Video: install ZX81 16K internally - 10 minute mod!

Discussions about Sinclair ZX80 and ZX81 Hardware
David G
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Video: install ZX81 16K internally - 10 minute mod!

Post by David G »

Old news to some of you I'm sure but I just watched this and got a bit excited
YouTube: How to install a ZX81 16K Ram expansion - 10 minute mod!
from November 2020 by backofficeshow

My questions during watching
  1. What chip is that? The mod is a single chip and four bodge wires
  • What PCB is that? Red on the back and yellow on the front. Looks like a ZX81 PCB Issue 3
I paused the video and the chip has marked on it "CY62256" which is well-known on here. And it is static RAM. The kind that was prohibitively expensive back in the day, but is now only $4 in USA
Lardo Boffin
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by Lardo Boffin »

The PCB is an issue three.

The chip is probably something like this:
3071296E-BFAD-4D44-A57F-CC237B3978B0.jpeg
I fit these inside my 16K modded zeddies.

I would say that 10 minutes is very optimistic unless you are a really, really good solderer!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
David G
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by David G »

he did seem to zoom right along with the soldering ... how long was the video

The 32K chip is CY62256NLL but i'd guess that any type 622256 with 28-pin package will work?

he used a "quite nifty Japanese engineering solder sucker" to remove the existing 1K chips (2 smaller chips). I can read the name in the video still ENGINEER SS-02 Solder Sucker. Followed up with using desoldering wick on some of the pins


There was one more step
  • solder in a jumper header for the 1K/2K option
Lardo Boffin
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by Lardo Boffin »

I have a couple of those desolder suckers. They are excellent.

I used one for my rebuild project viewtopic.php?p=27533#p27533.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Vorticon
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by Vorticon »

My board has a socketed SRAM chip. Is there a schematic of that mod so I can create an adapter?
David G
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by David G »

Vorticon wrote: Thu May 19, 2022 4:14 pmIs there a schematic of that mod
There is this old one from Peter Liebert-Adelt
http://www.zx81.de/english/32k-rame.htm
from http://www.zx81.de/english/32k-rame.htm
from http://www.zx81.de/english/32k-rame.htm
32k-ram.gif (3.23 KiB) Viewed 1787 times
the page has instructions and options for 16K or 32K or HRG (high res graphics) and battery backup
if you only want 16K-byte it is very simple:
  • remove the original 1or 2k memory chip(s)
  • solder a good ic-socket with 28 pins at the place of IC4
  • fit some short wire at the place of LK2 (on the right side of IC4)
  • take the 38Kx8 RAM-chip and bend pins 1, 2, 22, 23, 26 up, they are not allowed to have contact with the pins of the socket
  • put the ram (43256, 55257 or 62256 e.g.) into the socket
  • connect pin 1(A14) with ground (pin 14 of IC4)
  • connect pin 2 (A12) with A12 , this is the cathode of D3 on the ZX81 pcb
  • connect pin 22 with ground
  • connect pin 23 (A11) with A11, this is the cathode of D1 on the ZX81 pcb
  • connect pin 26 (A13) with A13, this is the cathode of D5 on the ZX81 pcb
  • if you want to run HRG8, connect pin 27 via a resistor of 10K ohms with 5 volts
that's all
* search for all posts: 62256
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Vorticon
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by Vorticon »

Thank you!
One question: will this modification affect the functionality of the ZXPand?
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mrtinb
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Re: Video: install ZX81 16K internally - 10 minute mod!

Post by mrtinb »

Vorticon wrote: Sun May 22, 2022 2:49 pm Thank you!
One question: will this modification affect the functionality of the ZXPand?
The ZXpand will occupy from 8k - 48k, so cannot work with this.

The only place left for memory expansion with the ZXpand, is between 48k - 64k. The only memory expansion I know of, that can be configured to only add memory to 48k - 64k, is Chroma 81 interface.
Martin
https://zx.rtin.be
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Re: Video: install ZX81 16K internally - 10 minute mod!

Post by Lardo Boffin »

I did the track cutting version internal 16K RAM mod on an issue one on Sunday and it took 45 minutes.
Of that around 20 minutes was safely (from the motherboard’s perspective) removing the original RAM chip. Clearly I need to practice!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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Paul
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Location: Germanys west end

Re: Video: install ZX81 16K internally - 10 minute mod!

Post by Paul »

I didn't dig into detail but the question if ZXPand works with this woke me up.
ANY RAM (specially internal) should obey the /RAMCS signal.
This means any part that drives the CS of a RAM chip should do this via a resistor and be connected to the external BUS so it can be overridden by 5V connected to this pin.
This way internal RAM expansions work with external devices.
Notice: using the existing /RAMCS of the ZX81 only gives 16K of RAM that can be disabled from external devices such as ZXPand.
I know my English is terrible so please correct my post to remove unclear parts.
Kind regards Paul
In theory, there is no difference between theory and practice. But, in practice, there is.
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