ZX Spectrum - can't get it repaired - help!

Hi,

I know I can buy a used one for EUR 5 or so, but that's not the fun ;-)

Problem is that I can't get the ZX to boot... black and white big vertical bars, in some horizontal lines all kinds of coloured flickering blocks and some thin (often red) vertical lines...

I have a ZX Spectrum issue 4A. I have now swapped almost all components with a working computer, that way I know (or at least I think I know, correct me if I'm wrong) that the following components are ok:

o Swapped Z80A - correct; o Swapped ROM (not 100% usre if that one works, but it came from another ZX Spectrum) - both seemed non-Hitachi, so the links could remain in the N-position, not the H-position, Rom says something like AMI bla bla and somewhere 128, so it should be allright. The original was a Nec ROM; o TR4, 5 and 6 were changed, although the 5V, 12V and -5V seemed ok (-5V seemed a bit lower). That should mean that the coil, the transistors, PSU, 5.1V zener diode, etc. are fine; o All 4116 RAM's are 100% ok; all connections were checked, upperside and bottom side are connected, pins 2 and 14 are connected and all other pins are in the "chain" (so pin5 of IC6 connects to pin5 of IC7, etc.); o 7805 is 100% ok; o ULA works (6C001E-7 - works great in a 3B issue); o Even changed the 74157 that is used for the basic 16K computer; o Two of the electronic capacitors were changed that play a role in the power part (9V, 12V, 5V, -5V);

Problem is that I forgot what caused this, it was years ago. Must have been a peripheral taken of while the power was on, but that usually damages the TR4/TR5 parts...

What else can I check / change? Can the 32K (the eight 4532's + five

74xxx) "extension" do any harm, do I need to take it out (all other IC's are in IC holders now and that was A LOT of work, so that would be 13 more chips times 16 pins!!).

If the "upper memory" does not need to be taken out, I would say it doesn't need to, what else can I check. The PCB looks, with an eyeglass, quite ok and replacing resistors doesn't make much sense. I can't imagine the 1N4148's are damaged either... Capacitors?

It CAN still be something in the power circuit, as the sounds of the coil is not 100% normal... Normally when you switch it on, it makes sound A and after a second or so, it makes sound B and it keeps sound B. In my case, it seems like the computer knows it can not boot and keeps on trying, although that's a wild guess, nothing spectacular changes on the screen, flickering blocks keep on being flickering blocks, no change after that first second either...

Well, that was a long story, please share your thoughts ;-)

Cheers,

G
Reply to
GdB
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It`s been a long time, but I used to repair Spectrums regularly, and what you describe is symptomatic of one of the memory ic`s being faulty, have you tried it with the 32k extension removed? are you absolutely sure that all the memory chips are ok? I might still have a load of old speccy bits laying around somewhere.

Ron(UK)

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Ron(UK)

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