I've got a beeb here that's not working. I've worked my way through the Service Manual, and all the bits I can check seem to be working ok. The various clocks are all working, RnW is working, reset is working. Horizontal and Vertical sync signals (IC39 pins 39 & 40) are there. CPU sync pin (pin 7) shows activity for a period, and then stops.
I've swapped out various ICs (6522s, 6845, 2023), but that's not made any difference.
Swapping out the OS ROM for Trickys test ROM shows some odd graphical behaviour, but the processor seems to be running ok - CPU sync pin (pin 7) keeps oscillating. The LEDs blink as they're meant to, and I get audible pings at the start of the test. The relay also clicks as it switches from a corrupt mode 7 screen to a corrupt mode 0 (?) screen.
I suspect a memory issue here, and I've tried the usual S25 jumper removal / reposition, but that doesn't seem to make much difference, so I'm thinking it might be related to CAS or RAS logic.
https://youtu.be/VfOJqNhWmu0
Was looking for some thoughts before I start pulling ICs...
Thanks!
I've swapped out various ICs (6522s, 6845, 2023), but that's not made any difference.
Swapping out the OS ROM for Trickys test ROM shows some odd graphical behaviour, but the processor seems to be running ok - CPU sync pin (pin 7) keeps oscillating. The LEDs blink as they're meant to, and I get audible pings at the start of the test. The relay also clicks as it switches from a corrupt mode 7 screen to a corrupt mode 0 (?) screen.
I suspect a memory issue here, and I've tried the usual S25 jumper removal / reposition, but that doesn't seem to make much difference, so I'm thinking it might be related to CAS or RAS logic.
https://youtu.be/VfOJqNhWmu0
Was looking for some thoughts before I start pulling ICs...
Thanks!
Statistics: Posted by KenLowe — Tue May 14, 2024 12:22 am