Still on the spectrum. Socketed loads of ics. Tested them. It's still flakey. Swapped TR4, TR5 with a different speccy. Still a massive 1.6v ripple on the 12v. Replaced most of the caps. Only wants to run the diagram. Refuses to run any other rom. I think this one is cursed.

@haitchfive @jonn_blanchard looking again at the TR4 and TR5, removed them and they still measure as being transistors. Scoped the voltage on the 4116s to look how it powerrail is. Found 2v ripple on the 12v, that can't be good

Bit forward, bit backward. Lower ram seems to test ok now. But now I get random nmi or random key presses without keyboard connected. ULA works fine in a different spectrum. I might end up socketing everything. But that feels a bit of a defeat. It also seems very picky about the eproms I use.

It doesn't make sense to me, why would it fail all 8 bits. Unless both ic4 and ic5 (both ls157) are bad. Or the diagrom isn't working properly (which i really doubt). (issue 4b board)

A bit more signs of life (or death?)

So 4 work, 1 faulty, and all broke keyboard membranes. Faulty one gives random screen output. Any one with a clue where to start? I'm still spectrum clueless.

anyway here's a patched z88dk that cleans up the ZX81 tape output, whoever wrote it clearly wasn't a DSP signals person

https://github.com/SegHaxx/z88dk

makes it all nice and symmetrical, no more DC offset

i'll make it nice sine waves someday

...i was unable to write readable tapes and after driving the SO nuts listening to screeching i determined the super clean outputs on a modern PC cause the vintage cassette recorders aggressive AGC to max out, resulting in mangling the signal when it comes in

the ZX81 has zero error correction and this fails the load

after adding a pilot tone inspired by the Apple II i can write tapes that load 100% reliably and it gives you something to orient on when scrubbing through a tape looking for the start of a file

it also shows as a solid white screen on the ZX81

near as i can tell a real ZX81 is so goddamn RF noisy the video buzz fulfills the same purpose of priming the recorder's AGC

80's tech, man ¯\_(ツ)_/¯

...with that i'm kinda burned out on the ZX81 lol

#retrocomputing
#sinclair #ZX81

using the incredible compatibility afforded by the front panel headphone jack, i can write a tape from a modern PC and load it into the ZX81

and i run headfirst in to z88dk's limitations, compiling with sccz80 renders in a lethargic 5.4 minutes, sdcc is much faster at 2.5 minutes

but sdcc doesn't seem to support the ZX81 compatible floating point library, so it glitches the video

the inner loop is 16bit fixed point but using it all the way through gets real raunchy precision wise so i thought i'd just use floating point on the outside hahahah

https://www.youtube.com/watch?v=XxWg5hIT2fo

#retrocomputing
#sinclair #ZX81