Search found 361 matches

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: 236

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: 236

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...
by RetroTechie
Wed Apr 17, 2019 5:52 pm
Forum: Hardware
Topic: Zx81 from death to life??
Replies: 17
Views: 397

Re: Zx81 from death to life??

@RS232: re-read my earlier post. You need to DOUBLE-CHECK modifications done since last working config. Looking closer at the pics, I noted: video signal comes from ULA (pin 16). Where to? Appears to be going nowhere. If indeed the case, then it logically follows that screen info doesn't make it to ...
by RetroTechie
Mon Apr 15, 2019 9:04 am
Forum: Hardware
Topic: Zx81 from death to life??
Replies: 17
Views: 397

Re: Help my zx81 mod has gone wrong!! 8 bit death

I think i cracked it i tried one of these diodes and a 1W-100R resistor no idea if it means 100oms (..) Brown-black-brown = 1, 0, x 10^1 = 100 Ohm indeed. Doing mods like this, one should a) know this , and b) preferably verify using a multimeter before putting the part in. Anyway - good job! :D Di...
by RetroTechie
Sat Apr 13, 2019 10:42 pm
Forum: Welcome Area
Topic: Hello from Fort Worth, TX
Replies: 2
Views: 163

Re: Hello from Fort Worth, TX

Welcome here, Glenn! :) First I tested the 9VDC power supply (..) Toss that! Well, not in the bin, but back into the box where it came from. It's giving out a fairly high voltage with light load (with just a TS1000 hooked up, around 12V is not unusual), its capacitors will be in poor shape by now, a...
by RetroTechie
Sat Apr 13, 2019 10:04 pm
Forum: Hardware
Topic: Zx81 from death to life??
Replies: 17
Views: 397

Re: Help my zx81 mod has gone wrong!! 8 bit death

When did you last get an image on screen from this ZX81? What mods where already on the board @ that time? What modification(s) did you do since then? A multimeter (voltmeter + something for checking connections between points on the board) is the very least you should have, I think. Modding a 35+ y...
by RetroTechie
Sat Apr 13, 2019 9:18 pm
Forum: Hardware
Topic: Zx81 from death to life??
Replies: 17
Views: 397

Re: Help my zx81 mod has gone wrong!! 8 bit death

It's always possible that some component was damaged during handling of the board, in-between powerups to test things, ESD damage etc. But for the NMOS IC's in a ZX81: highly unlikely. If not that, following method is practically foolproof: start from last configuration that you KNOW 100% sure to be...
by RetroTechie
Sat Apr 06, 2019 11:02 am
Forum: ZX BASIC
Topic: Difference between New rom and Old Rom
Replies: 19
Views: 1230

Re: Difference between New rom and Old Rom

Thanks for your trouble Moggy! :)

Works as-is as ZX81 ROM (in emulators at least). Simply rename .p -> .rom or .bin as desired.
by RetroTechie
Fri Apr 05, 2019 7:38 am
Forum: ZX BASIC
Topic: Difference between New rom and Old Rom
Replies: 19
Views: 1230

Re: Difference between New rom and Old Rom

Anyone know where I can find a copy of the old/buggy ZX81 ROM? Some emulators include a ZX81 ROM, and @ one point long ago I typed in the bytes one-by-one from a real ZX81 into a floppy-equipped machine. But that's all the 'new' ZX81 ROM, don't seem to have the buggy version around. (sigh) is there ...
by RetroTechie
Mon Apr 01, 2019 2:38 am
Forum: ZX BASIC
Topic: what is the largest number that can be stored on a zx81
Replies: 10
Views: 410

Re: what is the largest number that can be stored on a zx81

Andy Rea wrote:so we have 16384 * 8 bits to store our large integer (..)
Ah, so you put your code in ROM, do away with screen output (or interrupts for that matter :? :?: ), AND keep all your counters in Z80 registers. That's pretty hardcore! :lol: