Hello there, this is my first post.
I have a Brazilian Apple IIe clone, named Tk-3000. It was produced in 1986 and worked fine for many years, up to a few years ago. The first problem was that the disk drives began to fail, but I could still boot to AppleSoft Basic (but that is not the topic for today). Then it started to show memory corruption and loss of video signal. I have been trying to troubleshoot it for a long time now, with no success, so I found this forum hoping someone could offer some advice.
I have removed all addon boards, leaving only keyboard and display attached to the motherboard (pic0).
The video connector is a RCA composite video which I plug into a modern flat TV (more on this later).
- The computer boots ok, everything seem to be fine, I can run some simple BASIC programs (pic1 & pic2).
- After a few minutes, though, the video signal starts to flicker (pic3).
- It gets worse and sometimes the TV can not recognize the signal anymore (pic4).
- Sometimes the signal comes back to normal on its own, sometimes it comes back after a full reset (Ctrl-Apple-Reset), only to start flickering again.
- Sometimes, during the flickering, some garbage characters appear on the screen and/or the computer freezes (pic5).
- Booting into self test mode sometimes it works (pic6), sometimes not (pic7).
- After some point, the computer is no longer usable. If I turn it off for a looong time, it usually goes back to the begining (boots ok).
What I have tried:
- Measured the voltages with a multimeter while computer was on (all voltages seemed ok, even during the flickering).
- Visually inspected the capacitors on the motherboard, none showed obvious signs of leaks.
- Reseated all ICs (all of them are socketed), problem persisted.
- Tried three different display setups, all giving the same problem:
Computer -> RCA cable -> LCD TV with composite Input
Computer -> RCA cable -> RCA2HDMI adapter -> LCD TV with HDMI input
Computer -> RCA cable -> RCA2VGA adapter -> Monitor with VGA input
* I do not have an old tube TV to test with.
Does anyone recognize these simptoms ? What should I try next ?
(I have a bunch of spare 74XXX logic ICs, and have also ordered some 4164 RAM ICs)
(I do not have an oscilloscope)