ZX Spectrum (48k) - issue 4s - blue border on screen - failure to boot

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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Mfacer
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ZX Spectrum (48k) - issue 4s - blue border on screen - failure to boot

Post by Mfacer » Tue Oct 08, 2019 9:32 pm

Hi all - got hold of a 48k (I think!?) but when I plug it in, I get a flickering screen. There’s a blue border around a black box.

I’ve had a look at the circuit board (issue 4s) and it’s incredibly clean. No dust or any repairs.

Any idea what might be up?
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Mfacer
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Re: ZX Spectrum (48k) - issue 4s - blue border on screen - failure to boot

Post by Mfacer » Tue Oct 08, 2019 9:34 pm

Image

Image

Oh, and now this one!

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Ast A. Moore
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Re: ZX Spectrum (48k) - issue 4s - blue border on screen - failure to boot

Post by Ast A. Moore » Wed Oct 09, 2019 12:04 am

The CPU doesn’t seem to be communicating with the ROM. Either could be faulty. Since you have the schematic, test the data and address lines between the two. If possible, replace each—one by one—with known good parts. I’d start with the CPU, since ROM chips rarely fail. But first, make sure all the data and address lines check out (no shorts, no open connections).
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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