C64 repair question

[ianoid]

I have a C64 previously working motherboard that stopped working. I have tried swapping out the PLA and the SID, using the backbit.io clones and another PLA clone, as well as another (unknown working because it's from missing a 6582) PLA. I added sockets for the PLA and SID which look ok. Regardless of the combination of chips, I get this screen (on my LCD monitor with AV input) with checkerboard line on the right. On a 1702 I just get wavy lines.

 

No RAM chips get hot. I don't have a dead test cart. 

 

Any suggestions as to what to try next?

 

Thanks,

 

ian

 

[Virtualsky]

Have you looked through the Pictorial C64 Fault Guide for clues?  You could also remove the SID chip altogether and boot up to rule it out as a problem.  From what I understand, the C64 will boot up without the SID chip.

[ianoid]

1 hour ago, Virtualsky said:

Have you looked through the Pictorial C64 Fault Guide for clues?  You could also remove the SID chip altogether and boot up to rule it out as a problem.  From what I understand, the C64 will boot up without the SID chip.

 

 

Thanks for the link. The screen I get is not similar to any of the screens shown. The screens for repair all look like boot up screens when I'm unable to even see a boot screen. And when I use RF out it's garbled or black screen. I tried both channel switch settings on the 64. I might continue testing it using RF since the AV input of my TV is probably confusing me. 

 

I used my Chip Tester Pro to confirm I have one bad PLA but the other PLA tested ok but runs super hot, like ouch hot within a few seconds. My SID and backSID both test ok. I also tested the VIC-II and that tested ok. 

 

I use a modified power supply that worked fine with my VIC-20. I'm assuming that if it works with a VIC, it should work with a C64. 

 

What to try next?

[+scorpio_ny]

On 8/15/2022 at 4:38 PM, ianoid said:

 

 

Thanks for the link. The screen I get is not similar to any of the screens shown. The screens for repair all look like boot up screens when I'm unable to even see a boot screen. And when I use RF out it's garbled or black screen. I tried both channel switch settings on the 64. I might continue testing it using RF since the AV input of my TV is probably confusing me. 

 

I used my Chip Tester Pro to confirm I have one bad PLA but the other PLA tested ok but runs super hot, like ouch hot within a few seconds. My SID and backSID both test ok. I also tested the VIC-II and that tested ok. 

 

I use a modified power supply that worked fine with my VIC-20. I'm assuming that if it works with a VIC, it should work with a C64. 

 

What to try next?

On some of the repairs that I did on my C64 motherboards, was either MT RAM which is know to die and Commodore produced logic chips which are also notorious for failure. Could you post a picture of your motherboard?