ZX81 16k ram pack mod with 8118

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repetto74
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ZX81 16k ram pack mod with 8118

Post by repetto74 » Sun Mar 17, 2019 3:59 pm

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 ?

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 16k ram pack mod with 8118

Post by 1024MAK » Wed Mar 20, 2019 9:50 pm

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
Attachments
Sinclair 16K Issue 3 RAM pack modifications
Sinclair 16K Issue 3 RAM pack modifications
MB8118 pin out
MB8118 pin out

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RS232
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Re: ZX81 16k ram pack mod with 8118

Post by RS232 » Fri Apr 12, 2019 8:54 am

Have you looked into putting the 16k on the board of the zx81 itself? Anyone tried this?

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 16k ram pack mod with 8118

Post by 1024MAK » Fri Apr 12, 2019 10:50 am

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

Lardo Boffin
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Re: ZX81 16k ram pack mod with 8118

Post by Lardo Boffin » Sun Apr 21, 2019 9:52 am

I have successfully upgraded a number of zeddies following those links in the past. Top stuff!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

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