Sinclair ZX80 / ZX81 / Z88 Forums

So, I bought one of the Issue 3 replica PCB's by Franck78 and I've built it. I can say it has been a satisfying experience. The board is working like a charm . Building it was a breeze. I took some pictures while building it. It also has my own design internal 16K modification PCB and composite mod PCB which replaces the modulator box internals.

The ULA, CPU, modulator box and 6.5 Mhz resonator are from a duff ZX81 board, the rest is brand new. I rummaged through my parts bin and found some MPS2369 transistors and a brand new 7805 voltage regulator. MPS2369 is perfect on a ZX81, as a replacement for ZTX313.

The mounting holes are lining up and it fits in a ZX81 case.

I've also compiled a BOM:

Resistors:

R1 : 10K
R2 : 680R
R3 : Not used
R4 : 18K
R5 : 330R
R6 : 2K2
R7 : 470R
R8 : “
R9 : “
R10 : “
R11 : “
R12 : “
R13 : “
R14 : “
R15 : 220K
R16 : 1K
R17 : “
R18 : “
R19 : “
R20 : “
R21 : “
R22 : “
R23 : “
R24 : “
R25 : “
R26 : “
R27 : “
R28 : 680R
R29 : 1M
R30 : Not used
R31 : Not used
R32 : Not used
R33 : 4K7
R34 : 220R
R35 : 10K
R36 : “
R37 : “
R38 : “
R39 : 10K
R40 : “
R41 : “
R42 : “
R43 : 10K
R44 : “
R45 : “
R46 : “
R47 : “

Capacitors:

C1 : 47pF
C2 : 47nF
C3 : 22uF (Electrolytic)
C4 : 47nF
C5 : 1uF (Electrolytic)
C6 : 100pF
C7 : 47pF
C8 : 47nF
C9 : 47nF
C10 : 10nF
C11 : 47nF
C12 : 47pF

IC's:

IC1 : Sinclair ULA (or equivalent like vLA1)
IC2 : Sinclair ROM (2364 or equivalent)
IC3 : Z80A CPU
IC4 : 4118 or 6116 SRAM. For 16KB internal a 62256 with some rewiring can be used too. Or my own design 16KB internal PCB.

Diodes:

D1 : 1N4148 or 1N4448
D2 : “
D3 : “
D4 : “
D5 : “
D6 : “
D7 : “
D8 : “

Transistors:

TR1 : ZTX313 or MPS2369
TR2 : “
REG : 7805 Voltage regulator or switching mode DC/DC converter

Last but not least: Modulator box

Thanks for sharing this.
I'm going to start building it soon and your BOM comes at a perfect time
Kind regards Paul

Most of the components can be bought from Mouser or Digikey. Reichelt has the 6.5 Mhz resonator brand new. article nr. is SFE 6,5.

Is it possible to get a board still?

"The mounting holes are lining up and it fits in a ZX81 case."


I was unsure because I don't have any ZX81

Yes I have two or three pcb.

There is a resistor on the BOM that I just cannot locate on the board.

Is R40 the resistor under IC3, the Z80, it just says R, there is no number.

R41 isn’t marked either, I’m assuming it’s under R42 as that spot shows no marking at all.

redhawk668 wrote: ↑Thu Jun 17, 2021 6:19 pm Most of the components can be bought from Mouser or Digikey. Reichelt has the 6.5 Mhz resonator brand new. article nr. is SFE 6,5.

Reichelt don’t seem to sell to the UK, the country is missing from their drop down lists. Farrell in the UK have the resonators though.

I’m one capacitor away from completing the board……….and I ordered the wrong one!

Typical! Lol

Just ordered the correct one.

I’ll be over here, crying in the corner, lol

A new ZX81 was given life!

Thanks for the idea, the board and the help.

The pictures show it running and it just before it was completed with an original rom instead of its new one and no ram, couple of components missing.

No pic of it finished yet as I put it in a case to test it and I’ve not uncased it again yet.

Great little project, lots planned to do with it, watch this space as they say!

I would still love to get my hands on a PCB