Composite Mod

[PokeMon]

Yes - this could be an offset problem.
You could try to solder a resistor of 1.5k to 2.2k between pin 16 and ground.
This could reduce the output voltage of ULA due to a fixed significant load.

[rune]

Thanks Mark & Karl

I think you are saying that the Voltage from the ULA is too high and the modulator circuit lowers this.

I did try putting the 2.2K resistor between pin 16 and the transistor, but you say to take it to ground? Im too much of a novice with electronics to think about that. Basically Im too scared I blow something.

Still I have three more boards to look at. 3xCCBs on their way and the ULAs hiding in the cave somewhere safe in their anti static box.

Karl, is it best to adjust the trimming resistors with a non-metalic pop-trimmer?

[PokeMon]

Yes I mean between pin 16 and ground, not in series with the transistor.
This way there is significant current flowing at pin 16.
If you remove the modulator (=effective load) this may influence the signal level.
The transistor is amplifying the signal which means it takes only a little current from that pin and deliver a higher current (from supply voltage) to the video output.

You may use metallic screw drivers as well as I do myself. There maybe a change when removing the finger or screw driver due to a capacitive load given with the finger but then you just give a little more or less and proof the picture when removing it. This is more practice than science.

The ZX8-CCB has by the way a significant (but not too high) load through two 5k trimmers connected in parallel, giving a 2.5k resistor seen from the ULA side.

[dinosaur]

Instead of loading the ULA with a 2K resistor to the ground (which might not even be enough and will only make the ULA heating more), you can put a diode in series with the base of the transistor (i.e. pin 16, diode, base resistor, transistor base), this way you will lower the level of the signal to something more compatible with a normal TV video signal amplitude (1V peak to peak, normally), and will get much darker blacks.
Without the diode, the output signal will be over 2.5V peak to peak, which is too much, even if the automatic gain control of your TV video input may succeed to bring it to something manageable.
I personally got perfect results with two diodes in series (a basic small signal Silicon diode (1N14148) and a Schottky diode BAT85), which lowers the output signal by 1.2V or so. Using a fast switching transistor such as the 2N2369A is also a plus (Toff of less than 20ns instead of 500ns or so with a BC548).

[rune]

Cheers Karl and Dinsaur

I've got 3 more ZX81s to try and ressurect ands I have now got 4 working ZX 81s working and modded, so I can experiment a bit.

If it fails I've just received 3 new CCB boards today as backup.

[Urbancamo]

I installed a ZX8-CCB this evening and it is producing excellent composite video output. I installed it in the RF modulator case although I had to cobble together instructions from other installations to get a picture (no pun intended) of what I had to do.

The key thing I found, other than a clear picture of where the wires go, is that you have to remove the entire modulator and then unsolder 4 points where the PCB is attached to the casing, taking care when removing it to ensure that it moves in the correct direction given that there are push tabs holding it in place.

I'd also recommend leaving sensible loops in cables to aid in installation and just give a little 'slack' if things go wrong and you need to re-solder a connection. I ran the 'keyboard ribbon cable insertion' gauntlet and won this time

Here is a photo of the installation.

image.jpeg

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