ZX81 Troubleshooting

Discussions about Sinclair ZX80 and ZX81 Hardware
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citizenfish
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ZX81 Troubleshooting

Post by citizenfish »

I've got a ZX81 Issue one board with composite mod that is just giving a white screen without a cursor.

I've got access to a scope and multimeter where should I start looking? This board appears to be untampered with, the Rom version is 8111.

If it doesn't exist already I'd like to use this as the start of a troubleshooting guide for non-expert tinkerers like me.

Thanks
Dave
citizenfish
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Re: ZX81 Troubleshooting

Post by citizenfish »

One thing is that all chips are heating up apart from IC2 which I think is the ROM?
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1024MAK
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Re: ZX81 Troubleshooting

Post by 1024MAK »

Hi Dave :D
citizenfish wrote: Tue May 11, 2021 2:49 pm I've got a ZX81 Issue one board with composite mod that is just giving a white screen without a cursor.
Typically, that’s often the symptom of a RAM chip fault. But it can also be caused by a fault with the Z80, the ROM or any of the PCB connections.
citizenfish wrote: Tue May 11, 2021 2:49 pm I've got access to a scope and multimeter where should I start looking? This board appears to be untampered with, the Rom version is 8111.
Are you sure that’s the ROM? That number looks more like the date code printed on the first ULA version. See photo.
ZX81 issue one board with major parts identified
ZX81 issue one board with major parts identified
citizenfish wrote: Tue May 11, 2021 2:49 pmIf it doesn't exist already I'd like to use this as the start of a troubleshooting guide for non-expert tinkerers like me.
You’re far from the first...

IC1 is the ULA
IC2 is the ROM
IC3 is the Z80 CPU
IC4, or IC4a and IC4b is/are the SRAM chip(s)

The ULA runs rather warm. The Z80 runs slightly warm. The ROM runs cool. The SRAM varies depending on the type fitted.

First off, have you checked that the +5V supply from the 7805 voltage regulator is between 4.75V and 5.25V?

Secondly, have you used a ‘scope before? Do you know how to use it?

If yes, have a look at the waveforms / voltages you have on the following pins of the Z80 CPU.
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
6 Clock (should be a 3.25MHz square wave with not very sharp/clean edges)
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the Z80 CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Which type of SRAM chip(s) does your board have. And do you have a RAM pack? If yes, have you tried it?

Z80 pin-out here.

ZX81 schematic circuit diagram here (this has French video output circuitry, but that does not matter when fault finding the digital circuitry).

Mark
citizenfish
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Re: ZX81 Troubleshooting

Post by citizenfish »

Thanks Mark do we know each other? That’s exactly what I needed to get me started and you are right I had my chips mixed up .. just like my cooking. I’ll tinker further and answer your questions in due course. I’m teaching myself electronics as part of this journey including learning how to use a scope.
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1024MAK
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Re: ZX81 Troubleshooting

Post by 1024MAK »

citizenfish wrote: Wed May 12, 2021 9:17 am Thanks Mark do we know each other?
I’m on various other forums. I don’t recognise your user name though.

Mark
citizenfish
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Re: ZX81 Troubleshooting

Post by citizenfish »

Voltage reg outputting 4.9V

So far all of the Z80 test pass with the scope showing exactly what you described

There is activity on A14/15 and A0-13

D2 and D6 are not very active only tiny blips on the scope. The other data lines show activity.

The SRAM is labelled MOSTEK MK4118N-4 8037

I tried with a RAM pack but no dice, however I am not sure this pack works

Using the circuit diagram I've tested continuity from all pins on ULA, Z80 and ROM and that all looks good
bola_dor
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Re: ZX81 Troubleshooting

Post by bola_dor »

How did this go?
Ernesto
ZX80, TK83, CZ1000, CZ1500, TK85, CZ2000, CZ1000Plus, CZ Spectrum, TK90X 16Kb, CZ Spectrum Plus, TS2068, Sinclair QL. TS 1016, TS1040. And more to come :D
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1024MAK
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Re: ZX81 Troubleshooting

Post by 1024MAK »

citizenfish wrote: Wed May 12, 2021 4:56 pm Voltage reg outputting 4.9V

So far all of the Z80 test pass with the scope showing exactly what you described

There is activity on A14/15 and A0-13

D2 and D6 are not very active only tiny blips on the scope. The other data lines show activity.

The SRAM is labelled MOSTEK MK4118N-4 8037

I tried with a RAM pack but no dice, however I am not sure this pack works

Using the circuit diagram I've tested continuity from all pins on ULA, Z80 and ROM and that all looks good
It’s very odd that you have activity on A14 and A15 but D2 and D6 are not very active.

Do you have any spares? And which chips (if any) are in sockets?

Mark
citizenfish
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Re: ZX81 Troubleshooting

Post by citizenfish »

I’m away on holiday right now.

But I need to redo my tests with my new Hantek scope and report back. The cheap handheld I was using may not be giving decent results.

None of the chips are socketed which is a pain
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