After 30 odd years since getting rid of my ZX81 with wobbly 16K RAM pack I recently bought another one back from Ebay; the photo's showed a bit of a sorry state but when on arrival it was actually in rather good condition. I thought I document some of the mods I did in case some of you are pondering the same. I have attached a couple of photos to clarify.
USB Power: the dapper ZX81 gets a little warm with that small heatsinked 7805 and a busy ULA. After I did the 32K RAM upgrade (see below) I measured ~270mA at full load, well within the range of a typical USB device. I made a USB to 3.5mm jack cable, removed the 7805, soldered a fuse holder to the edge of the PCB (routed out an isolated space in the 7805 heatsink island) and included a 5.6V Zener diode between the + and ground. It runs happy and cool from any USB source and the Zener and fuse will save the day when somebody tries to plug in 9V
32K RAM upgrade: see PDF, these are great and easy to follow instructions from TOKABLN. I added 1K RAM sockets to the PCB to keep the option of going back to stock and one of them conveniently holds a 74LS00 for the additional selection logic
Composite Video: See the excellent tutorial from JoulesperCoulomb on YouTube (https://www.youtube.com/watch?v=1irH3KuGyl0). I removed the innards from the original modulator and fitted the new replacement circuit board inside. It works perfectly, really happy with the picture quality (even for the old ULA without the back porch signalling)
Power and Reset: Reset is a matter of shorting the right electrolytic capacitor to ground and +5V is available from many convenient points. I made a detachable flat cable connection between PCB and shell. I drilled 2x 1.5mm holes in the top right corner of the shell and glued a red LED and tactile switch beneath them. When powered up it shows a nice pinhole red light and for a reset I use a paperclip
MP3: I am sure you came across this frustration as well, the iffy loading from "tape". I didn't want to connect a bulky cassette recorder and I couldn't get it to load anything through the various devices I tried (MP3 players, phone, headphone outputs etc). After a closer look with the scope I devised a little circuit to shape the typical mp3 player output signal. It uses a LM358 opamp as a non-inverting amplifier to amplify the positive side of the audio signal and drive an AND gate. The AND gate produces a nice square wave that is used for input to the ULA (see scope screenshot). I disconnected C10 from the 3.5mm EAR input and connected the latter straight to the LM358 input. The AND gate output is connected to C10. I use zxtools p2raw and Audacity to convert .p -> RAW -> MP3. Even with a low bit rate of 64Kbits loading of any program works every time.
Hope this helps anyone'; just ping if you have any questions, happy to help!
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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 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Could you please give a bit more detailed instructions (I mean pin-to-pin connections, complete components list, perhaps bigger photos etc.), as I am not much into electronics theory and I'm not sure I completely understood your schematic...
Also, if some other fellow here in the forum has tried this mod, could he explain in a little more detailed fashion?