Updated: 7 Feb 2022
- On the pcb bottom of a ZX80 there is a soldering option choose between standard or inverted video.
Simple transistor mod for ZX80/ZX81
For Timex 1000 see underneath.
This transistor mod is enough most of the time for great picture output quality on a ZX80 or ZX81 (source: link).
This should only work on ZX81’s equipped with a 2C210E ULA, though I got the same good output with a 2C184E ULA.
Previous ULA versions (and by extension the ZX80) don’t output a proper back porch signal, and most people say in that case a ZX8-CCB or ZXVid board will be required (see underneath).
This mod requires only one 2N3904 transistor, a 100 Ohm resistor and often one 18 or 33 Ohm resistor:
First test it without the 18/33 Ohm resistor.
In case the output brightness is too high, add the resistor between the transistor emitter and video output to lower brightness to acceptable levels:
- With one European ZX80 I used 18 Ohm
- With one ZX81 I needed I used 33 Ohm
I usually cut the 5V line and video signal from the modulator board, since it can cause distortion.
Small clarification about this (7 Feb 2022): when modifying the ZX81 video output, I cut the 5V and composite video wires that go on the small board inside the modulator, so that the board doesn’t do / influence the video output anymore. I use the incoming 2 wires (that go through the plastic holes) for the modification obviously though.
First, move the middle wire at the modulator to the pad shown below, which will connect it to pin 16 of the ULA containing the video signal:
Second, remove this switch:
Replace the switch by one wire bridge as shown here:
Now cut the RF output lead to the RCA connector inside the modulator, where the arrow points:
Also disconnect the two parts hidden behind the RCA output socket (on the right on the photo):
Now add the transistor mod:
ZXVid / ZX8-CCB
- There are some advanced boards that do a c1omposite video mod: the ZX-Vid and the much cheaper ZX8-CCB.
A topic about ZX8-CCB and succesor: link
The ZX8-CCB doesn’t always work correctly without the transistor-mod (probably on ZX80 only).
- A USA ZX80 needs the same resistor values as with the European version when using the ZX8-CCB; the transistor mod isn’t needed then