ZX81 Internal 16K Upgrade - aka Wot No Wobble

TheCorfiot

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Hello Forum Chums

Another Amibay Exclusive for you...
Dedicated to Rob and Andy ;)

Having been put to the task I have engineered a very nice neat and cheap solution to banish Ram pack wobbles for ever and provide a nice upgrade path for std System owners unable to source a fully working reliable 16K Ram Pack for Sir Clive's original ZX81...

The modification involves the removal of the on board 2x2114L Ram chips which provide the ZX81's std 1K of RAM and populating the existing 28 Pin DIL socket layout that the boards already have provisioned with a 32Kx8 SRAM device, which is cool as no refresh circuitry is necessary and it also lowers the overall current consumption.

The 32K SRAM is a std easily available cheap 62256 or 42256 device and in this current application has been fixed to 16K operation only.

With some extra decoding ( one 74LS chip ) it is possible to fully utilise the full 32K capacity of the chip by utilising Address line A14 and the ULA's display file request line. But of course this would not look as neat.

Two very minor mods to the board are required and a few wire runs to incorporate the Extended Address Bus to the SRAM....

I have also added a 10K pullup to the R/W line to give it a bit more oomph when using the Expansion Connector.

And of course it works fine...and runs cooler.

Anyone wishing to upgrade their ZX81's just drop us a line ;)

Enjoy
TC :cool:
 

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Handsome job TC!(y) Looking forward to seeing a few units in action.

You got my order - but then you knew that anyway:roll::LOL:

BB
 
Hello Forum Chums

Another Amibay Exclusive for you...
Dedicated to Rob and Andy ;)

Having been put to the task I have engineered a very nice neat and cheap solution to banish Ram pack wobbles for ever and provide a nice upgrade path for std System owners unable to source a fully working reliable 16K Ram Pack for Sir Clive's original ZX81...

Enjoy
TC :cool:

Very nice work guys :D

The amount of times I lost data when the stupid Velcro-strips failed to hold the RAM steady still makes me angry to this day (even though it happened, like 30 years ago!).

:)
 
Awesome work, Bas! (y)

Is that a Memotech I see... :drool:
 
How about a guide for the CHR128 mod, Bas? HGR graphics FTW!
 
How about a guide for the CHR128 mod, Bas? HGR graphics FTW!

Um yeah...

Since I last visited the thread I have now added the additional 16K ram making a grand total of 32K and full HRG support :LOL:

CHR128 mod is doable but needs an additional 8K Static Ram chip sat on top of the Rom.
Maybe one day...

TC :)
 
Lemme see...

Piggybacking a RAM chip over the ROM or using a thin 8k x8 chip under the board...:whistle:

fan8200c.jpg


Just lift the CS line and that's it.:shhh:
 
Thecorfiot has just done my zx81 for me!

And a very nice job it is too, I cannot believe how good the composite video quality is,I have a ram expansion, and the hrg looks great and behaves impeccably... seems a really solid job inc timing mods etc etc ..

Very impressed :)
 
Thanks for sharing, When I resorted my ZX81 I fitted it with a 6116 to give 2k on board. I like the idea of 16k better, looks like a future project.

It might be your camera but the display looks a bit dark. I fitted a ZX8-CCB board to mine and the picture output is amazing. The board is small enough to fit in the modulator.

John


pce1.jpg

scrn.jpg
 
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You are indeed right John, the piccys were taken before I fitted the ZX8-CCB board I just used a straight emitter follower amplifier which of course does not reconstruct the back porch...
The output qualityof the ZX8 is very good indeed...

I should really update this thread with some new pics of the finished 32K machine with HRG and ZX8 Video.

TC :)
 
Please do update Bas. I'd love to see the finished article.

You've got me thinking if I could use one of these SRAM chips to upgrade the memory in my U101, or replace the power hungry 2114s.

John
 
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That is indeed doable john the uk 101 should be fine (y)

I did it for the Atom using a small carrier board that plugged into the 6502 socket and gave the full 32k ram.
Iainjh has the carrier board now and he reports it works fine...
 
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Can someone indicate me which 8k x8 SRAM uses the same pinout as the 2364C the ZX81 uses?

Don't willing to run lots of wires to/from the chip.
 
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