ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
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Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sat Nov 18, 2017 6:41 pm

Andy Rea wrote:
Sat Nov 18, 2017 6:39 pm
I would check for shorts near the data pins of the cpu, the track clearances are quite close around there, also the diodes near the keyboard connector some.of the traces run very close to the diode pads.

Other than it not working it looks very nice, good job so far.

Regards Andy
Cheers Andy. I will get the magnifying glass and continuity tester out later.
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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sirmorris
Posts: 2699
Joined: Thu May 08, 2008 4:45 pm
Location: oxon, uk

Re: ZX81 rebuild

Post by sirmorris » Sat Nov 18, 2017 11:08 pm

Logic probe £5 from Maplin. Nothing fancy, just shows if a pin is high, low, hi-z or pulsing. S'all you need.
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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1024MAK
Posts: 1803
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Sat Nov 18, 2017 11:44 pm

El cheapo logic probe for 5V TTL logic...

If you have a spare 74LS14 or 74HCT14 logic chip, connect a test probe to a resistor (100 ohms to 180 ohms for a 74LS14, 330 ohms to 820 ohms for a 74HCT), then connect to any of the chips input pins.
On the relevant output, connect a RED LED via a 390 ohm or 470 ohm resistor to +5V. LED anode to the resistor/ +5V side.
Then connect this same outputs to another input pin. On the relevant output, connect a GREEN LED via a 390 ohm or 470 ohm resistor to +5V. LED anode to the resistor/ +5V side.

Connect a ceramic capacitor (value not critical, preferred value 100nF) across the +5V (VCC) pin 14 and 0V (GND) pin 7of the chip. These also have to go via mini clip probes to the supply lines of the circuit under test.

Mark

Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sun Nov 19, 2017 1:59 pm

I wimped out and bought one :( - I would have had to buy some bits in order to make my own.
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sun Nov 19, 2017 2:50 pm

I got new sockets from Sell My retro:-

https://www.sellmyretro.com/offer/details/3473

They have four legs rather than the three in use on the zeddy. Do i just fold the unwanted one under or do i need to remove it? Currently I have reused an old one.
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

User avatar
sirmorris
Posts: 2699
Joined: Thu May 08, 2008 4:45 pm
Location: oxon, uk

Re: ZX81 rebuild

Post by sirmorris » Sun Nov 19, 2017 3:30 pm

Lardo Boffin wrote:
Sun Nov 19, 2017 1:59 pm
I wimped out and bought one :( - I would have had to buy some bits in order to make my own.
No shame in that! You have enough on your plate ;)
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

User avatar
1024MAK
Posts: 1803
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Sun Nov 19, 2017 7:46 pm

Lardo Boffin wrote:
Sun Nov 19, 2017 2:50 pm
I got new sockets from Sell My retro:-

https://www.sellmyretro.com/offer/details/3473

They have four legs rather than the three in use on the zeddy. Do i just fold the unwanted one under or do i need to remove it? Currently I have reused an old one.
Remove it or just cut it off. It's not fitted on the sockets that Sinclair used. Hence not needed.

Mark

User avatar
PokeMon
Posts: 2171
Joined: Sat Sep 17, 2011 5:48 pm

Re: ZX81 rebuild

Post by PokeMon » Sun Nov 19, 2017 9:14 pm

Yes - I think this product have been upgraded with a fourth leg in the last 30 years. ;)
Cut it with a side cutter.

Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sun Nov 19, 2017 11:08 pm

PokeMon wrote:
Sun Nov 19, 2017 9:14 pm
Yes - I think this product have been upgraded with a fourth leg in the last 30 years. ;)
Cut it with a side cutter.
:D
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Nov 24, 2017 1:49 pm

So I’m finally armed and dangerous with a logic probe (that thing has a wicked sharp tip!).

Its an LP-2800 from Maplin.

Now what?.... :?: :)
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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