The TS 1500 video signal includes the back porch, a part of the video signal important to setting the black level and overall contrast. This is an improvement over most Sinclair ZX81 and some Timex/Sinclair 1000 computers, which did not produce the signal.
Composite video output is available on pin 53 of the TS 1500 standard cell logic device (SCLD). It is connected to diode D19 and then to a voltage divider formed by R1 and R2. The signal from the voltage divider the signal goes to the RF modulator, which produces a signal for older analog televisions.
The modulator has 3 inputs. Starting at the top, they are: video input, power (+5V, middle), and channel 2/3 selection, driven by pin 54 of the SCLD.
Modifying Your TS 1500
You’ll need a single NPN transistor like the 2N3904 and a short length of wire.
The modification is simple:
- Remove the top from the modulator.
- Cut-off the top (video signal) and middle (+5 volt) connections to the modulator.
- Disconnect the RCA output jack.
Next, solder the transistor, inside the modulator, like this:
- Pin 1, the base, to the former video signal input from SLCD.
- Pin 2, the collector, to the +5V input.
- Pin 3, the emitter, to the RCA output jack.
- Replace the top on the modulator.
Last, bypass R1 and D19 by soldering a wire like the image below.
Thank you to Claudius Fatla for the detailed instructions, photo and schematic illustration.
