Search found 379 matches

by RetroTechie
Tue Jul 02, 2019 1:41 am
Forum: Hardware
Topic: Z80 cursor but no text
Replies: 12
Views: 1533

Re: Z80 cursor but no text

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'...
by RetroTechie
Sun Jun 30, 2019 8:24 pm
Forum: Hardware
Topic: Z80 cursor but no text
Replies: 12
Views: 1533

Re: Z80 cursor but no text

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...
by RetroTechie
Wed Jun 19, 2019 5:04 am
Forum: Hardware
Topic: Unusual PSU
Replies: 4
Views: 870

Re: Unusual PSU

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...
by RetroTechie
Wed Jun 19, 2019 4:44 am
Forum: Hardware
Topic: French FrankenZX81, unique?
Replies: 31
Views: 4612

Re: French FrankenZX81, unique?

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 ...
by RetroTechie
Mon Jun 10, 2019 5:23 pm
Forum: Hardware
Topic: HELP i was trying to replace caps now i have issues!
Replies: 2
Views: 1779

Re: HELP i was trying to replace caps now i have issues!

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...
by RetroTechie
Sat Jun 08, 2019 8:38 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 38
Views: 5762

Re: White screen and no cursor at all

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.
by RetroTechie
Sun May 12, 2019 10:52 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 38
Views: 5762

Re: White screen and no cursor at all

(..) 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...
by RetroTechie
Sun May 12, 2019 2:01 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 38
Views: 5762

Re: White screen and no cursor at all

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...
by RetroTechie
Mon Apr 22, 2019 3:05 pm
Forum: Hardware
Topic: Composite mod done Zx81 great picture but somethkng is wrong
Replies: 14
Views: 2485

Re: Composite mod done Zx81 great picture but somethkng is wrong

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...
by RetroTechie
Sun Apr 21, 2019 10:50 pm
Forum: Hardware
Topic: Composite mod done Zx81 great picture but somethkng is wrong
Replies: 14
Views: 2485

Re: Composite mod done Zx81 great picture but somethkng is wrong

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...