The picture via RF from a Spectrum is relatively decent, however a composite signal offers a much better quality image, and as televisions with an analogue RF tuner become rarer, composite input may be the only input avenue available in the future.
Modifying a Spectrum to output composite video sounds difficult, but the actual mod is beautifully simple when you realise that the Spectrum already produces a composite video signal internally, which feeds the RF modulator input. The modification essentially bypasses the modulator circuitry entirely, and because RF and phono leads have the same socket type, no external changes are required to keep the modification looking factory.
The specific style of modification described here requires soldering, but can be easily reversed at any point in time afterwards to restore RF output. It also disconnects the power feed to the modulator.
Tools required for this job:
- Soldering iron
- Solder wick or desoldering pump
- A short piece of wire
Step 1: Disconnect the Modulator
Open the case and remove the PCB from the lower case by undoing the small screw in the centre of the PCB.
Locate the modulator at the top left, and remove the modulator lid. Using your soldering iron and desoldering pump/wick, disconnect the two wire connections from the PCB to the modulator (circled on left is the composite input to the modulator, on the right is the +5v feed). Once removed, apply further wick or desoldering actions to clear the holes in the PCB.
Step 2: Disconnect the modulator feed to the RF socket
Inside the modulator in the top left corner, you’ll see a resistor installed on its end, encased in a clear plastic insulator with one end soldered to the centre connection to the RF socket. Using your soldering iron, desolder this connection, moving the top of the resistor away from the socket entirely (no need to remove it).
In the picture to the right, the resistor end is shown already desoldered.
Step 3: Install new Composite connection
Taking a piece of wire, feed it through the spare hole on the left hand side of the modulator, and then solder this wire to the centre connection of the RF socket (shown in picture). Lastly, draw this wire through the desoldered hole for the video input and solder in place, then cut the excess.
Step 4: Tidy up
Replace the modulator lid (trapping the two disconnected leads inside the can, between the lid prongs), refit the PCB to the bottom case, reconnect the keyboard membrane and close the machine case up.
You should now have perfect composite video to your TV.
Note: In some rare cases, your TV may not like the signal being produced by the Spectrum and refuse to display a picture. In this case, it’s recommended to install a 100uf radial electrolytic capacitor instead of the wire to make the connection between composite output on the PCB and the RF socket. In this case, install the capacitor outside of the RF modulator can, positive lead to the PCB and negative lead to the RF socket.
This problem is due to the ~2V DC voltage offset present by default on the Spectrum’s composite signal. The capacitor removes this offset, and as a handy by-product, reduces the current draw of the Spectrum by about 15mA (input resistance of a TV is 75 ohms, so by Ohm’s law: 75/2 = 37.5mA theoretical maximum, but obviously a TV signal’s voltage levels vary quite a lot).
What if I don’t own a soldering iron?
It is possible to perform the modification without soldering, a video detailing the process is shown below.