Sinclair ZX80 / ZX81 / Z88 Forums

Well some further development while i am waiting for parts to hopefully fix the video noise issue, i thought i would turn to tape input. i couldn't get a simple resistor network to work well, even though in my project setting i have pins set to "float" instead of "keeper" it was still sticking at 2.4 volts... very odd behaviour indeed. so a simple single stage bjt amp was built with a gain of around 15 so the output is very square which was exactly the intention , i think the gain is a little high as for the first time ever i have had to use a substantially lower volume on my cassette player than normal

anyway hopefully with the space freed up by chaging the video circuit i should have enough room for on board transistor amp.

regards.

Hi Andy,

I've faced the same issue when I built the resistor network with the 1.2V bias. It worked well when isolated on a breadboard, but when I plugged the ULA in to the Zeddy's PCB I started reading 2.4-2.5V. Odd. So I gave up with it and just tried to load anything directly with the circuit as it was. I tested with the TZXduino. Everything was fine, software was loaded without errors.

Thanks for the confirmation that I wasn't going mad, my circuit is very similar indeed, I need to tweak the gain a little but it's all good stuff ready for the next prototype.

Regards Andy

nothing beats the feeling you get when a bodge comes together !

Introducing ... DeadBug sot-23-6



big improvement on the video noise, but still have close to 50mv p-p noise ( spike every 1/2 cpu cycle ) , i want to retain the emmiter follower since as i have a 100ohm load on board, this give compatibility with low-z and high-z input monitor / tv making small improvements.

Thanks go to PokeMon and Claudius for hints and ideas.

i haven't had this much fun since.... the last ULA

regards Andy

"Dead Cockroach Mark II"

Is slow mode working yet?

BarryN wrote: ↑Wed Mar 28, 2018 6:27 pm Is slow mode working yet?

In my ULA, still work in progress. Unfortunately I had to pack my toys and locked them up for a few weeks.

@Andy: Are spikes in your case similar to these in red circlers in the attached picture?

Andy Rea wrote: ↑Tue Mar 27, 2018 11:37 pm nothing beats the feeling you get when a bodge comes together !

Introducing ... DeadBug sot-23-6

In the best Sinclair tradition

Keep the bodging ticking over guys

Mark

McKlaud wrote: ↑Wed Mar 28, 2018 6:37 pm

@Andy: Are spikes in your case similar to these in red circlers in the attached picture?

similar as in they are spikes... but way more frequent, i can tell you exactly when, every 1/2 clock cycle

all the following are video signal, probe and ground at the ula pins, noise is worse at modulator socket.

this is white video x10 probe so noise is around 80mv p-p

this is a vertical row of inverse spaces rest of the screen is white, the burst of noise at the left coincides with the force nop



lookd ok as a bigger picture levels are close to perfect but still get that horrible verticle banding,


forwards.... its the only way to go