Found a ZX81 and fixed it

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Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Found a ZX81 and fixed it

Post by Moggy »

Read and digested. :D

My EPROM jobbies will be a lot easier from now on.
rmzalbar
Posts: 12
Joined: Mon Jun 22, 2020 5:07 am

Re: Found a ZX81 and fixed it

Post by rmzalbar »

Moggy wrote: Wed Jun 24, 2020 8:14 pm The 81 was meant to be used with the "shoe box" type cassette player recorder, using the earphone socket for loading which as you may appreciate has a reasonable amount of "punch". One method that has been suggested By f/m 1024MAK is to send your WAV into say a pair of PC desktop speakers then run a suitable cable from the EAR socket (that these speakers generally have fitted so as to regain said "punch") into the 81.

Obviously the stereo needs slimming down to mono but I get a feeling that's well within your capabilities.
As for the resistor I cannot say whether or not it would work but certainly worth a shot and I cannot see it harming anything.
I made a stereo-to-mono cable and got it load from .p files (after converting to WAV) when output through my computer speaker's headphone jacks. Yay! Hopefully my smartphone will provide enough punch to do the same. I'll pick up an old tape recorder, as I really like the experience of using the old equipment. I do use a datasette with my VIC-20.

EDIT: Cell phone success!! (with the volume maxed) Ho ho hahahaha!
20200627_181935_1.jpg
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Found a ZX81 and fixed it

Post by Moggy »

Superb news, another one back on the road. Well done Sir!! :D
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1024MAK
Posts: 5103
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Found a ZX81 and fixed it

Post by 1024MAK »

So having got interrupted with work, I did not have time to post this, but better late than never, here is a diagram showing the ZX81 socket pins and their signals compared to the "standard" 28 pin ROM pin-outs (the 27 series EPROMs have similar pin-outs).
Image

Mark
Attachments
ZX81 ROM socket pin-out vs. JDEC 28 pin ROMs
ZX81 ROM socket pin-out vs. JDEC 28 pin ROMs
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
rmzalbar
Posts: 12
Joined: Mon Jun 22, 2020 5:07 am

Re: Found a ZX81 and fixed it

Post by rmzalbar »

Tested tape saving tonight. It was a real P.I.T.A getting a usable audio file recorded; When I made my video amplifier, I used a direct input to the transistor with pullups to +5v and pulldown to GND to set the voltage. I think it attenuates too much of the ULA output, reducing the MIC level. The amplitude was almost nothing vs noise. I had to screw around with software amplifying and filtering a lot before finally getting an audio file that could load back in again, and be recognized by ZX tape converter. I should redo my video amplifier with a proper input resistor for high-z and do my dividing from that.

Here is the dumb little program I made for testing. I used ZX Tape Converter to normalize it and leave my audio issues behind. Surely you remember Etch-a-Sketch toys? Arrows drive the stylus and S clears the screen.

Etchasketch.p
(1.32 KiB) Downloaded 326 times
Etchasketch.tzx
(1.54 KiB) Downloaded 303 times
20200710_002754.jpg
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