I'm dusting off my ZX81 with 16k memory pack (the pair cost me $200 in 1981!) and have already discovered that the Z80A chip has definitely gone bad. (The HALT\ output pin sits at an indeterminate-level 1.2 volts even with the logic IC removed, and the CPU is halted). I had a Z80L (good for only 1 MHz and the clock is 3.5 MHz) in the junk box, but even so, swapping it resulted in lots of activity on the address, data, read and write lines, etc. as well as video pulses coming out of the custom logic chip, pin 16. http://www.mainbyte.com/ts1000/good_schematic_hi.jpg I'd like to take the output from that pin, which currently goes to a "canned" RF modulator through a couple of resistors and a diode, and instead convert it to NTSC video, as I also have a 9" monochrome CCTV monitor (75 ohm input) which would make a compact display. Can someone help me with a simple circuit to generate the appropriate RS-170 1 volt p-p video signal? thanks Charles