Sinclair ZX80 / ZX81 / Z88 Forums

whoah!!!!

I don't know, you wait ages for a ULA replacement to come along, then two turn up following one another!

Well done to both of you

Mark

Mark,

Andy is miles ahead of me. His ULA is working perfectly fine My version is nearly there.

McKlaud wrote: ↑Sat Mar 10, 2018 6:34 pm Mark,

Andy is miles ahead of me. His ULA is working perfectly fine My version is nearly there.

Not really miles ahead... I need to sort out tape input, i guess i have some experience because I already did some ula's before. The video out has some noise in it, which needs addressing.

Anyway what problems are you having ? Pretty sure myself or mark can help out.

It is look like that I should assemble the board again, because in this one I've got some synch issues. When I press the board in the socket hard I am getting the "K" prompt on the screen. There is some noise on the video out too. I've been thinking about different type of a NPN transistor there. At the moment I use BC847B one.

What about the tape input? Maybe a bias would help.

Ah yes i have a motherboard like that, pretty sure a trace is broken somewhere but try as I might i can't find it so it only works when pressure is applied at just the right spot...

Yes bias is required I have provisioned on the PCB a spot for 2 resistors one to rail the other to ground currently they are unpopulated as i was more interested in getting other things working first. I also use npn transistor as emmiter follower bc817 gain is a non issue in emmiter follower mode but current handling can be because of the relatively low load ( 75ohm ) input of most composite inputs, I also have a 1k load on the board so that high impedance composite input monitors can be used without additional work ( a lesson from my altera based solution ) I have a 0.1uf cap right next to the transistor between collector (+5 ) and ground. I was thinking about either a small smd inductor immediately before the collector. I also put an extra 33pf between base and ground this helps a little.

Nice work for both, congratulation !

Now I have to dig out my board and continue working on it

viewtopic.php?f=6&t=445&p=3796&hilit=superfo#p3796

Superfo: welcome to the club

Andy:

The small smd inductor (e.g. 1uH) and than a 0.1uF capacitor between supply rail and GND might be a good idea. I will give it a try.

Mine has had a bit of a torture test today, had to go out to see my mum of course left the house at 10.30am and also left a test zeddy with my ula installed running maxdemo... Got back at 6.15pm ( 7hours 45minutes later ) and......

It's still running !

I'm gonna pat myself on the back and call that stable...

Now then where did I leave the trumpet?

Andy

Congratulation Andy!!!

My ULA was also tortured and I did a few minor mods earlier today. Now Zeddy boots up every time to the 'K' prompt. Screen and border inversion works as it should. Anyway, still works in progress.