Yet another ZX-81 Restoration

dave-collinsDave Collins wrote 12/22/2022 at 16:59 • 3 min read • Like

Well here's another year :) 

I've made this as a space to manage my thoughts on this Zx81.  To be honest there's probably better examples out there as to the correct way to do this. I would direct you to these places first. But if you're interested here you go.

I have this little side project going on:

A friend recently sent this ZX81 from the UK.  its in decent enough shape to be from 1983 wouldn't you say?   I have a lot of irons in the fire so This is kind of working on the side of other projects but I thought it would be nice to do a little restoration (modification) work on it.

First issues are it's PAL and this is a NTSC region.  fortunately the ULA can be coaxed to output NTSC by clearing out a few solder points and adding some resistors and a diode. 

Secondly it is missing all of its sub-miniature jacks. I want to make a little saddle PCB that sits in the place of those and provides a mini USB for power, and stereo 1/4 inch jack (for mic. and cas. lines no sense in separating these since the existing tape emulators don't. ) There's no Keyboard and case at this point, I am thinking about a open concept one like this:

clone zx81
Clone ZX-81

GitHub for whole clone ZX81 -

Also the ULA has issues creating a proper back porch modern TV's seem to need to work properly.  There's a few options to dealing with this; I will be trying to build one of them up and see if i can make a usable signal.

Additionally it needs a little general maintenance and clean up.  The CPU is TTL (could be replaced with CMOS).  The main reason for wanting to do this is to lessen case heat (the ULA is a big source of it to begin with) and lessen current draw on the already aging outputs of the other components.  I know of at least one other restoration attempt (done by the byte attic) that used this approach and I already have a handful of these CPU's to hand.  There's some corrosion (mostly oxidation of the 1980's HASL), I need to address this.  There's a period correct 16k memory expansion which looks immaculate; beyond simply looking at the solder points under magnification if this is working well I will very likely leave it be.  The existing silk screen is flaking.  Any rework on the top of the board is causing issues with this - I don't know that much can be done but for this reason I'm trying to concentrate all of the rework efforts to the back side.  The two electrolytic caps are in good condition, and actually have signs of rework I don't know if I will replace them (unless they come back as very bad on ESR or I have some other issue related to them).

That's about it for now.  I don't know if ill do a dedicated hack-a-day page for this one;  a ton of the work is going to be manual vero-board soldering.  Most of the mods I'm doing are going to be the kind of thing you really only need just one piece for.  maybe if there's interest I'll do a custom flexible PCB for the port replication (to make it all thin enough to fit in a standard Zx-81 case.  I don't know if that will be soon as I am still trying to get my sound card build done (so close!)