ZX81: What's going on with this processor mod?
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ZX81: What's going on with this processor mod?
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.
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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Re: ZX81: What's going on with this processor mod?
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
ZX81 Versions/Issues
Topic with photos of the extra board to correct the ROM bug
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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- Posts: 30
- Joined: Sun Apr 11, 2021 5:31 pm
Re: ZX81: What's going on with this processor mod?
Thanks does that make it more collectible?
- 1024MAK
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- Location: Looking forward to summer in Somerset, UK...
Re: ZX81: What's going on with this processor mod?
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
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: ZX81: What's going on with this processor mod?
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.
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- Posts: 30
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Re: ZX81: What's going on with this processor mod?
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.
- 1024MAK
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- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: ZX81: What's going on with this processor mod?
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
Do you have a digital multimeter?
It’s worth checking any socketed chips are firmly pressed into their sockets.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: ZX81: What's going on with this processor mod?
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.
- 1024MAK
- Posts: 5118
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- Location: Looking forward to summer in Somerset, UK...
Re: ZX81: What's going on with this processor mod?
With a white screen, it could be any of the four or five chips. Or a poor connection or dry joint.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.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
-
- Posts: 30
- Joined: Sun Apr 11, 2021 5:31 pm
Re: ZX81: What's going on with this processor mod?
I do.
There were a few dry joints which I've soldered and am going to work through it component by component.