ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: ZX81 rebuild

Post by 1024MAK » Fri Nov 24, 2017 2:29 pm

A sharp tip lies in wait... :shock:

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1024MAK
Posts: 1799
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Nov 24, 2017 2:46 pm

1024MAK wrote:
Sat Nov 18, 2017 5:55 pm
If you have a 'scope or logic probe, check Z80 address lines A14 and A15.
Mark
Or go the whole hog and test every single pin on the Z80 CPU. See the link in my sig for the pin-out.

Most pins should be pulsing at various speeds.
Not normally used control inputs, or inactive inputs should be logic high (for example /RESET).
All the address and data pins should be pulsing if the CPU is running the normal ROM code and the RAM is okay.

Address line A14 tells you if the Z80 is attempting to access ROM (A14 low) or RAM (A14 high), but keep in mind that after every instruction fetch, there is a (DRAM) refresh cycle and the top half of the address (A8 to A15) comes from the current value in the I (interrupt vector) register. Address line A15 tells you if the CPU is accessing ROM or RAM during normal operation (A15 low) or reading data to generate the display (A15 high).

As you have some working Zeddies, compare the signals on a working one to the rebuilt one (don't forget to swap the power and ground leads over).

Mark

Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Nov 24, 2017 8:06 pm

Thanks! Which settings on the probe should I be using? I have the choice of TTL and CMOS. :oops:
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
1024MAK
Posts: 1799
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Nov 24, 2017 9:33 pm

For 5V logic use normally it's best to use TTL mode. Most 8 bit CPUs (made with NMOS technology and their CMOS replacements), along with their support chips normally use TTL compatible levels. As does most bipolar and NMOS ROM, EPROM, SRAM and DRAM chips that run at 5V.

74xxx, 74Sxxx, 74Lxxx, 74LSxxx, 74ALSxxx (these are all bipolar TTL technology) and 74HCTxxx (high-sped CMOS with TTL input switching levels) chips all use TTL levels.

So, in general, use TTL. The exception being if you are measuring inputs on 4000 series logic chips (which are CMOS technology), or on 74HCxxx logic (HC means high-speed CMOS).

The difference is in the detection window of the logic levels.

Mark

Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Thu Nov 30, 2017 6:12 pm

1024MAK wrote:
Fri Nov 24, 2017 9:33 pm
For 5V logic use normally it's best to use TTL mode.

Mark
Having finally found some time...

I have tested pins 4 (A14) and 5 (A15) on a working zeddy - A14 has a slow-ish pulse, A15 has a fast pulse.

On my rebuild both are stuck high. None of the pins on the Z80 pulse. Some are high and some are low.
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: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Thu Nov 30, 2017 6:51 pm

Below are the resistors and capacitors I ordered from CPC (www.cpc.co.uk) if anyone fancies having a quick look to see if they are correct? :D

2C610EE1-96BB-4954-BDCF-35C01BAC8345.jpeg
2C610EE1-96BB-4954-BDCF-35C01BAC8345.jpeg (394.34 KiB) Viewed 117 times
DC90F0FB-E8EE-4C32-901F-BA862F9A71E4.jpeg
DC90F0FB-E8EE-4C32-901F-BA862F9A71E4.jpeg (1.54 MiB) Viewed 117 times

I have checked I labelled the bags correctly (I have) and checked that the resistors in the bags have colour bands that match the original description. I can check the placement of resistors but not capacitors as they are helpfully not marked... :cry:

Thanks!
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
1024MAK
Posts: 1799
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Thu Nov 30, 2017 8:39 pm

Does the ULA have power to it's power pins (34 and 40)?
Does the ULA CPU clock out have a 3.25MHz signal (pin 14)? (Pulsing)
Does the Z80 CPU have a 3.25MHz signal On pin 6? (Pulsing)
Does the /RESET pin (pin 26) go from low to high (very quickly) after a power on?

If the Z80 has no clock signal on pin 6, or /RESET is low, then the CPU will just sit there doing nothing and some of the outputs will have random non-logical and invalid states.

Mark

Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Thu Nov 30, 2017 10:08 pm

1024MAK wrote:
Thu Nov 30, 2017 8:39 pm
Does the ULA have power to it's power pins (34 and 40)?
Does the ULA CPU clock out have a 3.25MHz signal (pin 14)? (Pulsing)
Does the Z80 CPU have a 3.25MHz signal On pin 6? (Pulsing)
Does the /RESET pin (pin 26) go from low to high (very quickly) after a power on?

If the Z80 has no clock signal on pin 6, or /RESET is low, then the CPU will just sit there doing nothing and some of the outputs will have random non-logical and invalid states.

Mark
Thanks as ever. :)

The ULA has power to the pins.
Pin 14 of the ULA pulses.
Z80 Pin 6 stays low.
Z80 pin 26 goes from low to high quickly after power up.
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
1024MAK
Posts: 1799
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Thu Nov 30, 2017 11:36 pm

IMG_5794.JPG
Clock circuit
IMG_5794.JPG (257.15 KiB) Viewed 86 times
Check / test R5, R6, C7 and TR2.
R3 should not be fitted.

Mark

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1024MAK
Posts: 1799
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Dec 01, 2017 12:11 am

What type of transistor is fitted in TR2 position?

Mark

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