Continuing from my last post, I had some time waiting on parts to finish the analog prototype testing for HBSound so I set up the new scope and ... 

It lives:

I like to think about the video output as the consciousness of these old home computers. Generally speaking enough of the gears need to be spinning to make this output correct (memory, clock ocilator, CPU and reset).  It's also important that it shows line data being sent as this further improves chances we have a working computer. 

The scan looks good but does verify the 'back porch' signal is missing. 

Here's an example of a properly formed scan for NTSC the example is with color burst so you can ignore that bit for the ZX-81 we're working on here:

(Graphics stolen from this excellent resource https://www.analog.com/en/technical-articles/basics-of-analog-video.html)

What we do have is a horizontal blanking pulse that sits at about 1v. This is indictive of some of the late release ULA's which do not actually generate a complete blanking pulse. This is probably fine for a crt displaying modulated RF.  Today we need an appropriate back porch signal to make sure the black levels (among other things) display properly on a lcd.  For this I will need to build up a small PCB with a 555 timer and some transistors to amplify the signal and ad back the back porch so it will properly display on a lcd. 

I also took the time to clear out all of the needed points of solder and put a 'zero' ohm resistor in at R30 to activate NTSC mode in the ULA.  The ZX-81  installation and assembly guide indicates it's value as a Zero ohm resistor, but I have read elsewhere it can simply be grounded. I opted to eliminate the additional video conditioning for NTSC and simply build a brand new video circuit in the same place the can modulator was installed. My hope is that the video quality will be better by keeping the trace length down.   For now I've ordered a few things I need to continue and I'll be looking to finish up HBSound testing phase by the end of next week. 

If you found this interesting or have a question please feel free to leave a comment I hope everyone has a fantastic holiday weekend!