ZX81 Video problem

Discussions about Sinclair ZX80 and ZX81 Hardware
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Philsan
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ZX81 Video problem

Post by Philsan »

When my ZX81 gets warm (after 15 minutes), the upper 2 pixels of the first line of text are bended to the right and all first line is unstable.
Next lines of text are perfect.
I found the problem in three different televisions with two different ZX81 power supply.

I read that ZX81 have often video problems.
In another thread Moggy kindly explained that it's a normal problem of the first ZX81s.

Is it possible to solve this issue?
Will a composite mod solve it?

EDIT
A picture is worth a thousand words.
IMG_6129.JPG
IMG_6129.JPG (78.17 KiB) Viewed 5835 times
After 15 minutes:
IMG_6139.JPG
IMG_6139.JPG (76.98 KiB) Viewed 5832 times
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zx81jens
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Re: ZX81 Video problem

Post by zx81jens »

hi!

this could be an ULA-problem! solution: change it (if you can)...

greetings
jens
eyerything will be okay in the end.
if it´s not okay, it´s not the end.

and: uıɐbɐ ʎɐqǝ uo pɹɐoqʎǝʞ ɐ ʎnq ɹǝʌǝ ɹǝʌǝu ןןıʍ ı
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Philsan
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Re: ZX81 Video problem

Post by Philsan »

zx81jens wrote:hi!

this could be an ULA-problem! solution: change it (if you can)...

greetings
jens
Hi Jens.
Could you please explain what is the ULA problem?
I only know that ULA is a chip inside ZX81.
gozzo
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Re: ZX81 Video problem

Post by gozzo »

You will need a can of 'freezer spray' - one at a time, squirt the middle of chips inside the zx81 (don't get it on the keyboard membrane tails, though, it will make them brittle and crack)- including the regulator - to see if it makes any difference - when it does, thats the dodgy chip! , if not , open the cover of the modulator and squirt this from a distance of at least 30cm (the modulator could be drifting in frequency or other operating levels slightly as it warms up) - in any case, it's probably a good idea to stick a heatsink on the ULA as they do get rather hot! I suppose it is POSSIBLE for other components around the ULA circuitry to cause this, but dodgy chips are more likely - especially the early ULA's.
Last edited by gozzo on Sat Aug 06, 2011 9:21 pm, edited 1 time in total.
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zx81jens
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Re: ZX81 Video problem

Post by zx81jens »

hi Philsan!

ok, i´ll give it a try, too :-)

the ULA is a custom made chip (made by ferranti) and combines some logic - with that the ZX81 can be build with only four (or five if you have two RAM-chips) IC´s - the ZX80 has 21 chip´s !!

these IC´s can be found on your ZX81:

the ULA (on the left side)
the CPU (a z80 from zilog or NEC)
the ROM (8k)
the RAM (one or two smaller IC´s)

the problem with old hardware is always the same - time nibbles ;-)

you can try this: open your ZX81 carefully (the keyboard-foil might break - be very carful!!) and see if the ULA have a socket - if not... :-(

if there is a socket, remove the ULA carefully and put it in again - maybe the contacts are better than.

if this doesn´t help, maybe there is a ZX-user near to your place who can hlep in person ?!

don´t hesitate to write!!

greetings
jens (german zx-team)
eyerything will be okay in the end.
if it´s not okay, it´s not the end.

and: uıɐbɐ ʎɐqǝ uo pɹɐoqʎǝʞ ɐ ʎnq ɹǝʌǝ ɹǝʌǝu ןןıʍ ı
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Philsan
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Re: ZX81 Video problem

Post by Philsan »

Thank you guys.

If I am not wrong, it seems that a video mod will not solve the problem (unless gozzo's tests solve it).
Perhaps an heatsink over the ULA could be a good idea.
BTW, my ULA doesn't have the symptoms displayed here (third page):
http://www.thesinclairshop.com/docs/ula.pdf
gozzo
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Re: ZX81 Video problem

Post by gozzo »

Looking more carefully at the second of the 2 pictures above, it looks a bit like a tuning drift problem -the tell tale sign is the lighter 'ghosting' outline on the right of the characters - try manually 'fine tuning' your TV - if you can!, some modern TV's have no facility for this - if this 'cures' it ,it's probably excessive modulator drift - they all drift a little when warming up. A TV with a good AFC action is better for ALL 'vintage' home computers with RF TV out , preferably a good old 'analogue' CRT type, not a modern LCD thing! You could try sticking an extra decoupling capacitor or 2 on the modulators 5v and ground connections. I had a problem with an Amstrad CPC combined power supply and modulator box - it gave symptoms similar to this and the problem turned out to be a 'noisy' 5.1v zener diode on the modulator supply - sticking extra decoupling didn't cure it but did reduce it a lot.
zx80nut
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Re: ZX81 Video problem

Post by zx80nut »

Hi.
I had a similar problem with one of my ZX81's. Wasn't a tuning issue for me, definitely a heat issue.
Freezer spray would return the top line to be vertical for a while until it warmed up again.
Puting a heatsink on it cured it for me. Strange how it only happens on one of the machines.
I glued it on at each end, with a smearing of heatsink compound in the middle bit.
Works a treat now.

Grant.
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Philsan
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Re: ZX81 Video problem

Post by Philsan »

I don't think it is a tuning problem because it apperead on three different TV (LCD, color CRT, b&w CRT - I have two more LCD TV to test).
Perhaps I should try this cheap heatsink: http://cgi.ebay.it/ws/eBayISAPI.dll?Vie ... K:MEWAX:IT
ksng36

Re: ZX81 Video problem

Post by ksng36 »

Hi Philsan,

I think gozzo hit the hammer on the nail with this one. I have already heatsinked everything up, including the ROM, and have added a butt-load of extra filtering on the ZX81. I even have been using a modified 12V battery eliminator, but I can still tap the video modulator on the ZX81 and watch the video display go wavy motion. I think a close look of your video modulator is in order - I know I'm going to take a second look at mine this week. At some point in the future, I intend to replace it with something better, but I haven't decided what I want to do yet.

Regards,

ksng36
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