ZX8-CC8 and 2C158E ULA (PokeMon to the courtesy phone!)

[PokeMon]

I think the ULA is defective and gives only two voltage levels where three are needed. Those are mixed with two different drivers and resistors, similar to the ZX8-CCB output. So the white/black driver seem to work but the sync driver not. It has to reach a level lower than black and if not it wouldn't be possible to use the ZX8-CCB correctly. The point why the TV shows a picture anyway is, that the sync position is more or less "fixed" means is interpreted valid in a small timing window only of +/-1us per line, something like that. But this results in a poor picture with low contrast/brightness.

So I have not the solution for this problem. I would say the ULA partly died (the sync driver internal). I have similar ULA where the sync level works but the black level died (only sync and white).

[dinosaur]

I have a few ZX81s, three are V3 motherboards and work perfectly with the CCB board. My fourth will tune into a 22" flat screen Phillips TV giving a fuzzy but recognisable cursor. When I added the ZX8 CCB I get the same image as Balfords Fairly sharp cursor but about 4 or 5 characters wide. No amount of tuning will get it right.

Im using the latest ULA (210E?) on a very early V1 board. Could the problem be caused by the motherboard rather than the ULA or ZX8CCB?

With the 210E ULA, which provides already a back porch, you do not need any such circuit... With my French ZX81 (which comes modified with an inverter transistor (a ZTX313) at the entrance of the TV modulator, so to make the TV signal SECAM-compatible), I simply *removed* stuff to get a perfect composite video output: I removed entirely the internal components/board from the TV modulator. I also removed the 270 Ohms resistor on the ZTX313 collector and replaced it with a strap (only letting the 150 Ohms emitter resistor in place), then wired the emitter of the transistor to the output of the RCA connector (which is all what is left of the modulator). Done !

If you don't have a French ZX81, simply use a 2N2369A (fast switching NPN transistor, equivalent to the ZTX313), wiring the emitter to the ground through a 100 or 150 Ohms resistor, the +5V to the collector, fit a 2.2Kohms resistor in the R32' slot and a 1N4148 in the D9 slot (they should already be present for USA models: in this case, simply remove R31'), then wire the base of the transistor to the R32' resistor (opposite to the D9 diode). The composite video output is picked up from the transistor emitter.

To get a proper picture, simply make a cable (use a shielded, one wire cable) with a RCA male connector on one side, and a SCART/PERITEL connector on the other side. The SCART pins to connect are pins 17 (composite video ground, connected to the shield of your cable) and pin 20 (composite video, connected to the signal of your cable). If you also want the sound from the tape input/output (especially useful with modern TV sets as those will blank the display when they don't recognize a valid video sync, which happens when SAVE/LOAD-ing), then simply wire a 10 kOhms resistor between pin 20 and 6 of the SCART connector (left/mono audio channel) and add a wire between pins 17 and 4 (audio ground).
Be careful with SCART inputs on your TV: some TVs with multiple SCART connectors may not have a composite video input on all of them.

[Moggy]

@Balford

I have a couple of these older ULA's that work in a normal set-up. If you want to retain that particular zeddies integrity PM me an address to send it to and I will double check one is working by wiring up one of Karls wonderful devices and if all is in order send you the ULA with my blessings.


Regards.

Moggy

[PokeMon]

With the 210E ULA, which provides already a back porch, you do not need any such circuit...

Well - the circuit does more than just add a backporch. It removes the "shit" (noisy white background) that comes out of the ULA and can not be removed with a simple transistor emitter follower (which repeats the white noise). It recreates the video signal from a fresh 3.6V supply. The noise comes from the poor power supply lanes inside the ULA which was developed for digital signals and not analog signals (which the TV out in fact is even if only two colours used - black and white).

That was the actual reason for me to develop this board ZX8-CCB.

[Moggy]

With the 210E ULA, which provides already a back porch, you do not need any such circuit...

Well - the circuit does more than just add a backporch. It removes the "shit" (noisy white background) that comes out of the ULA and can not be removed with a simple transistor emitter follower (which repeats the white noise). It recreates the video signal from a fresh 3.6V supply. The noise comes from the poor power supply lanes inside the ULA which was developed for digital signals and not analog signals (which the TV out in fact is even if only two colours used - black and white).

That was the actual reason for me to develop this board ZX8-CCB.

I for one am glad you developed this karl as it takes the bother out of sourcing the newer ULA and also as well as looking professional the price is excellent.

I also find it very stable in use, even when not used for a while my ZX80 needs no recalibration just the same clear picture 10/10.

As you say in Germany..."es ist schön und preiswert"


Regards.

Moggy.

[stefano]

+1, I couldn't use my TS1000 anymore without that brilliant circuit !
I must thank twice this group because I got it as a gift

[dinosaur]

With the 210E ULA, which provides already a back porch, you do not need any such circuit...

Well - the circuit does more than just add a backporch. It removes the "shit" (noisy white background) that comes out of the ULA and can not be removed with a simple transistor emitter follower (which repeats the white noise).

I see no such "shit" here... See by yourself. The white is pure, without any noise whatsoever (which is normal: the white level simply saturates the video input of the TV), and there's just a slight amount of dark grey vertical lines in large, black areas. So yes, there's indeed some noise, but it's quite bearable (barely visible, most of the time) and nothing comparable to what I got with the TV modulator where the "white" background presented, quite literally, all the shades of grey (you could even tell what the CPU was doing by looking at the grey patterns above and below the useful screen area !)

Note that I could probably add a small RC filter on the transistor collector (10 Ohms and 220µF, for example), to help and filter out the +5V supply noise and get a better black, even though it won't remove the noise coming from the ULA signal itself.

Note also that the transistor choice does matter. Sinclair used a fast switching transistor (ZTX313), and if I had to use another (i.e. one that is still produced and available), I'd use a 2N2369A (almost an exact equivalent). People using BC547, BC107 & Co are going to be disappointed with the result...

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It recreates the video signal from a fresh 3.6V supply. The noise comes from the poor power supply lanes inside the ULA which was developed for digital signals and not analog signals (which the TV out in fact is even if only two colours used - black and white).

Nice ! I didn't know it had its own power supply/regulator (it's probably the only one such circuit to have it).

[balford]

Moggy - PM incoming

[Moggy]

Balford. Ditto.

[PokeMon]

Note that I could probably add a small RC filter on the transistor collector (10 Ohms and 220µF, for example), to help and filter out the +5V supply noise and get a better black, even though it won't remove the noise coming from the ULA signal itself.

Note also that the transistor choice does matter. Sinclair used a fast switching transistor (ZTX313), and if I had to use another (i.e. one that is still produced and available), I'd use a 2N2369A (almost an exact equivalent). People using BC547, BC107 & Co are going to be disappointed with the result...

Well - it depends on the quality of TV/monitor you use to display. I was not happy with the transistor solutions and regardless of the speed of transistor and filter you do - this is not noise from the power supply this is voltage drop inside the ULA power lanes which is measurable and viewable at output pin 16. Keep in mind that in the video signal 700mV is the difference between white and black and already 5-10% noise (35-70mV) can be seen on a white background. This can't be removed from the signal at pin 16 except sampling it and creating it new. And it is of course high frequent noise.

You can reduce this with individual contrast/brightness settings but when you connect it to a 42 inch plasma TV as I do, you can surely see those thin line pattern in the background and it is true that it is depending on the CPU usage during video display and you can see when the data is delivered for a character, when it is latched and so on. These actions do slightly voltage drops on the ULA output which is not as steady as it should be for a good analog signal. And of course I am not happy to change the default/custom settings of brightness and contrast on my home TV for just the ZX81 display.