ZX80 - very faint video/cursor.

[gmc]

I've got a ZX80 that I've built from scratch using a clone PCB based off issue 5.

Finished building it and after first thinking this thing is never going to work I powered it up and to my surprise there is (barely) a cursor on the bottom right. It's very faint but it's there. Black/grey with a faint outline. See attachment.

I've not connected up and RF modulator and have the composite video mod wired (schematic attached) up to the video.

Is the video mod I'm using just rubbish and this is causing my issues? Is there a better one I should be using?
I have tried both 18R and 33R and both didn't seem to make much difference.

I've got the video mod output connected a retrotink (Which works well with my ZX Spectrum, also with a different video mod)

Since there is some kind of video I'm guessing the CPU/RAM/ROM is all functioning. Have checked the address/datalines, clock, supply voltages - all look good.

I've also attached a screenshot of the video signal on the scope before it goes into the video mod. Not sure if this is good or not.

Any tips would be appreciated. I must be close to getting this working.

[1024MAK]

The ZX80 mixes the video picture and sync signals with two resistors to generate a baseband signal with non-standard voltage levels. Hence why that composite video circuit is not working very well.

Try replacing R30 (1kΩ) with a 2.2kΩ, replacing R32 (330Ω) with a 1N4148 diode (anode to the PCB track that joins to R30), adding a 4.7kΩ resistor from the +5V rail to the junction of R30 and the anode of the diode (in the position of R32). From this same junction, feed this to the base of a small signal NPN transistor. The collector goes to the +5V rail. The emitter goes to a 75Ω resistor. The other end of which goes to another 75Ω resistor which goes to the 0V/GND. The junction between these two 75Ω resistors is the composite video output.

That may improve matters, however, it depends on how your "retrotink" handles a video signal that is lacking the "back porch" part of the video signal. Most (but not all) colour receivers ( TVs/monitors etc.) use the "back porch" part of the video signal to determine the correct level for the most black part of the signal. They then automatically calibrate so that the black, shades of grey and white parts of the picture are displayed correctly plus the various colours.

But the ZX80 was designed for use with a black and white monochrome TV. The ZX80 does not generate the "back porch" part of the video signal.

Please note, I have not tried the modification described above on a real ZX80 myself yet, because I use a ZX8-CCB.

Mark

[gmc]

Thanks. I'll try this out and let you know how it does.

Otherwise that ZX8-CCB does look like a good option.

[gmc]

Just to be clear - the modifications should look like the attached?

Not seeing much improvement - but can make out a K now.

Beginning to think my retrotink is not liking this setup.

[1024MAK]

Either R33 should be 4.7kΩ with no extra 4.7kΩ resistor, OR the extra 4.7kΩ resistor is fitted with R33 not being fitted. Both should not be fitted.

R31 should not be fitted. Sinclair produced the board with various options in order to help cater for the different video requirements in various countries.

The rest of the circuit is correct.

Mark

[gmc]

Thanks, made those modifications and didn't have any effect.

A few hours of trail and error later I dug out my video mod board I use in my ZX81. Its the mod with the 555 timer/2 transistors/2 diodes, and installed it. Booted up and get a almost perfect output with a crisp K.

However as I type on the keyboard the screen flashes off and on briefly - so it's not perfect, and there is also some character corruption.

I think the retrotink is not happy. Do you think the ZX8-CCB will fix these issues?

[1024MAK]

A few hours of trail and error later I dug out my video mod board I use in my ZX81. Its the mod with the 555 timer/2 transistors/2 diodes, and installed it. Booted up and get a almost perfect output with a crisp K.

That confirms that the lack of the back porch part of the video signal is the problem.

However as I type on the keyboard the screen flashes off and on briefly

That's normal for a ZX80. It does not have the "SLOW" mode / compute and display mode of the ZX81. It's operation is similar to a ZX81 running in "FAST" mode. Hence unless the ZX80 BASIC is waiting for keyboard input, or otherwise just waiting for user input, it will discontinue executing the code that generates the video signal and instead devote all processor cycles to running the program code.

Try this program to see what I mean:

Code: Select all

10 FOR A=1 TO 1000
20 NEXT A

- so it's not perfect, and there is also some character corruption.

I suspect that the 555 video board needs one of the present resistors adjusting slightly. At least, I assume the version you have has this feature. Various people produce this type, which makes giving specific advice a non-starter.

Do you think the ZX8-CCB will fix these issues?

I think it's a better option than the 555 based video board, but it cannot ever solve the intentional design limitation of the ZX80 not producing a video picture when BASIC is not waiting for user input.

Having said that, the 555 based video board does produce and add in the back porch part of the video signal, hence it does what it is supposed to.

Whereas the ZX8CCB removes noise (more important with the ZX81 maybe, the earlier version was called ZX8CCP with CCP meaning crystal clear picture ) and produces and adds in the back porch part of the video signal. And the creator/producer is a member of this forum (although he no longer posts these days).

Mark

[Moggy]

Can confirm the ZX8-CCB is ideal for the ZX80 fitted mine a long time ago and still going strong.
Also with this device you have also the option of inverse screen and border with white letters which I find better visually.

[gmc]

Thanks for all the information - it's very useful. Very interesting about the SLOW mode. Will do some more reading up on the operation of the ZX80 out of interest.

I'll continue messing around with the resistor values but in the mean time will also order the ZX8-CCB board.