Sinclair ZX80 / ZX81 Forums

[iturbez]

Hi everybody!

This is my first post in this forum, and I'm impressed of knowing there are still people that owns Sinclair's computers ("It was twenty years ago today..."). I´m an spanish lover of Sinclair's computers. When I was young, I owned a ZX81. I had to sell it in order to buy a new Spectrum (still own it).

I've buyed a ZX81 at eBay. I've don't tested it yet, but the seller said it worked... I want to upgrade the 1K memory with this:

http://www.zx81.de/english/32k-rame.htm

I've opened the ZX81, and it's a ISSUE ONE. It has two memory chips, instead of one. They are 2114 (1K x 4 bits), and my question is:have I to desolder both memory chips and add the new 62256 in the place of IC4?

Thanks in advance, and sorry by my poor English.
Merry Christmas, and a happy New Year.

[Mike3]

My 2114's were in sockets.. you need to remove them both.. if yours are soldered.. I would just use diagonal cutters and snip the pins close to the board,, and if your zx81 board is like mine the holes for the NEW 28 pin socket should be clear of solder.. and remember that D1-8 are from left to right.. on the board .. not like in the ZX81 diagram... I made that mistake.. it took me a while to figure that out...

Have fun...

Mike

[yerzmyey]

Hi, Iturbez. Welcome to the Forum.
I wish You luck with the RAM - there are many cool progs for expanded ZX81.

[XavSnap]

Hi all,

Yes... You add to remove both 2114 chips.
add a 28pins support on CI4 and link (strap) the LK2 to redirect the A10 to the good chip pin.

on the Z80 CPU:
pin 1 A11
pin 2 A12
pin 3 A13
pin 4 A14

on the Ram chip (62256-32k):
pin 1 tie on A14 , D7 diode.
pin 2 tie on A12 , D3 diode.
pin 23 tie on A11 , D1 diode.
pin 26 tie on A13 , D5 diode.


I had to change the Ron Reuter's Zx81 circuit diagram !
Several users get the wong address line on mother card.
I added the edge connector points on the circuit too.

[iturbez]

Thank you very much.

I Love ZX81!
I Love this forum!

When I had made this mod, I'll show here the job. I'll take some photos.

Have a happy new year.