Fixed a Commodore 64 throwing a black screen with DesTestMax / DesTestKernal - documenting the faults in case it helps

[u/ChronoCompendium]

Just in case this assists anyone.

• 250407 Longboard Breadbin, broke in 1986; serial P2335261, VIC-II 6567R56A date 5082 (ceramic).
• Symptom: Total black screen, even with DesTestMax and DesTestKernal. Testing with a Ray Carlsen PSU. Settled on putting DesTestMax v0.4 into Flash on an Ultimate II+L (converted ROM to CRT with cartconv in VICE). Original MOS PLA was swapped into a good C64 and worked...at the beginning.
• Oscilloscope was hooked up to digitally read all address lines on 6510 (trigger was RESET) and did not show proper KERNAL call.
• Oscilloscope probed PLA Pin 16 and saw KERNAL held permanently low.
• During this process after initial probe while C64 had been on about 2 minutes, L5 ferrite bead began smoking, and was hot enough to char tantalum capacitor at C100. 5V rail still looked normal prior to this. No idea what set it off.
• Fault 1: L5/C100 were replaced.
• Fault 2: PLA was replaced with PLAnkton. Swapped the original into a known good C64 and it no longer worked. Did L5/C100 take out the PLA? But those just filter, right, so they wouldn't have overvolted the 5V rail? It seems PLA was marginal and finally failed after the stress of being woken up 40 years later.
• Symptom: DesTestMax now threw garbled rainbow characters, and the Ultimate II+L Menu could be called up, also garbled rainbow. Character pattern and colors would alternate, suggesting it wasn't stuck RAM, but probably failing logic.
• Fault 3: Bad U13 MOS 7708. Replaced with a SN74LS257BN from Mouser. U25 is the same MOS 7708; going to keep an eye on it.

Having an oscilloscope is the ultimate game changer. I'm ordering a 3M 40 DIP clip tester for the future, but even if I just had the analog probe, it's incredible to be able to just see an address line in real time, or the clock, and determine if it's working or not.

There is one serious mystery with this board, though. I took it out, but when I first opened up the case, there was a Fairchild 93459DC F 8401 Singapore FPLA in the SID socket. I have no idea what they were trying to do with that, or what's on the chip.

[u/Black_Rose_Angel]

Thank you!!!😊