GdB wrote:
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

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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