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Help on resurrecting a Spectrum 128.

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flaviosr

Experienced Member
Joined
Aug 27, 2016
Messages
266
Location
Turin, Italy
Dear All,

again I have to ask for help.
I have bought a Sinclair Spectrum 128 and I am trying to resurrect him.
What I have found up to now:
° the CPU was dead;
° the EPROM (I have the Spanish one) was dead;
° one low RAM chip was dead.
After this replacing I have been able to arrive to the attached picture!!! :)
I replaced UC17 and NOTHING changed (meaning that still the diagnostic are telling me to replace ALL chips) => I have decided to replace also IC15 and IC16 and still NOTHING changes from the diagnostic.
At this point I have to surrender... I do not understand why (I am sure the "new" chips are good).

Any ideas?

Another point: at the moment I have written the diagnostic on a EPROM but I cannot find a BIN for the Spectrum 128 (I would like to have the Spanish one but also the original one): any link?

Thank you
 

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The ZX8401 deals with RAM logic I believe. Maybe that is at fault and could explain why it shows all the RAM in that block as bad. Is it socketed? Could you swap it? I think there is a modern replacement for those, although personally I've found them to be hit and miss.
 
Half your ram appears to have failed. You have some working, or it wouldn't show, and your screen would likely be funny.

I'd suggest desoldering and replacing it - but it coulod also be the Zx8401. They are relatively cheap to obtain ( compared to the ULA ) and you can remove and socket one if you have the skills - otherwise you should be able to find someone who can.

Could also be a broken line, since all are showing as dead - eg, faulty circuit trace. Check that you have continuity from the ZX8401 to each of the relevant pins on the 6164 ICs. Each pin of these chips should make it back to the ZX8401 OK and the data lines are probably OK or all are not likely to show as faulty. Could also be a problem with the select logic between the z80 and the zx8401 or the RAS/CAS and MUX address lines of the zx8401 and the RAM.

It also could be the PAL chip which selects banks, since if it has failed, then you're going to end up with bank issues, and it's not impossible to lose them all.

But if suspect, as I mentioned, they are one of the cheaper chips to replace - They are just a single chip replacing the six that normally exist between the z80 and the ULA on other boards.

Regards
David
 
Today I am going to check the traces and, in case this is not the issue, I am going to ask a tech to desolder the ZX8401. I will not that fast but I hope I can reach the goal of having a working computer.
Do you have a link to BIN of the various 128s ROMs?

Thank you very much, Flavio
 
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