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The other thing to remember is that, in 1-bit mode, the SQD01 shouldn't be doing anything (at least as far as I know), since there is no graphics acceleration - not using accelerated transfers. So, if there is an issue with U19.23, it could be that it is related to the SMT02 instead. Also, the...

No shame there - for either of us - hard to know anything about a 35-year-old de-identified build. ;) But great that you now have a spare...and hopefully it works! You can retire the other one. :D

The MT42C4256 VRAM pins you are looking at have different functions. Some are serial data and others are just data. Also, SDQ is a Serial Data pin. SQD is Squid - careful with those acronyms, buddy. ^_^ Maybe write out Squid instead or use a little squid icon. :D When you're comparing U19.26...

Well...I told you they were GALs. Alas, I did my best. :P ^_^

Yep - GALs. That board was probably produced later after nobody cared about secrecy. You can compare the serial numbers to check. But, same hardware, later build.

Did you hook up the entire VRAM to the logic analyzer? Try to look at the entire transfer waveform to see what might be wrong with the timing, strobes, rise/fall times, etc. Maybe there is something hanging on the line or a bad discrete. It could also just be a single failure in one of the ICs...

Good job on the debug schematic capture! I would really suggest hooking up a logic analyzer to all the pins on U19 and U21 to see what the RAM waveform/cycle timing looks like. You don't have the functional spec for the SuperMac devices, but you do have specs for the RAM. If there is bad data...

Good job!! You should probably just reflow all the RAM pins and maybe the SMT02 -- but carefully and only after thorough cleaning -- you could have microscopic debris shorts that have worked their way behind the pins. Blast it out with compressed air. Check all of the SMT02 pins for shorts --...

If the traces are damaged, then they will be resistive and/or intermittent. The problem you are seeing with the broken column is 100% reproducible, so not intermittent. Try not to flex the board too much -- you could do more damage. Just carefully check resistance on the VRAM traces and look for...

Good job. It could have been bad solder, debris or electrolytic corrosion. Unless you thoroughly clean the hole (and the pins -- to remove any oxidation/surface issues, there could be residual problems (and it might depend on the degree of via-trace dendrite invasion, etc.). If there is debris...

The interposer should be helpful. Anything directly connected to a NuBus line on the board could be ESD'd from handling (buffers, etc.), since people sometimes handle the NuBus connector from the bottom and touch (and maybe shock) the pins. It's an unknown. Also, if you have a PLCC puller, you...

Yes - there needs to be a crystal installed. There are no clock synthesizers on the early boards. Those came later. I think Spectrum/24 originally shipped with 64.00Mhz oscillator installed on the board and a 30.24Mhz in the box. If users wanted to use NTSC, they could call in for a 14.31818Mhz...

Cool - original Spectrum/24. Make sure the pin 1 orientation of the oscillator is correct when you are trying it and that you are using a standard monitor - like SuperMac 19" 60Hz or Apple 13" with a known good sync-on-green cable. The standard oscillators for that board (and also the...

Seems like you're making progress. It's certainly possible that the BSR and DAC are OK. If you have identified a bad VRAM, you could just substitute or replace it. I think you have other boards that use the same parts. Or, you could use (or write) a board VRAM R/W tester. If any RAM is bad...

Yes - CLUT/DAC - same. I shouldn't call it that - mostly just legacy metonymical slang. It's a DAC (that includes CLUT functionality). Within the DAC, drivers set up the CLUT values and/or luminance mapping for mono modes. The 473 is a triple-channel DAC, but I thought the board used it in...

Looks like you've been busy! The reason you are seeing the common green channel is that I think it uses the green channel for up to 8-bit mode -- typical of SuperMac boards -- although I don't think the Bt473 cares which channel it uses (configurable) in lower modes. It only uses triple channel...

@eharmon Yes - correct - more configs -- I was forgetting. Thanks. Eventually 1360x1024 and 1600x1200. The DSP board connections go all the way back to Thunder/24. The original Thunder/24 had a max of 1152x870. I don't know as much about later boards -- after '93/'94 -- but performance-wise...

The V may have had more supported monitors or other hardware changes to consolidate manufacturing. And not sure if the DSP modules are the same or not. I know the Thunder/PDQ+ swap works, but I haven't tried the others. However, I think I have all of these boards.

- There was only the original Spectrum/24, then Spec/24 III and beyond. - The Thunder/24, PDQ+, Spec/24 IV and Spec/24 V use all the same parts and the acceleration is SQD01-based. - I don't know if the Spec/24 V ROM is backwards compatible (haven't tried it), but Spec/24 IV should be (just a...

You don't have to remove the port. Just make sure there are no solder voids and test from the connector with pin jumper wires.