looser, less reliable fit for directly inserted ICs, so this is something you only do once you have given up on the original socket and intend to replace it anyway.
 
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