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Gah, I think I found the culprit: RF14, I read one of my composite resistors backwards meaning I'm off 216k by a couple of orders of magnitude :confused: I don't have the right value to hand so I'll await the next parts delivery…

Sorry to bump such an old thread, but it seems like a good place to ask questions about this mod. I've got a 820-0360-B Color Classic analogue board that I've tried this mod on. I think it is the same revision as @Bolle 's. Unfortunately I'm seeing a very different issue after making the...

Thanks for the input. OSes: 7.0.1 and 7.5.3, SuperMac control panel installed. It doesn't look like there's a monitor sync signal coming from the card; the same VGA+ adapter setup works fine with the on board video and the E-Machines card but gets nothing from the Spectrum. I'll give it...

Hi Folks, I managed to nab a Supermac Spectrum/24 PDQ+ from Buyee, ROM version 1.292. Unfortunately I'm not having much luck getting it running in my (otherwise seemingly functional) iici. With the card in place the machine does not boot. The symptoms vary: iici hangs with the Happy Mac icon...

It's alive, Jim! I managed to lay my hands on a second LC575 board, albeit a fully functional one, and compared the CLUT connections to the ones I had already mapped. It turns out that there was indeed a dead-pin, the one I investigated back in post #35 (bottom-right but one on the CLUT). It...

That's odd; on my board that pin connects to R100 (which measures 48Ohms) and then to the top-left pin of U19. Where 'top' is having the ports facing away from the viewer. I count the pins starting from '1'. I haven't progressed this much further but I think I need to transfer my notes into a...

I haven't put the new chips on yet; having the board boot with the SIMM is enough for me to try to get to the bottom of the video issue. Unfortunately I haven't had time to do much more investigation; there's what looks like sensible continuity between U26, the VRAM SIMMS, RAM (data), MEMCjr and...

I actually had enough time to remove the RAMDAC/CLUT/U26 and give it a clean and even get it back onto the board. Underneath it didn't actually look too bad, a bit of corrosion that I cleaned up but not much else. Unfortunately no change to my symptoms. So, I'm guessing that the problem...

Right, I'm back to where I started; with the suspicious RAMDAC/CLUT/Antelope chip (the functioning boot but crazy video). From probing the signals I was convinced that my SIMM hack should have worked. Funnily enough: I found a break in A4 between the on-board RAM and MEMCjr; in the same place...

Yup, I'm still waiting on the RAM chips. Though I had an idea and did a little experiment: The logic board RAM chips have their own RAS line, separate from the SIMM. All of the other lines are shared. I have a functional 4MB SIMM I didn't mind temporarily mangling, so I disconnected its RAS0...

Yup! As the chips are taking their time to arrive, I did some more chip-removal practice on the board with the last 3 dram chips (I think I did it much more cleanly this time). I decided to test the board just in case it would work with just a SIMM (no on board memory). This is what I...

I think I've verified all of the lines I may have affected with my work. Still have 3 chips in the top-3 ram spaces (nearest the ports) and still have the sad mac chimes. I'm waiting for some fresh RAM chips to arrive before I continue testing as I don't want to risk damaging the board some...

Okay; thanks. I'll do some experimentation and re-check what I've got at the moment. I don't think I should have damaged the three remaining chips at the top of the board so there might be something else that I've damaged in the process.

I removed all 5 so far. It's getting to the point where I'm damaging the pads (I've managed to remove 2 and get wires back to where they need to be) so I was hoping to do it in one iteration and maybe later replace the chips with ones bought on ebay. Looking at the specs you could buy these...

Thanks; I'm giving the ram chip removal a go. Currently I've removed the bottom 5 ram chips as these were the ones I think most likely to be fried. The board isn't booting beyond the death chimes yet. If I remove the last 3 chips, would it be able to boot just from the SIMM?

Thanks, I'll give that a go. I'm surprised that the board should cope with ad-hoc RAM chip removal; the data lines looked like it was geared up for 32bit read/write access spread across the chips. Is the MEMCjr smarter then that, then? And any tips on how to prevent chip frying? I removed with...

So I haven't tackled U26 yet as I wanted some practice at removal/replacing chips. I did a few SOJs on a scrap board as practice before deciding to tackle the on-board RAM chips on the 575 board (due to the slightly mysterious failures post-reflow attempt). Unsurprisingly I did find and clean up...

Thanks I'll give the cleaning a go. What hot air settings do you use to remove a chip that size? I've just got a fairly standard temp-controlled station with a few different nozzle sizes.

Whelp, the fresh VRAM showed me that it probably isn't (just?) the vram to blame. The symptoms have changed a bit - the screen will now eventually start to flicker with different colours. If I move the mouse cursor to select a menu (blind) you can see obviously correlated changes to the...

Thanks for checking! The VRAM is due to arrive sometime tomorrow. I think I'll also see if I can repair the damaged traces 1:1 rather than using the via-to-pin bodges. Looking at the clock and data lines it may make some difference.