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Commodore 64G - need help

flash951

flash951

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Joined
Aug 18, 2017
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17
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Norway
See screen picture, display with scrambled distortion. Every IC i. socket and checked out, also recapped machine and checked all diodes and ceramic caps for shorts.

Any idea?
 

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Funny... yesterday I took a C64G that I got a while ago out of storage for testing/restoration. Guess what? It has *exactly* the same fault!

Text mode looks garbage, just like in flash951's screenshot. The system works, however. You can type and run BASIC programs, load games from disk with perfect sound and graphics etc. It's just the text mode that is messed up. Also, Dead Test just shows a pink screen. Though I'm not sure if the version of Dead Test I have actually supports that board revision.
 
Timo W. said:
Text mode looks garbage, just like in flash951's screenshot. The system works, however. You can type and run BASIC programs, load games from disk with perfect sound and graphics etc. It's just the text mode that is messed up. Also, Dead Test just shows a pink screen. Though I'm not sure if the version of Dead Test I have actually supports that board revision.
Click to expand...
@flash951 This does raise some questions. What happens when you try to load games from disk or cartridge on your 64G? Do they load and play fine like they do for Timo? Also, what was done to confirm that the sockets and Character ROM's were functional?

Is there any change if you gently push down on the Character ROM while the computer is running? To me it seem that both may be good (The ROM is working and the socket has no broken leafs) but something is not making full contact/connection. Have you tried DeoxIT or something similar in the socket, and check for any cracked solder joints on the board, especially the Character ROM socket, but I would check the CPU and the PLA as well for broken joints, especially if disks and carts load funny as well.
 
Can't speak for him, but pressing on chips has no effect on mine. Only the SID and VIC-II are in sockets on my board and the contacts are fine.

When pressing and holding the reset switch on the Dead Test cartridge, the garbage characters go away and I have a perfectly (frozen) screen as long as I hold the reset switch.

I'm pretty sure the MOS 8701 is shot. Have to check if I can find a spare from a working machine to test.
 
All sockets pins are checked for continity, sockets are mostly new as chips was soldered originally. Char rom got the correct checksum and works in other c64, as explained, all ic's checks out. Ive checked all socket pins for shorts as well. All diodes and resistors are checked, caps are new, power switch is new as the old one was bad. Im using a good c128 recapped and adjusted psu.

Next thing will be to check for break in traces on pcb. And then try to scoop and follow signals, cpu - rom etc.


robert_sissco said:
@flash951 This does raise some questions. What happens when you try to load games from disk or cartridge on your 64G? Do they load and play fine like they do for Timo? Also, what was done to confirm that the sockets and Character ROM's were functional?

Is there any change if you gently push down on the Character ROM while the computer is running? To me it seem that both may be good (The ROM is working and the socket has no broken leafs) but something is not making full contact/connection. Have you tried DeoxIT or something similar in the socket, and check for any cracked solder joints on the board, especially the Character ROM socket, but I would check the CPU and the PLA as well for broken joints, especially if disks and carts load funny as well.
Click to expand...
 
Timo W. said:
Can't speak for him, but pressing on chips has no effect on mine. Only the SID and VIC-II are in sockets on my board and the contacts are fine.

When pressing and holding the reset switch on the Dead Test cartridge, the garbage characters go away and I have a perfectly (frozen) screen as long as I hold the reset switch.

I'm pretty sure the MOS 8701 is shot. Have to check if I can find a spare from a working machine to test.
Click to expand...

Cpu/mpu usually dont fail in short board models. Check char rom.
 
Timo W. said:
Funny... yesterday I took a C64G that I got a while ago out of storage for testing/restoration. Guess what? It has *exactly* the same fault!

Text mode looks garbage, just like in flash951's screenshot. The system works, however. You can type and run BASIC programs, load games from disk with perfect sound and graphics etc. It's just the text mode that is messed up. Also, Dead Test just shows a pink screen. Though I'm not sure if the version of Dead Test I have actually supports that board revision.
Click to expand...

Only in text mode on my c64g as well, games loads and looks good in bitmap mode.

Checked continuity on traces from char rom, all ok.
 
I'm pretty sure it's the MOS 8701. It must be some timing issues while reading out the lines from the character ROM.

I have a C128 with a 8701 that I can swap for testing, but I'm busy with other projects now and the C128 is in storage anyway.

btw, does the Dead Test run on your machine?
 
Timo W. said:
I'm pretty sure it's the MOS 8701. It must be some timing issues while reading out the lines from the character ROM.

I have a C128 with a 8701 that I can swap for testing, but I'm busy with other projects now and the C128 is in storage anyway.

btw, does the Dead Test run on your machine?
Click to expand...
I've ckecked all IC's, including 8701. Dead test runs, got it in Flash bank of MMCReplay. Also just got the new DesTestMax cart. Ive now checked all traces from U8 mem controller/mmu/pla and char rom. Will check traces from 8701 tonight.

Got a c128 with same error, but only in 40col C128 mode, not in C64 mode. And display is correct during freeze/reset. I suspect defect in CPU IC on the C128.
 
After some more testing, I fear it's a fault with the SuperPLA on my machine. The cartridge port does not work at all - not just the Dead Test. Depending on what cartridge I try, I either get a black screen or the system just starts BASIC, but with reduced free memory (so it sees the ROM and maps it into memory, but code execution does not happen).
 
Timo W. said:
After some more testing, I fear it's a fault with the SuperPLA on my machine. The cartridge port does not work at all - not just the Dead Test. Depending on what cartridge I try, I either get a black screen or the system just starts BASIC, but with reduced free memory (so it sees the ROM and maps it into memory, but code execution does not happen).
Click to expand...
I am not sure which revision of the board you have, but if you have a 250469 rev B, you can buy a new old stock SuperPLA here:


I had that link saved when from I tried to repair board that I suspected was the PLA, but it was the VIC-II that was dead. I guess never cleaning up my bookmarks folder may be have been handy for once.
 
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