I have had numerous requests on how to add direct video to the TS-1000 & TS-1500 computers so as to drive a composite video monitor. This has been covered many times in the past in other newsletters and magazines, but I feel that for those who are just now starting to use their computers and those who missed the other articles, here is how to do it.
The only way to do it is to go inside the computer and make some extra connections and add extra circuitry. On the two monitors that I have, one works quite well from just tapping the video signal before it goes into the modulator and connect it directly to the monitor (don’t forget the ground). This method will work on almost all monitors that have a switchable input impedance — labeled Hi-z and 75 ohm. If you connect the video from the computer directly to the input of the monitor, you should use the Hi-z position.
For monitors that come only with 75 ohm input, you will need to build a small driver circuit to the 75 ohm input of the monitor. This driver circuit can be a simple transistor emitter-follower circuit using the input impedance of the monitor as the load. Here again, a few monitors have a slight DC potential on the input, so some sort of isolation must be used. A 0.1 micro-Farad capacitor will work fine.
The video in the ZX81/TS-1000 can be found on one of the wires leading into the RF modulator. The lead to the left is video (facing from the front) and plus five volts is the center wire (+5 volts is needed for the transistor circuit).
On the TS-1500, the video signal can be found on pin 11 of U1 inside the modulator and +5 volts can be found on pin 8. The video can also be found on the left side of R30 and +5 volts on the right side of R30 (facing the front of the computer). These second two places are easier to get to. As far as adding a connector, I just clipped the existing leads at the RF connector and ran my new circuitry over to the existing connector which gives a cleaner look from the outside, but you will not be able to use a regular TV again.
