Spectrum Video Mods

It is possible to modify the original ZX Spectrum’s video output to give a composite video signal through its standard phono jack – this gives a much improved picture and is more compatible with modern TVs (which may not have an analogue RF tuner).

There are two main preferred methods of doing this: One is to use a capacitor, the other is to use a transistor. The capacitor (100uf 16v recommended) mod is simpler and is probably the most TV-compatible version but the picture can be a little dark for some. The transistor (BC549C recommended) mod gives brighter picture (and buffers the Spectrum’s video circuitry), but it may not work on all TVs. If care is taken, both mods can be easily reversed if necessary.

The components are available from store.retroleum.co.uk



capacitor_mod1

Capacitor version:

1. Pop the lid off the modulator box and de-solder the top end of the resistor that is connected to the jack socket pin.

2. Desolder the modulator’s input and power leads (on the left of the box) and fold these wires over the top of the modulator out the way. Make sure the holes in the PCB pads are clear of solder.

3. Bend the capacitor’s leads so that it can mounted as shown. You can pre-trim the +ve lead (the one without the white stripe on the capacitor’s body) so it is easier to fit.capacitor_mod3

4. Install the capacitor, passing the -ve lead through the topmost grommet in the modulator.

5. Solder the -ve lead to the phono jack pin where the resistor was originally connected and the +ve lead of the capacitor to the video out pad on the Spectrum motherboard .

6. Pop the lid back on the modulator, trapping the original wires.





Transistor Version:trans_mod1

1. Pop the lid off the modulator box and de-solder the top end of the resistor that is connected to the jack socket pin.

2. Desolder the modulator’s input and power leads – tuck these wires over the top of the modulator out the way. Make sure the holes in the PCB pads are clear of solder.

3. Bend the transistor’s leads so it can mounted as shown – The flat side of a BC549C transistor body must face the modulator (the pin layout of other transistor types will vary!)

4. Install the transistor and solder the collector and base leads, trimming off the excess on the underside of the board. Please note: Some Spectrum PCBs have exposed tracks running below where the transistor is mounted – make sure these are not shorted by the transistor’s leads!trans_mod2

5. The emitter wire will be too short to pass through the gommet in the modulator body so use a small piece of wire to extend it. It may be easiest to pass this through the grommet, soldering one end to the jack socket and the finally soldering the free end to the transistor’s emitter.

6. Pop the lid back on the modulator, trapping the original wires.



Obviously, a composite video cable is now used to connect the Spectrum to the TV.

http://www.retroleum.co.uk/electronics-articles/repairing-a-zx-spectrum/spectrum-video-mods/