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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 30th Apr 2020, 12:31 pm   #301
Slothie
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Default Re: Zx81

If you need to remove a turned pin socket then you can carefully break off all the plastic with wirecutters then remove one pin at a time. It takes care not to damage the pcb but its possible.
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Old 30th Apr 2020, 12:39 pm   #302
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These are all good points - thank you.
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Old 1st May 2020, 11:56 am   #303
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Hi guys,
Just to let you know that I stuck with the 'cheap' socket and repaired it as per the single strand of wire technique and... it worked! All connections were there and it seems fine now! Thank you so much.

Quick question re: the schematic. It refers to IC4 but my board only has IC4a and IC4b. There is certainly an outline for IC4 but not IC and there are no spare holes anywhere for them - any ideas?

Thanks again.
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Old 1st May 2020, 12:04 pm   #304
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Default Re: Zx81

IC4 must be a dual something and the two sections are being referred to as A and B.
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Old 1st May 2020, 12:07 pm   #305
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IC4 must be a dual something and the two sections are being referred to as A and B.
Ah yes. It is odd - I have two ZX81s and they both have IC4 a and b but no IC4.
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Old 1st May 2020, 12:10 pm   #306
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It is not quite so odd. It is just like double triodes where the sections are referred to as A and B.
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Old 1st May 2020, 12:43 pm   #307
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I have an idea that IC4 was a 1K*8 RAM (1K byte) possible a 4118. IC4a and IC4b were 1K*4 RAMs so you needed 2 for the 1K bytes. I think they were 2114 chips, a device that I regard as being 'change it if you get odd symptoms', the 2114 is possibly the most unreliable IC I've ever seen. A given board would have one or the other type of RAM, I think the pair of 2114s is a lot more common.
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Old 1st May 2020, 1:19 pm   #308
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A quick shufty at the diagram linked to in #286 shows that the RAM can either be a single 8-bit wide IC (IC4) or two 4-bit wide ICs (IC4a and IC4b).

It so happens that both of my ZX81s originally had a single 4118 fitted, although both have now been replaced with 62256 SRAMs to give the machines 16K of internally fitted RAM. (The 'spare' 16K in the other half of the 32K IC is not utilised).

Apart from making it usable while still retaining Rick Dickinson's classic design looks - the basic ZX81 is not a bad looking machine at all - doing it that way spells an end to the horrors of RAM pack wobble.
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Old 1st May 2020, 1:27 pm   #309
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Originally Posted by SiriusHardware View Post
A quick shufty at the diagram linked to in #286 shows that the RAM can either be a single 8-bit wide IC (IC4) or two 4-bit wide ICs (IC4a and IC4b).

It so happens that both of my ZX81s originally had a single 4118 fitted, although both have now been replaced with 62256 SRAMs to give the machines 16K of internally fitted RAM. (The 'spare' 16K in the other half of the 32K IC is not utilised).

Apart from making it usable while still retaining Rick Dickinson's classic design looks - the basic ZX81 is not a bad looking machine at all - doing it that way spells an end to the horrors of RAM pack wobble.
Yes! I certainly know what that is like! I've just bought a TZXDuino to load games - exciting to use that rather than cassette
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Old 1st May 2020, 3:56 pm   #310
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As you have already modified that particular ZX81 quite a bit, you might like to consider making a 16K internal RAM mod your next project.

Once again, your friends at Tynemouth Software have made it look easy:

http://blog.tynemouthsoftware.co.uk/...e-version.html

They have intentionally done it in a non-destructive way (no track cuts) so the machine can be returned to its historically correct configuration if that is ever required.

As you are starting with two-chip RAM you would have to remove those ICs first, then fit a socket in the holes designated for the single RAM IC, IC4, before you could proceed.
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Old 1st May 2020, 4:14 pm   #311
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Originally Posted by SiriusHardware View Post
As you have already modified that particular ZX81 quite a bit, you might like to consider making a 16K internal RAM mod your next project.

Once again, your friends at Tynemouth Software have made it look easy:

http://blog.tynemouthsoftware.co.uk/...e-version.html

They have intentionally done it in a non-destructive way (no track cuts) so the machine can be returned to its historically correct configuration if that is ever required.

