How would you like reverse video on your TV screen when using your TS1500? Not only is it possible, but it’s also ridiculously simple. All it involves is installing an SPST switch, and soldering two wires.
Locating the switch is perhaps the hardest part of the mod, as there isn’t too much spare space available. Probably the best place is right in the centre of the rear apron of the upper case half. Run two wires from the switch.
Locate resistors R20 and R30. These are between the ROM, SCLD and modulator case. Connect the left end of each of these resistors to your switch wires. Voila!
When the switch is closed, your TV picture will be reversed. The contrast is lower than optimum, but on most sets you can get a quite acceptable inverse video picture by turning the contrast up, and the brightness down. By the way, the left side of R20 is also a good place to pick up composite video for a monochrome monitor (high-impedance input preferred; if you have a lo-z 75 ohm monitor, use an emitter follower as described in SWN Vol. 1). Also, the video reverse switch will NOT reverse the signal to the monitor, only to the TV.
How does it work? Beats me, I just found out about it by accident. I can’t even find R30 on the schematic! Needless to say, additional experimentation is encouraged, and additional discoveries are welcome.
TS1500 MEETS OLIGER VIDEO
Yes, you can use the TS1500 with John Oliger‘s TMS9918A video upgrade. The required modification to the computer’s NMI line is MUCH easier than with either version of the ZX81/TS1000 board.
First, check that your machine has a 2200 ohm resistor (red-red-red), in parallel with a small capacitor, tacked on near the top end of the CPU. If so, you won’t even have to add the 4.7K resistor. If not, run the 4.7K resistor between the donut pad going to edge trace 12A, and the top end of capacitor C17.
You only have to make one trace cut, and bridge it with the 1N914/1N4148 diode. This is the first trace below the CPU chip (pin 20-21 end) on the component side. Carefully scrape away the varnish on this trace with the tip of your Xacto knife, and cut the trace. Now bridge the cut with the diode, cathode (banded) end towards the left (away from the heat sink). This trace is very narrow, so be careful when soldering. If you use too much heat, the trace could lift off. Bend the diode leads at right angles to the body, then bend the ends (about the last 1/8″) into an “L” shape so that a larger area is available for soldering. Carefully clean the flux from the area using methanol, to insure that you have a good solder joint and that you didn’t accidentally solder to the adjacent trace.