Is my Comp Video signal correct?

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Luc
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Is my Comp Video signal correct?

Post by Luc »

Hello,

To be able to use my ZX81 again, I connected the collector of a BC547B to 5v, the base to pin 16 of the ULA and the emitter to the composite video output and to ground via a 100 Ohm resistor. I did not put a resistor between the emitter and composite video out.
IMG_0150.jpg
IMG_0158.jpg
But I get no picture on the AV of my LCD (with board with HDMI, VGA and AV1 and 2).
IMG_0154.jpg
On my oscilloscope (I don't know a better name for this board) this signal comes from the composite video output, but I don't know whether this is good or not.



Is this the signal I should see on the composite video please?

rgs

Luc
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1024MAK
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Re: Is my Comp Video signal correct?

Post by 1024MAK »

I recommend resistor between the ULA output pin (16) and the base terminal of the transistor to both protect the ULA and limit the amount of current going to the base of the transistor.

In fact, I recommend the circuit that I link to in my sig below.

A video signal has a bandwidth (range of frequencies) up to 6MHz. Do you know the specifications of your ‘scope? We normally recommended a bandwidth of at least double that of the signal that you are trying to look at.

I’ve posted traces of the ZX81 video signal here somewhere…

Mark
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1024MAK
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Re: Is my Comp Video signal correct?

Post by 1024MAK »

There is a ‘scope image of the first part of a TV line shown in this post. Keep in mind this is just one part of the video signal.

In order to get the image in the linked post, I was very careful in setting up the ‘scope trigger control to get the part of the waveform that I wanted.

Mark
Luc
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Re: Is my Comp Video signal correct?

Post by Luc »

Thanks Marc,
After my ZX81 came a Spectrum, followed by a QL. So my ZX81 has been stored for a very long time and I wanted to get it working again.
The keyboard flat cables fell apart, but replacing a keyboard seems not that difficult. Because some capacitors, in the RF box, looked rather weird. I decided to build in that resistor and transistor and test if my ZX still works.
I connected it to an LCD screen that I use on my Raspberry Pi (via HDMI), but that also has a VGA and AV input. On the AV input I got some stripes, and when I tried to change the settings of my screen I felt that there was some high voltage on the control board of my LCD screen. I probably touched the ZX board at the same time. I lost signal on my LCD and noticed that the ULA, Z80 and ROM got "cold".
The 8705 has passed away. I then gave my ZX, 5 volts directly with a lab power supply and the chips heat up again, but I still don't see a signal on my LCD.
My 'scope' has a bandwidth of just 0 - 200 KHZ. That's much faster than I can measure, but of course it's ridiculous compared to the 6MHZ you're talking about.
Chances are I killed not only the 7805 but one or more of the other chips as well.
Your video converter looks much better than the one I implemented and if I can still get my board to work then I will definitely install it. For the moment there are too many uncertainties, I don't know if my screen is suitable and I don't know what still works from my ZX81 board.
Plus I don't know how to fix it.
Anyway, thank you very much for your help
Luc
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1024MAK
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Re: Is my Comp Video signal correct?

Post by 1024MAK »

Luc wrote: Thu Aug 05, 2021 4:23 pm The 8705 has passed away.
I think you mean the 7805 voltage regulator :mrgreen:

Do you have, or do you have access to a digital multimeter, a logic probe or a logic analyser?

Note also that the ULA you have is the ULA2C184E version. This version does not generate the part of the video signal known as the ‘back porch’. The result is that a colour TV may not automatically set itself to the correct brightness and contrast levels, so you get a very dark (dark grey/black) picture. If you do get this, turn up both the brightness and the contrast controls.

Mark
Luc
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Re: Is my Comp Video signal correct?

Post by Luc »

Yes I mean 7805 and I have access to a multimeter (and a ' scope' ;) )
And my ULA is indeed a version without back porch.

My next step is testing my ROM (it seems possible to test a rom with an Arduino)
I have ordered a simple Z80 board ( https://www.budgetronics.eu/nl/bouwpakk ... a-24691-20 ) on which I can test my Z80. But Z80 are cheap that is not a problem.

But the problem remains the ULA even if it is OK it is still a version without back porch. And ordering one with back porch) from New Zealand ( https://vdrivezx.com/vla81/ ) Due to the transport costs + import duties + administration costs, I fear that the price for 1 ULA will get out of hand.

I'll ask my friends if someone has still an old mono monitor and eventually change the video circuit on my ZX.

Thank you for the answers

Luc
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