As you are starting with two-chip RAM you would have to remove those ICs first, then fit a socket in the holes designated for the single RAM IC, IC4, before you could proceed.
Wow! Yes that does look interesting. This is the same issue as mine it seems - issue one. Luckily those ICs are socketed rather than soldered as in the other ZX81 I have!

Thank you - i will certainly have a good look.
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Old 1st May 2020, 4:22 pm   #312
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I've just typed into my ZX81 the PEEK Command on the Tynemouth Software blog:
PRINT PEEK 16388+256*PEEK 16389
The return number surprised me. It is 17408 - shouldn't it be closer to 1024?
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Old 1st May 2020, 4:46 pm   #313
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That figure is telling you where the topmost byte of RAM it found is.

The first 16K of the address range (up to about decimal 16383) is occupied by the ROM, then the RAM starts at 16384. 17408-16384 = .... 1024!
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Old 1st May 2020, 4:47 pm   #314
John Earland
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Originally Posted by SiriusHardware View Post
That figure is telling you where the topmost byte of RAM it found is.

The first 16K of the address range (up to about decimal 16383) is occupied by the ROM, then the RAM starts at 16384. 17408-16384 = .... 1024!
Ah - got it.
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Old 1st May 2020, 6:07 pm   #315
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I have an intriguing new project (in addition to the thought about the 16k RAM and replacing the three ICs on the PET) - I have a TFT Monitor for a car (7inch) which I'd like to use with the ZX81 like a laptop. I have to work out how to power it! Lots of research me thinks!
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Old 2nd May 2020, 9:04 pm   #316
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Just dug out my original 'normal' ZX81, the one which is still in the original casing / keypad, and fired it up only to find that only about 5 of the keys were working, even though it was put away fully working probably not more than 2 years ago.

The problem is the usual, the ribbon connectors have hardened with age and they fell apart into little bits of plastic confetti as soon as I tried to separate the PCB and upper casing. It's like that all the way back to the keyboard entry point on this one so I can't employ the usual dodge of snipping the bad ends off and making new ends a few mm further back. It's a complete goner.

A replacement key membrane is already on order - if anyone else has one of these safely stored and believed working, better check it out while the membranes are still available. They aren't too badly priced, around £14 to £17 including post. Available from a number of sources.
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Old 3rd May 2020, 7:17 pm   #317
John Earland
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Quote:
Originally Posted by SiriusHardware View Post
Just dug out my original 'normal' ZX81, the one which is still in the original casing / keypad, and fired it up only to find that only about 5 of the keys were working, even though it was put away fully working probably not more than 2 years ago.

The problem is the usual, the ribbon connectors have hardened with age and they fell apart into little bits of plastic confetti as soon as I tried to separate the PCB and upper casing. It's like that all the way back to the keyboard entry point on this one so I can't employ the usual dodge of snipping the bad ends off and making new ends a few mm further back. It's a complete goner.

A replacement key membrane is already on order - if anyone else has one of these safely stored and believed working, better check it out while the membranes are still available. They aren't too badly priced, around £14 to £17 including post. Available from a number of sources.
Itís a shame isnít it but at least a replacement is fairly easy to find at the moment. Iíve tried the TZxduino and it works perfectly! Iíve been playing 3D Monster Maze-havenít quote worked out what Iím doing and have only amassed 80 points so far!!
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Old 3rd May 2020, 9:38 pm   #318
SiriusHardware
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An Atari / Spectrum style switched joystick is not too hard to add to a ZX81 as long as the game uses the arrow keys 6-7-8-9 for movement and zero for fire, as many do.

In 3D monster maze, just not getting eaten is a pretty big step forward. I don't think I have ever bothered to keep score. The aim, obviously, is to get out of the maze without being caught by the monster.
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Old 4th May 2020, 2:41 am   #319
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Default Re: Zx81

@SiriusHardware To be pedantic, the first 8k is ROM, the next 8K is spare, then the RAM starts at 16k.
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Old 4th May 2020, 9:02 am   #320
SiriusHardware
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Pedantic indeed.

I try not to make explanations more complicated than they need to be at the point of delivery. When looking at one of my '81s yesterday, I noticed that the ROM socket was bigger than the ROM, so I assume that 0-16383 is mapped as ROM space even if the physical ROM used never occupied it all.

I suppose I should have just said 'The RAM starts at 16384'.
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