Zx81 from death to life??

Discussions about Sinclair ZX80 and ZX81 Hardware
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RS232
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RS232 » Sun Apr 14, 2019 11:52 am

Ok good news folks back in business again...

I found i had accidentally shorted a wire inside the metal box housing where the plus 5v wire was snipped. So now the computer powers up

So here is something weird. That 22uf i replaced only the minus side actually had a real connection and that was going to the external connector the plus side was just a dead end solder point. Very odd no?

Problem with my composite out. I get a steady picture running a BC108 transistor

C to +5v
B to ULA in
E to middle pin ULA video out

But the problem is that it is a bit washy grey white.
I read that soldering 100ohm to transistor E and earthing its other end helps with two diodes in series.

What diodes do i need?
Does it matter which way round to solder diodes and resisters?

Much thanks to you all (may your ram packs be solid as rock)
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RS232
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RS232 » Sun Apr 14, 2019 12:07 pm

Holy moses !!!!!

I think i cracked it i tried one of these diodes and a 1W-100R resistor no idea if it means 100oms but bingo

Its nuts thanks everybody !!

See pics
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RS232
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Re: Zx81 from death to life!

Post by RS232 » Sun Apr 14, 2019 2:22 pm

Ok just need a new keyboard membrane for my poor old zx81.. gulp

While risking life and limb i decided to fit a power on led to my zx..

Tapping 12v to a pre made resistor amd LED drilled hole and dab od solder... all good
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RetroTechie
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RetroTechie » Mon Apr 15, 2019 9:04 am

RS232 wrote:I think i cracked it i tried one of these diodes and a 1W-100R resistor no idea if it means 100oms (..)
Brown-black-brown = 1, 0, x 10^1 = 100 Ohm indeed. Doing mods like this, one should a) know this, and b) preferably verify using a multimeter before putting the part in. Anyway - good job! :D

Did you stick some insulating material on top of the wiring in the modulator box? Otherwise it may short out against the metal cover.
Modulator wiring.jpg
Also consider sticking a cooling plate on top of the ULA.

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RS232
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Re: Zx81 from death to life??

Post by RS232 » Tue Apr 16, 2019 10:32 pm

My Zx81 has a death wish..

I attempted to add a LED supporting a resistor tappin the 12v from the boad. From that point i got no signal from Tv composite out.

1. Desolderd power to LED
Problem continues
2. Replaced transistor
Problem continues
3. Replaced diode
Problem continues
4. Replaced 22uf cap
Problem continues


Wonders if i have killed my dear old zx81.
Wondering could i have soldered two
Joints the one next to the transistor in the pic
Together by accident? I have an issue 3 board..

Please someone confirm was this my error
On changing out kit?
Does my own memory have ram wobble
Do i have to plunge a hot soldering iron
In my left arm?
Ahhhhhb
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dessony
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Re: Zx81 from death to life??

Post by dessony » Wed Apr 17, 2019 2:41 am

RS232,

You gave your ZX81 a complicated and hard life. :o WOW! Keep it simple, s______ by gutting a RF modulator's innard. And put a very cute little circuit board inside the metal box. See a topic here: 1024MAK/DesSony Video Transistor Buffer Amplifier Board, viewtopic.php?f=7&t=2405&p=28600&hilit=veroboard#p28600

Remember that Sir Clive Sinclair really kept his computer design simple. i.e. from ZX80's chips counts: 22 to ZX81's chips counts: 4 or 5.

Cheers,
DesSony

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RetroTechie
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Re: Zx81 from death to life??

Post by RetroTechie » Wed Apr 17, 2019 5:52 pm

@RS232: re-read my earlier post. You need to DOUBLE-CHECK modifications done since last working config.

Looking closer at the pics, I noted: video signal comes from ULA (pin 16). Where to? Appears to be going nowhere. If indeed the case, then it logically follows that screen info doesn't make it to the monitor. :lol: Let's hope 12V supply and 5V were not short circuited at any time while powered as that would do a lot of damage.

And yes, diodes have polarity. So do transistors (base, collector, emitter). And NPN vs. PNP. Note that none of my composite modded ZX81's have a diode in their video output. Just transistor, resistor, and a series capacitor (the latter also with polarity ;) ). Something like this but without the 2k2 series resistor (probably doesn't hurt to add that, though). Polarity of the capacitor in that circuit is wrong btw, + of capacitor should be on the transistor side as that carries a positive DC offset w/ respect to the monitor's video input. And I have 220 uF in there (but 100 uF also does the job).

Edit: for a quick 'n dirty check, you can hang ULA pin 16 directly on the center pin of the RCA connector, through a series resistor (eg. in 220 Ohm - 1kOhm range or so). That will likely show something on most monitor's screens. If the familiar "K" shows there, proceed with the transistor buffer circuit on the output.

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RS232
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Re: Zx81 from death to life??

Post by RS232 » Thu Apr 18, 2019 12:47 pm

Omg i have much to learn. Thanks will try the points raised. :shock: :o :idea: :D

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