Issue 4a docs needed

Posts: 8
Joined: Tue Dec 24, 2019 10:37 pm

Re: Issue 4a docs needed

Post by eccles » Wed Jan 15, 2020 10:17 pm

Not getting too far here - not convinced the cheap and nasty Hantek USB oscilloscope I purchased is either working or any good, since it doesn't ever appear to show square waveforms, only a constant DC power level - will see if I can test it on something known good

But some voltages:
ULA IC1 pin 14 - 4.94v (pin 13 is 3.6v if relevent)
14,15,16 on LM1889 IC14 all 11.6V

R64, LHS 4.74v, RHS 4.94v

Also attached, long awaited images of the 4a (bad lower ram/dc-dc), and the 4s (repaired dc-dc, no image)

Lardo Boffin
Posts: 1118
Joined: Sat Nov 26, 2016 2:42 am

Re: Issue 4a docs needed

Post by Lardo Boffin » Thu Jan 16, 2020 7:44 am

Can’t really help with the repairs (and that 4S will never work with all those chips missing! :lol: ) but there are people who can repair these for you. I have used Mutant Caterpillar Games quite a few times over the years.
Also you can get a replacement ULA from here ( if it is faulty.
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

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