TFW8bit composite mod board

[1024MAK]

The 5V DC in is getting about 4.95V.

That’s fine

The video in is showing about 3.5V.

Thats about normal if there is not much load on the ULA output pin.

The video out is showing about 2.85V.

That’s a bit high for a composite video signal

And here it is working on a far more picky TV. Go figure.

Every TV may be different. Some can cope with an out of specification video signal better than others...

Either use two or maybe three 1N4148 diodes to reduce the signal level as detailed in my post above, or maybe try putting a 47uF or 100uF capacitor on the output of the TFW8bit composite mod board.

Mark

[Lardo Boffin]

Either use two or maybe three 1N4148 diodes to reduce the signal level as detailed in my post above, or maybe try putting a 47uF or 100uF capacitor on the output of the TFW8bit composite mod board.

Mark

Thanks Mark. I have some 47uF electrolytic capacitors - how do I connect one up?

[1024MAK]

Thanks Mark. I have some 47uF electrolytic capacitors - how do I connect one up?

Connect the positive lead of the capacitor to the composite video output of the TFW8bit composite mod board.
Then connect the negative lead of the capacitor to the centre terminal of the Phono/RCA socket on the modulator.

Normal Trotters Independent Trading terms (No income tax, no VAT, no money back, no guarantee) apply...

It may work well, or it may not help... But it’s worth a try

Mark

[Lardo Boffin]

Thanks Mark. I have some 47uF electrolytic capacitors - how do I connect one up?

Connect the positive lead of the capacitor to the composite video output of the TFW8bit composite mod board.
Then connect the negative lead of the capacitor to the centre terminal of the Phono/RCA socket on the modulator.

Normal Trotters Independent Trading terms (No income tax, no VAT, no money back, no guarantee) apply...

It may work well, or it may not help... But it’s worth a try

Mark

I have that song stuck in my head now!

Does the capacitor replace the wire or is it as well as?

[1024MAK]

Replace

Mark

[Lardo Boffin]

That definitely helped thanks.

The TV that had the rolling picture and very faint K now has this:

Stable but distorted. Ignore everything except the K.


The big TV now has this:

No longer faint and washed out.

And my everyday retro TV has a superb picture:

Would varying the uF rating make much difference?

[1024MAK]

It’s unlikely that reducing the value of capacitance will do any good. It’s possible that using a 100uF capacitor may help a little. Using a larger value above 100uF will not likely make much difference.

As I said earlier, the main problem is the signal level being incorrect. Some TVs will be happy, while others will not. The Wikipedia article gives a reasonable diagram of a typical composite signal.



Notice that the sync level is at 0V and peak white is at 1V. Although it can also be described with the black level at 0V, peak white at 0.714V and the bottom of the sync. pulse at -0.286V. Which again gives an overall signal voltage of 1V.

The ULA in a ZX81 was designed to feed the modulator. IIRC, the TFW8bit composite mod board is just a buffer amplifier (emitter follower), so it provides current gain and no voltage gain. The output voltage however will be a little bit lower. This is because of the transistors base-emitter junction, hence the output voltage will be about 0.6V lower compared to its base (input) signal level.

But as the ULA outputs a voltage of about 3V to 3.5V, the TFW8bit composite mod board will produce an output with too high a voltage.

Adding the capacitor stops (blocks) the DC voltage, only allowing AC signals through. But some TVs still may not be happy

Mark.

[Moggy]

Just to add to what Mark is saying re certain TV's not being happy, I have a Roadstar small portable B/W CRT TV which is odd insomuch it has a composite input. The picture when used with pokemons video board is like Lardos, all washed out. My solution was to shunt some of the signal output down to gnd via an adjustable resistor,take reading of said resistors value then replace it with a fixed resistor of a near enough value(100 ohm if memory serves) and this works well enough for me,my conclusion being the input signal was too powerful for this particular TV.

[Lardo Boffin]

Thanks for the excellent explanation as ever Mark and thanks for idea Moggy.

Did you do a voltage divider type of thing? Would be interested to see how you did it. I am working my way through a Complete Idiots Guide to Electronics but its slow going!

[Moggy]

Thanks for the excellent explanation as ever Mark and thanks for idea Moggy.

Did you do a voltage divider type of thing? Would be interested to see how you did it. I am working my way through a Complete Idiots Guide to Electronics but its slow going!

The simplest way just for starters is to solder a 100 ohm resistor on the video out pad then the other end to the gnd pad,similar in the way the transistor mod works. This just taps off some of the signal to gnd.

In my case once I found the value required (100 ohm in my case)I soldered a miniature resistor across the positive and gnd pins inside the phono plug.