ZX81: What's going on with this processor mod?

Discussions about Sinclair ZX80 and ZX81 Hardware
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citizenfish
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ZX81: What's going on with this processor mod?

Post by citizenfish »

I picked up a couple of ZX81's recently, both have composite mods but both are showing white screens only so I've got some work to do. I think this is going to be a steep learning curve!

One of them has several chips socketed and this modification to the Z80, any idea what on earth it is? This board is soldered to the legs of the z80.

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1024MAK
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Re: ZX81: What's going on with this processor mod?

Post by 1024MAK »

That’s actually a fix for a bug in the ROM chip. See these links:

ZX81 Versions/Issues

Topic with photos of the extra board to correct the ROM bug

Mark
citizenfish
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Re: ZX81: What's going on with this processor mod?

Post by citizenfish »

Thanks does that make it more collectible?
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1024MAK
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Re: ZX81: What's going on with this processor mod?

Post by 1024MAK »

Yes, as does the date code on the first version of the ULA that you have there. But don’t get your hopes up that it will be significantly more valuable and you won’t ever have to work again. There is a fair few of these out there.

Mark
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BarryN
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Re: ZX81: What's going on with this processor mod?

Post by BarryN »

To some people the buggy rom and the older ULA make it less desirable, so, well, it's worth what it's worth. At least now you know what it is.
citizenfish
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Re: ZX81: What's going on with this processor mod?

Post by citizenfish »

I’m not selling! This is part of my own personal collection. Now how do I get it working, might be time to invest in an oscilloscope.
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1024MAK
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Re: ZX81: What's going on with this processor mod?

Post by 1024MAK »

Getting or making a logic probe is worthwhile.

Do you have a digital multimeter?

It’s worth checking any socketed chips are firmly pressed into their sockets.

Mark
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BarryN
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Re: ZX81: What's going on with this processor mod?

Post by BarryN »

If it's not working, it's often a bad ULA. :( The good news is someone may have a machine that they upgraded the ULA in and they have the older ULA, presuming you want to keep it original. If you do want to keep it original you will need a B&W analog TV to act as a display, as the older ULA does not like color TVs or newer TVs.
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1024MAK
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Re: ZX81: What's going on with this processor mod?

Post by 1024MAK »

BarryN wrote: Mon Apr 12, 2021 3:42 am If it's not working, it's often a bad ULA. :( The good news is someone may have a machine that they upgraded the ULA in and they have the older ULA, presuming you want to keep it original. If you do want to keep it original you will need a B&W analog TV to act as a display, as the older ULA does not like color TVs or newer TVs.
With a white screen, it could be any of the four or five chips. Or a poor connection or dry joint.

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Re: ZX81: What's going on with this processor mod?

Post by citizenfish »

1024MAK wrote: Sun Apr 11, 2021 9:13 pm Getting or making a logic probe is worthwhile.

Do you have a digital multimeter?

It’s worth checking any socketed chips are firmly pressed into their sockets.

Mark
I do.

There were a few dry joints which I've soldered and am going to work through it component by component.
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