Finally got my first C64 back to working order, writing this to the Wizball title music 😁
25 years ago or something like that, it would suddenly not read disks anymore. Was sure it was the computer, and not the disk drive, got three 1541 drives at the time, and none of them would work. Got another C64, but kept the faulty one through all those years. Finally decided to try to do something about it. Could of course not remember exactly what the symptoms were this many years afterwards, so wired it up to test it. Now it would not boot at all. Checked the fuses in the PSU, both OK. Opened up the computer, the internal fuse OK too. Removed all socketed chips, cleaned the sockets and pins, and reseated them.
Connected everything back again, and powered the computer up. This time it booted just fine. The disk drive got the reset signal when booting the computer, but would not respond at all to any other commands, which pointed at a faulty CIA chip. Luckily there are two of those in the C64, controlling different ports. Desoldered the CIAs, soldered in sockets, and swapped the chips to see if the serial port would get back to life. No dice. Good thing that wasn't the problem, those chips are expensive.
Took the computer apart again, and desoldered the HD7406P inverter chip for the serial line. Soldered in a socket. Borrowed the 7406 from one of my 1541 drives to test with. No dice this time either. Had no idea what to try from here on, so simply started following traces and checking all components between the inverter and the serial port. Found two shorted diodes, which luckily happened to be regular N4148 ones. Anyone with the slightest interest in DIY electronics, including me, always got a stock of those at home 😀 Replaced the diodes, installed the original 7406, put everything back together, and tested again. Same behavior. Crap. Opened the computer up yet another time, replaced the 7406 with the one from the 1541 drive, put it together again, booted it, and... THE SERIAL PORT WORKED 😁 😁 😁
So, all and all, at some point something killed the inverter chip and those diodes. The only thing I can think of is if I or my kid brother managed to plug in the PSU in the serial port by mistake, or perhaps unplugged/plugged the disk drive while the computer was powered.
Now there's only one thing left to do; trying to figure out why the text sometimes, seemingly at random, is replaced with "noise blocks" when booting the computer. It's always been like that, but back in the day, a simple reset would usually do the trick. Now it seems to be worse, got to power it off and let it sit for a while before powering it on again for it to show the text correctly. It looks like this:
Any ideas?
By the way, anyone here got a C64 with a lower serial number? 😁