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ZX81 16k ram pack mod with 8118

Posted: Sun Mar 17, 2019 3:59 pm
by repetto74
Hi guys,

I have a 16k ram pack issue 3 for the zx81 which has a bad coil. I would like to get rid of the 4116 ram and swap them with 8118 as they require only 5v. The Vcc pinout is slightly different and the 8118 have the Vcc on pin 8 i/o pin 9 on the 4116. What do you suggest ? Dessoldering the coil? Where do I need to bridge to get the 5v on pin 8 of each ram socket ?

Re: ZX81 16k ram pack mod with 8118

Posted: Wed Mar 20, 2019 9:50 pm
by 1024MAK
For an issue 3 board, I suggest you do the following:
Remove: R3 (2.2k), D1 (BA157 or BA221), D2 (1N4002), D4 (12V Zener), D8 (5.1V Zener) and the coil.
Remove all the existing 4116 (or equivalent) DRAM chips.
Confirm by visual inspection that the only components connected to the former +12V supply rail (pin 8 of the DRAM chips) is capacitor C13 (100uF) and any small decoupling capacitors (black plastic axial types on most boards).
Use a multimeter to confirm that the +9V rail does not feed any power to pin 8 of any of the DRAM chips.
Solder in the 8118 DRAM chips.
On any of these, solder a link wire between pin 8 and pin 9.

Power up. Test that you have +5V on pin 8 of all the 8118 DRAM chips.

Good luck.

Mark

Re: ZX81 16k ram pack mod with 8118

Posted: Fri Apr 12, 2019 8:54 am
by RS232
Have you looked into putting the 16k on the board of the zx81 itself? Anyone tried this?

Re: ZX81 16k ram pack mod with 8118

Posted: Fri Apr 12, 2019 10:50 am
by 1024MAK
RS232 wrote:
Fri Apr 12, 2019 8:54 am
Have you looked into putting the 16k on the board of the zx81 itself? Anyone tried this?
Easier these days to replace the existing 1K (or 2K if a TS1000) with a 32k byte SRAM chip (such as a 62256) and only use half of it. For example, browse these links...

I showed how I did an issue 1 board here ;)
Note that my mod is designed to allow a further modification to 32k bytes. Hence the cutting of the +5V track, then fitting a wire link across it.

Dave Curran of Tynemouth Software shows how he does an upgrade on an issue 1 board here.

RetroTechie's site shows how to do the mod on an issue 3 PCB. See the archived web page here.

So take your pick. They all do the same. Expand a ZX81 to have 16k bytes of internal RAM.

Mark

Re: ZX81 16k ram pack mod with 8118

Posted: Sun Apr 21, 2019 9:52 am
by Lardo Boffin
I have successfully upgraded a number of zeddies following those links in the past. Top stuff!