![]() So it's not worthwhile damaging a good ULA. There is a modern recreation, but it's currently only available in very limited quantities from one person. Two reasons: Replacement ULA chips are no longer available. I don't think it's wise to go below 150 ohm, and even that may cause problems. If I'm right here, then you should find that RAM tests to the upper bank of RAM pass, as they are not going through these resistors.Īlso, you could try bypassing just one resistor (by soldering a 120R resistor in parallel), and see if that bit then starts working. Mark, do you have any thoughts on whether it would be safe to reduce these resistor values? 100 ohms would give 0.74V, which should be OK 220 ohms would give 1.08V, which is still a bit marginal Reducing the values of R1 to R8 might help: It's quite likely this will be read as a one. The problem is, there are strong pull ups (1K5) on the GODIL inputs, and also pullups (8K2) on the Spectrum, which means the voltage the GODIL sees when the a zero is being read will be raised, actually quite a bit.Īssume the RAM is able to pull down to 0.4V. These are present to prevent any possible contention between CPU write cycles, and the ULA reading the display memory. When the ULA and/or lower 16K of RAM is driving the CPU's data bus, it's doing so via 470 ohm resistors (R1.R8). I've just been looking over the Spectrum 48K schematic: ![]()
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