ZX81 ULA

[Moggy]

I am going to put a heatsink on mine!

My attack moggy (Keith) loved this so much he asked me to post this.

He also muttered some thing about booking a flight to the lone-star state for him and his M16 to enjoy a short hunting holiday.

[fizzyade]

The ZX81 I purchased did indeed turn out to have a faulty ULA! Was so happy when this replacement bought the "K" to the screen!

Many thanks Andy!

[Andy Rea]

The ZX81 I purchased did indeed turn out to have a faulty ULA! Was so happy when this replacement bought the "K" to the screen!

Many thanks Andy!

great stuff. Happy that its working for you.

Regards Andy

[mrtinb]

So tonight I repaired the PCB which had been damaged in the ULA removal process. And added the 1K resistor and ground to the ULA to get it working through the modulator.

Unfortunately it's an unstable gray screen so something has changed. Debugging will continue when I receive my cheap oscilloscope from China.

I have now received my DSO-138, soldered it and got it working. I am ready to debug my ZX81.

Is there thread here, that describes how each of the ULA pins should be on my oscilloscope?

[Andy Rea]

Hmmm a while ago i saw these build it yourself scope kits... i thought they had really low band width like 200Khz low ! since as the many of the signals on the ula are switching at Mhz speeds i dont think you are gonna have much joy with that.

i would first check for shorts on adjacent pins, and then check for continuity of all pins to relavent parts of the circuit.

also what modulator is fitted to your ZX81... i only tested the 1k thing with the UK um1233 modulators.

regards Andy

[mrtinb]

I see your point. The oscilloscope has to be faster than the circuit it measures. (I know people say these oscilloscopes were bad, but now I know why.)

I’ll debug with my digital probe then.

[TMAOne]

(I know people say these oscilloscopes were bad, but now I know why.)

They're not all bad, just not suited for everything. I have a DSO201, a 1 MHz "older brother" of yours that looks like a cell phone. I love it. I keep it in a box of "away mission tools", (just the bare essentials), and it serves quite nicely for any kind of audio problem. I think you'll enjoy yours, just not for this particular application.

What you might want to spend a couple of bucks for is one of those cheap Chinese knock off Logic Analyzers. USB hookup to your computer, free software, and 8 channels at 24 MHz. They're great fun too.

Ian

[mrtinb]

What you might want to spend a couple of bucks for is one of those cheap Chinese knock off Logic Analyzers. USB hookup to your computer, free software, and 8 channels at 24 MHz. They're great fun too.

That's a great idea. Thanks.

[Andy Rea]

Bumpety Bumpety...

20 ULA's up for grabs on SMR...

link in forst ppst

Thank you.

Andy

[Lardo Boffin]

Got one!