Yes, those have all been checked. I'll post up a marked up schematic showing everything I've checked in the morning.Have you confirmed the address bus connections between the 6502 and the IC12/13, particularly A5 and A12?
I had already removed all the DRAM ICs before I saw this post. I tested each DRAM IC in a working beeb, and every one seems to be working fine!Also, If you measure the resistance between each DRAM address line and ground, do they all give the same(ish) value. Do this first with the 81LS95 removed, then again with these in place.
I've stripped the machine back to a minimal install and, so far I've also dropped out and tested / swapped out the following:
IC1 - 6502
IC2 - 6845
IC3 - 6522
IC6 - VC2023 (Video Proc)
IC14 - 74LS245
IC37 - 74LS04
IC40 - 74S00
IC43 - 74S04
IC44 - 74LS74
IC45 - 74S139
IC8..IC13 - 81LS95 (6 x Octal Buffers)
IC53..IC68 - HM4816AP-11 (16 x DRAM)
I've not put any of the Octal Buffers or DRAM back into the board yet as I need to order up a bunch of sockets first. But with these ICs removed, it does make it easier to remove others. So what next??? IC41 got a mention earlier, so I might try that. It feels like I need to socket everything in that area of the board, since I've already got it stripped this far back.
Some of the decoupling caps disintegrated when I removed them (to gain better access to the ICs) so I'm going to have to order some replacements for those too.
With the Tricky test ROM still installed, it's amusing to still hear the 'bongs' and see the alternating LEDs when powering the board up in this part built condition. At least I haven't completely broken the board. The CPU still seems to be doing something!
Edit: Found a bunch of 7, 8 & 10 pin dual wipe sockets, so I've got everything I need to put all the ICs back onto the board. That's a job for tomorrow! I now just need to locate some decoupling caps. I've got lots with 5mm lead spacing, but very few with 2.5mm spacing.
Statistics: Posted by KenLowe — Tue May 28, 2024 12:43 am