(This was originally posted to Twitter on February 3, 2015.)
February 3, 2015
Today, I’m going to tackle this new-to-me Commodore SX-64 computer. I bought it cheaply from a seller that said it wasn’t working. It has a few faults that we’ll take a look, including the keyboard which has a few unresponsive keys.
To start, I’m using the Commodore 588220 Diagnostic Cartridge. Here I don’t have the diagnostic harness attached, which will show some false BADs. However, the RAM, PLA, and ROM are showing as problematic so we’ll start there.
February 4, 2015
Swapped in a known working C64 Kernal ROM in the SX and get the same result (diff startup color tho) so I think it’s good. Next is the PLA.
Wasn’t expecting to find a 28 pin socket for C64 SX-64 Kernal either. But it’s not 27xxx compatible, it’s based on 23xx/25xx layout :-/
Ok, so SX-64 PLA was definitely bad. I pulled one from another C64 to test. RAM U22 still complains as does kernal.
What’s wrong with this picture? (Hint: There should be more BASIC bytes free than that.)
February 7, 2015
So what’s the current preferred method to replace the #c64 PLA? Original pull? EPROM replacement? SuperPLA?
Had a @go4retro C64 PLA ROM adapter so I put it together with a M27C512-12F EPROM and so far it works!
2023 Update: There are now a multitude of PLA replacements available such as the PLA20V8 which uses two readily available GALs.
Now to tackle that SX #c64 RAM problem. Someone’s attempted a repair before. Two jumpers and flux everywhere.
Swapped out UB6 with a socket and new RAM and it passed the RAM tests. Now, why does the Kernal error persist?
Swapped the original SX-64 251104-04R Kernal ROM with a 2764 EPROM (w RETRO Innovations adapter) with confirmed new burn of the SX-64 Kernal and still get the BAD Kernal error with the Diagnostic cartridge.
I tried diagnostic cart 586220 in my other SX-64. It recognizes it’s in an SX, it also reports the stock Kernal as bad. So, no problem..?
I also tried the C64 diagnostic 586220/58820 in VICE with the SX-64 ROM. It still reports BAD. It knows it’s in an SX-64 yet checksum still fails..?
2023 Update: The original C64 diagnostic cartridge will show false positives on the SX-64. There are now updated versions of the diagnostic cartridge 586220 that resolves these issues.
Side note: ever notice blue painters tape matches SX-64 handle caps? Temporary fix until I can locate or 3D print.
February 8, 2015
Let’s table the keyboard next. Cleaning the SX-64 keyboard is tricky. Patience will reward. It’s essentially a thing, delicate rubber-composite membrane that rests on top of a circuit board. The key plunger pushes against the rubber and causes the black conductive material to come in contact with the circuit board, closing the circuit. This hook took is useful for lifting sticky bits
The rubber membrane sticks around the contacts from years of pressure. Lift gently! Afterwards, clean the circuit board well with IPA.
All back together and working well. Calling this one done. Now for Prince of Persia on EasyFlash 3!
Do you have a SX-64 with a fault? Let me know in the comments!