Hello from Moscow

[doperst]

Hello everyone!
Sometime ago i found a rusty Memotech 16K RAM pack in my father's garage in country place, and that was kind of a trigger for me.
First computer in my house was an Ohio Scientific C1P in 1978, then came ZX80, ZX81 and finally ZX Spectrum.
My father saw sir Clive Sinclair in person when he was in Sweden and had a conversation with him at Beckman's.
Father made a decision not to buy QL, later he brought Enterprise 128 for a couple of days, but it did not stay either.
So ZX Spectrum was the last retro computer at home. Later, few years ago, Spectrum died and was replaced by 'Pentagon', a local clone with 128K RAM and two floppy drives. I used to write quite a lot for ZX Spectrum, machine code, BASIC extensions, copy pretection stuff etc. Participated in local ZX printed magazine as an author, tho only one issue finally hit the shelves and two more or so were in production. This mag is still available online.

I started refurbishing my ZX81 amonth ago or so, here are things that i found at my dad's home:

[doperst]

...more pics:

34 years ago my father made a control box out of ZX80, i still have the book that he used.

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Googling the internet for info, i found this forum and the German forum, tlienhard.com with a lot of information on current ZX upgrades and repairs.

Here is what i have done so far:

1. Composite mod, based on a 555 chip version because it fit neatly into the stock modulator box:

2. I have no old TVs at home, and the only TV with a composite input is dedicated to videos, so i made a composite to hdmi mod, using a chinese us$5 box placing it inside with a shielded signal cable from backporch mod:

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3. Next came ULA heatsink:

4. and power convertor replacement. Large heatsink went to junkbox (not really):

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My original ZX81 had only 1K inside using a single chip without a socket, so i had a hard time removing the chip.
I have chosen a "reversible 16K upgrade" by Tynemouth, so i bought a 32K chip and make this

5. RAM upgrade:

The 'reversible' mod turned not so reversible, because i have broken two lines leading to RAM from CPU and had to patch them with wires.
Anyway, this worked.

By the time i made this, opening and closing my ZX81 50 times killed the keyboard membrane, so i had to make

6. A keyboard repairs mod:

I did not have a conductive glue at that moment, so i used a thin Akasa 0.20 solder piece, put it along a broken membrane line, fixed it with scotch and it helped. Fixed everything and left at this state so far. I have now a conductive glue, but i prefer not to touch the membrane while it lasts.

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7 Next came multiROM:

Bought some chips, grabbed schematics from forum and there it was.

8 ROMs i used to inhabit the chip are as follows:

Original ZX81 ROM
Improved ZX81 ROM
Sg81 ROM
Open81
Big Bang
Big Bang m (not sure about difference yet)
H4T
Thee4th

I tried to use Coral BASIC interpreter ROM but it was hard to find it as a ROM, and the ROM that i took from XuR emulator, didn't work for me.

[doperst]

Looking for a cassette loading replacement (i have checked, the EAR in works on my ZX81), i saw few options, VDrive being one of them, but it required buying rather expensive VDrive itself and modifying ROM or RAM severely to include drivers. And then i have found a miracle,

8 WESPI mod:

It worked like a charm. I've been really impressed by the simplicity and power of WESPI.

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9. Playing games on ZX81 could hurt the original keyboard, so i started to look for a replacement keyboard.
There were different schematics, i have already bought some chips and other parts, but my decision was to make an 'internal' keyboard mod without use of ZXBUS and external boards. So this is what i have come to, finally:

I have made a couple of 'BUSes' on the edge of ZX81 board, one for powering and signalling WESPI and HDMI, and another for keyboard cable, including 5V, ground and Reset signals, that took 16 lines.

[doperst]

...continued...

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...even more...

In the process of making keyboard, i noticed a strange behaviour of SHIFT button: pressing Rubout once worked, but pressing it twice did not, it led to 'stalling' of KB0 line completely. Same was the FN function (SHIFT+NEWLINE).
I had no resistor networks at hand, so i made one from 5 single 10K resistors, put them in parallel with stock KB0...KB4 resistor network and it solved the trouble.