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Re: Dead ZX81.
Posted: Fri Oct 26, 2018 10:12 pm
by flaviosr
1024MAK wrote: ↑Thu Oct 25, 2018 11:32 pm
A faulty Z80 CPU, a faulty ULA, a faulty ROM or a faulty RAM chip could and would prevent start-up.
A poor connection (socket, dry joint, broken/cracked track) in any of the many critical circuits could also prevent start-up.
Mark
The two transistors, unmounted and professionally tested, are good. So they are now again mounted.
Now I am going to socket the two RAM chips and but two new ones... let's cross the fingers...
I hope it's not a more subtle issue...
Re: Dead ZX81.
Posted: Wed Nov 14, 2018 9:34 pm
by flaviosr
Let's wait for the RAM chips and let's cross the fingers...
Re: Dead ZX81.
Posted: Sun Feb 03, 2019 4:53 pm
by flaviosr
Obviously the RAM is ok.
Having no more ideas I have decided simply to change the two transistors and the two capacitors and... the ZX81 is not perfectly working!
I have chenged them all together so I will never know if it was a cold welding and which component was the bad one.
The strange issue is that, after unmounted and tested, all of the components are... ok! Or at least many multimeters say so.
Just wanted to clase the story if this can help somebody else...
Re: Dead ZX81.
Posted: Tue May 07, 2019 2:53 pm
by Papalapa
flaviosr wrote: ↑Sun Feb 03, 2019 4:53 pm
Obviously the RAM is ok.
Having no more ideas I have decided simply to change the two transistors and the two capacitors and... the ZX81 is not perfectly working!
I have chenged them all together so I will never know if it was a cold welding and which component was the bad one.
The strange issue is that, after unmounted and tested, all of the components are... ok! Or at least many multimeters say so.
Just wanted to clase the story if this can help somebody else...
Thank you for the information, I'm in the same situation than you with a white screen without showing the cursor on my ZX81. I replaced the capacitors, the voltage regulator and installed a video+memory MOD but the problem remains and next step before to replace some IC will be to replace both transistors. It's an easy and quick operation.
I hope to be as lucky as you...
Re: Dead ZX81.
Posted: Fri May 10, 2019 9:04 pm
by flaviosr
Heppy to help!
Tell me the results!
Re: Dead ZX81.
Posted: Sat May 11, 2019 11:29 am
by Papalapa
flaviosr wrote: ↑Fri May 10, 2019 9:04 pm
Heppy to help!
Tell me the results!
Unfortunatelly after replacing both transistors the problem remains. Now I'll start to check the ULA and CPU chips and see what happens..