My first ZX80.

[Muttley Black]

Today I got this beautiful screen, with no reason. I mean I didn’t do anything new.




But after I hit something on keyboard, half texts missing:





After a reset (on/off) I got this. look at the black vertical line at the left:





And here, is the signal of the video output. You can see the back porch that is there. But also I see much noise.

[bola_dor]

Sadly my zggy 555 mod is not working,, but what I see is that it makes that kind off noisy signal at the Q1 emitter (I have no usefull signal at Q2 collector after r5,r4)

How is the signal at the RF modulator input? You can measure at the point of R32 near the metal box. Then at IC 19 pin5 (SYNC) and IC 20 pin 6, here you will see the K cursor or type O (PRINT) to see something bigger as this is the CHAR output only..

[Muttley Black]

The modulator is out from the motherboard, but this doesn’t matter.

So, here is the output of R32:



Here is what I get from IC19 pin 5:



And here is IC20 pin 6:



But hear that: To get this measurement’s need to remove the 555 circuit from the board, cause with the circuit on, I get a small back porch signal on "Video in"....





If I just connect the video in to my tv i got full texts, not the best screen and not so black "K":

[bola_dor]

Looks that IC output are a bit noisy. My oscilloscope may not show as much as yours

I have far less amplitude at the R2 ending

[bola_dor]

I am actually using a simple transistor mod without any hardware modification to the original board. Is not the best but works fine.
I couldn't make the 555 work on the ZX80.

[Muttley Black]

I really don't know. I am confused...

looking at the following video, after 9:40, the guy use the same zigg circuit on a zx81. If you see the first test was on a mini tv. he got very nice picture but with missing texts, like me. At the and he test it on a big tv and all was great.

https://www.youtube.com/watch?v=5OQuJ-GMwF4

[bola_dor]

This how it looked like at a TS1000 with newer ULA with backporch, (at the video he has an issue1 zx81 likely with an older ULA). I thought that the black left border was due to the preexisting backporch in this one but I see you had the same effect in the ZX80 at some point. Then I could never make it work on the ZX80. (Mine is a 60Hz USA version BTW)
Now it looks like my 555 board us not working at all..

[Muttley Black]

me point. Then I could never make it work on the ZX80. (Mine is a 60Hz USA version BTW)

Me too, but in my case D11 is missing.


Next step:




And we will see!

[1024MAK]

With the NE555 circuit design, the video signal is still ‘buffered’ by a NPN transistor (Q1j, so is still subject to some of the problems that a simple video ‘buffer’ circuit has.

Depending on the quality of the signal (level, edges and the amount of unwanted noise), the input to the trigger input (pin 2) of the NE555 may need to be adjusted (but the presented design has no provision for this) and/or the amount of time it times for (pins 6 & 7) may need to be adjusted (replace R1 with a fixed 2.2kΩ resistor in series with a 4.7kΩ preset).

Reference from zx.zigg.net, ZX81 Video Conditioning.

Mark

[Muttley Black]

(replace R1 with a fixed 2.2kΩ resistor in series with a 4.7kΩ preset).
Mark

I did that. Not exactly with the same values, but close. This let you play with back porch length, if I understand well.

Here is some outputs by play with the resistance:





By play with this resistance screen make changes. The text "GO TO” start to clear, and at the end it was extremely clean:











But after a hard reset, I had a bigger black vertical line: