Just brought an issue 6A spectrum back from the dead with some faulty lower and upper RAM chips. I've socketed all of the RAM chips and tested all the DRAM in another test machine using the Retroleum Diagnostics ROM and I'm sure I've got a good set of 48k chips now. Spectrum boots fine and runs all the games I've tried.
Strange issue is, if I plug in any expansion peripheral (tried the Retroleum card and two other diag boards I have, all which work fine in another spectrum) I just get a screen with different coloured borders (sometimes white, sometimes green, magenta, cyan) and either black and white broad stripes or flashing squares - like a RAM/ULA issue.
Now, it gets a bit more interesting.. I have a board that boots Brendan Alfords Diag Rom and it seems to run - in that I can see it performing tests but all of the text on the screen appears garbled - but you can sort of see the correct stages of the test running.
This is a Spectrum with the PCF1306P - which I've replaced from another Spectrum - and I've also replaced the 74LS04 near the ULA too. No change.
I haven't done a full recap but all voltages on the RAM look good - the upper RAM has 5.04v and the lower RAM has 12.3v/5.02v/-5.04v on them.
Whenever I plug something into the edge connector, it stops the spectrum working properly. The voltages are also the same with anything plugged in.
Anyone with any ideas where to go next?
Also should say that I've tested both the ULA, PCF, Z80 and ROM in another machine and all is good...
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Yep - the edge connector was pretty dirty and corroded but I cleaned it up and re-tinned it with solder - then removed the excess with my desoldering gun. Now all the contacts are clean and fresh. It's still the same.
I think someone has socketed the Z80 at somepoint, so I think checking that may be a good place to start - and checking all the connections back from the edge connector to each chip as you say...