You know what, I think you're absolutely right! I was using a modern 9v/2a PSU and have just changed to an original old Sinclair brick and certainly using the +2 with a RGB/scart connection is now much better. I still need to test it with the QL-VGA device as the interference with that was certainly a lot more pronounced. I'd already changed the internal voltage regulator from a switching one to an original 7805 but that didn't seem to do much.Tiger wrote: ↑Thu Nov 19, 2020 9:54 pm Today I had nearly the same problem with "crawling dots scrolling over the screen". Tested an ZX Spectrum Issue 6a PCB. But an other Speccy, wich worked fine before also shows these dots. I used an adjustable bench top power supply and after I took an other power supply every thing works fine. Seems that the (two years fine working) bench top power supply suddenly produces interferences . So I think it is a good way to test your spectrum with an alternative power supply too.
When I start it I get variations of this:
Sometimes there are a lot of random colour boxes, but the white horizontal bars are always there. The border is usually purple, sometimes other colours but rarely white.
Such a shame as this is my main Speccy and it was fine and very handy as it all worked. I wonder if the original PSU I reverted to is a problem, so I'll re-cap/check that. But in the mean time I have a broken +2.
Yes. You think it's the ROM? Or you mean to burn a diagnostic ROM? I do actually have the original saved ROM data somewhere.
I have the plug in DiagRom and that is obviously more focussed on 48k models but I did manage to get through to the 128k menu and it reported a couple of bad RAM chips, but I've ruled those out.
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If diagnostics are at least running, then the Z80 is mostly working. The ULA is also mostly working. It could be one of the chips that multiplexes the address lines to one of the groups of DRAM chips.