White screen of death

Discussions about Sinclair ZX80 and ZX81 Hardware
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PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: White screen of death

Post by PokeMon » Sun Sep 03, 2017 6:03 pm

nama wrote:
Sun Sep 03, 2017 9:32 am
I just built up Marks circuit using a 74LS02.
Have a look at this...It's even more dynamic

https://www.youtube.com/watch?v=zg1Gymf ... e=youtu.be

Phil
What happens if you put your finger on the /ROMCS signal ?
Does it get more stable ?
I had similar effects on other hardware (ZX81 compatibles) and good solved with a 22-47pF capacitor between /ROMCS and GND. 8-)

nama
Posts: 124
Joined: Fri May 09, 2008 4:49 pm

Re: White screen of death

Post by nama » Sun Sep 03, 2017 11:21 pm

I was just trying out the thought about touching the ROMCS line, unfortunately it didn't seem to work....However while doing this I had another serendipitous moment. The lead going to pin 6 of the 74LS02 dropped off, and the 'K' cursor instantly cleared up. Then I connected this line to GND and again the mess came back. I then connected it to the +5V line and the cursor cleared up again!!!

So, I guess this has fixed it??? I'll test a little more, but otherwise I build a proper little circuit mounted to the top of the ULA.

Any thoughts on why this would work are most welcome.

***UPDATE*** actually connecting both lines pins 5 and 6) or just one line to +5V produces the same result. The output (pin 4) in both cases is always +5V, and this produces a nice 'K'. So it seems there is no need to connect this the the /RFSH line.

Phil

nama
Posts: 124
Joined: Fri May 09, 2008 4:49 pm

Re: White screen of death

Post by nama » Mon Sep 04, 2017 8:32 am

Ok, I spoke a little too soon.
After about 20 seconds the pixels started to creep.
Adding a capacitor to GND helped fix this (as Pokemon suggested)
Fortunately it works well in the issue 3, but unfortunately not in my issue 1 where the characters are still all weird. So It'll stay in the issue 3.
IMG_6166.JPG
IMG_6166.JPG (41.89 KiB) Viewed 746 times
Anyway, I've spent enough time on this. I'll try to track down a new ULA for the issue 1.

Thanks for all your help.

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Andy Rea
Posts: 1530
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: White screen of death

Post by Andy Rea » Mon Sep 04, 2017 10:36 pm

Most perplexing.... reminds me of the time i could not for the life of me get an issue 1 board to work correctly, i changed all chips and even swapped out the resonator, checked most of the other components and yet it still would not produce a steady image ( jittery )... eventually i gave up...

anyways good that you have managed to salvage something so at least the issue 3 can live on for a bit longer :D

regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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