Sinclair ZX80 / ZX81 / Z88 Forums

To protect my ZX80 I'm thinking of introducing an optocoupler into the video signal path. Most important for protection against ESD damage or such, is to have series resistance in the signal path. Something that limits a current spike to values that eg. a protection diode in a 74xx IC's output won'...

Welcome to this forum, albertusj! :D ZX80 (and -81) need most things to work in order to get anything looking like normal screen output. Behaviour suggests that CPU is working, and both ROM and RAM as well. At least enough that CPU doesn't crash. :mrgreen: At first I thought you might have some timi...

I've seen several marked "EURO 1400" as well. Not sure if other values like 1200 or 700 were also used in combination with that. Iirc some of those supplies had the same transformer inside, just different value capacitors (to 'upgrade' a "1200" to "1400" or similar). And/or same as some Spectrum psu...

Those RAM boards could very well be off-the-shelf RAM expansions. With 16KB on each, I suspect one of those is modified to fill some other memory area than the 4000-7FFFh for a normal 16K pack. That last expansion board is certainly some custom (or at least low-volume) job. Wire-wrap no less. Great ...

Rule #1 of modding: check that machine works before modifying anything. Rule #2 of modding: if after that you find machine doesn't work anymore, it will be because of whatever was modified just now . Double-check, triple-check, or undo if necessary. Sure there's ESD, pcb tracks where a crack develop...

Isn't that mini-pcb supposed to go into the modulator can? (after you rip its contents out). That would very much tidy up the wiring since all signals / power supply you need is right there.

(..) although I'm not very condifent with my probe. Just poke it into some other apparatus. There's a cpu in everything these days! :lol: (and clock oscillators that go with it) The resistor that you mention is R3 that is marked as "not used" in the electronic schemas No it's not. Afaict from the p...

After testing with a digital probe I've found that the CLOCK signal on the pin 14 of the ULA is not present, its state is LOW and no PULSE. That would indicate a problem with the ceramic resonator, surrounding passive components, or ULA. But have a look at the board: there's some bodge resistor the...

I notice it's true the ULA does still gets a bit hot, but only in the center of the chip which surprised me given i have replaced the regulator. Hmm yeah, center of the chip is where the heat gets released... ;) The ULA's dissipation is not changed by a regulator replacement, only the temperature o...

That light-colored band on the electrolytic capacitor (facing resistor), is that the band indicating "-" (minus)? If so, you've got cap reversed. :x Most positive voltage will be on the transistor/resistor side, lower voltage on RCA center pin (monitor input). So + of capacitor goes onto resistor/tr...