DrAM19
Experienced Member
Hi Everyone,
I am repairing an NTSC C64 breadbin (205407) for a friend and I have encountered a very strange situation.
When I first got it, it displayed a black screen. Checking the PLA in a working C64 of mine showed it was bad. Swapping it with a working PLA did not get me a picture on the screen.
I next noticed the reset line did not release to 5v on startup. I desoldered the reset legs from the CIAs (not socketed chips and I didn't want to cut the legs; I verified they were isolated after desoldering) and confirmed the MOS 7707 Hex Inverter at U8 was bad, so I socketed U8, replaced it with a 7406, and resoldered the two CIA legs.
At this point I got the normal blue startup screen, but had an "Out of Memory Error 0" message, along with some garbage on the screen. Following the errant characters, I started desoldering and socketing RAM one at a time. Each time, one class of junk character disappeared. This continued until I socketed and replaced all 8 4264 RAM chips. (MT RAM unfortunately; and I checked them in a chip tester, all 8 were bad). I then finally got a BYTES FREE message. So now I highly suspect a bad power supply or major static situation.
However this is where a very unexpected symptom appeared.
Sometimes the BYTES FREE message has the wrong value, and sometimes the screen stays black on boot. If I get the BYTES FREE message on screen, and my hand goes near the RAM bank (and I do not touch it), I will get some of the junk characters on the screen again. And they do seem to correspond to which chip I am near (U21 puts a bunch of ! on the screen for example).
Has anyone else ever seen this? What was the cause? My inclination is bad capacitors (C14 is electrolytic, C25-28 and C40-43 are ceramic), but I do not have great evidence to support this.
Thanks!!
I am repairing an NTSC C64 breadbin (205407) for a friend and I have encountered a very strange situation.
When I first got it, it displayed a black screen. Checking the PLA in a working C64 of mine showed it was bad. Swapping it with a working PLA did not get me a picture on the screen.
I next noticed the reset line did not release to 5v on startup. I desoldered the reset legs from the CIAs (not socketed chips and I didn't want to cut the legs; I verified they were isolated after desoldering) and confirmed the MOS 7707 Hex Inverter at U8 was bad, so I socketed U8, replaced it with a 7406, and resoldered the two CIA legs.
At this point I got the normal blue startup screen, but had an "Out of Memory Error 0" message, along with some garbage on the screen. Following the errant characters, I started desoldering and socketing RAM one at a time. Each time, one class of junk character disappeared. This continued until I socketed and replaced all 8 4264 RAM chips. (MT RAM unfortunately; and I checked them in a chip tester, all 8 were bad). I then finally got a BYTES FREE message. So now I highly suspect a bad power supply or major static situation.
However this is where a very unexpected symptom appeared.
Sometimes the BYTES FREE message has the wrong value, and sometimes the screen stays black on boot. If I get the BYTES FREE message on screen, and my hand goes near the RAM bank (and I do not touch it), I will get some of the junk characters on the screen again. And they do seem to correspond to which chip I am near (U21 puts a bunch of ! on the screen for example).
Has anyone else ever seen this? What was the cause? My inclination is bad capacitors (C14 is electrolytic, C25-28 and C40-43 are ceramic), but I do not have great evidence to support this.
Thanks!!