ZX81 White display

[inaki]

I am trying to get to life my old ZX81 in kit.
Apparently it is working but display is white. No cursor, no characters. Just a white background.
I have checked the video output with a scope and it looks right. The display is a well formed white rectangle.
The Z80 apparently is running fine. I have checked that It is able to run the ROM code startup, it checks the memory top, checks keyboard, etc.
I have not checked the ULA though. But, as I had another kit I have replaced the ULA with another one, but the result is the same.
I have replaced as well the RAM chips and the Z80 itself, but the white display persists.

All I have read about a problem like this points to the ULA. So I wonder if there any specific check I can perform on the ULA itself.

At this moment I have no clues about where to look at. Any pointer is welcome!

[blittled]

Are you connected to a modern tv? Some of the ULA's did not generate a good back porch signal so the inverted K can appear extremely light or may not be there at all on newer tvs. I finally found a portable B & W TV from the early 80's that gave a good display.

[gozzo]

The missing back porch normal causes an overall 'dark' picture..well, it does with me!..Have you tried replacing the ROM, as it's the only chip you haven't mentioned as being changed? - failure is less likely than the other chips, but still possible! If still no results need to check if the oscillator is working (would have thought so if there's an overall 'white' screen..??) and that the clock is getting to the CPU.. How do you KNOW it is running ROM code OK, checking RAM, keyboard etc. if there is nothing appearing on the screen?? As for other things, modulator failure is unlikely, but possible(if you are using the modulator's output.?) The two transistors need checking as one inverts the clock signal to the CPU and the other is part of the NMI circuit..

[inaki]

I have not replaced the ROM because apparently it reads well and the code matches the one I can see on the available ZX81 ROM listings.
I do know the system is working because I see it working when I connect it to a Z80 ICE. I can actually see the startup code running.
The TV modulator appears to work well. And I have checked the video signal at the modulator input as well. There is a video signal that is identical to another, working, ZX81.
The first check I did was the input clock. It gives the expected frequency and reaches the Z80 clock input.

I have not checked the NMI circuit. I will take a look.

[1024MAK]

The display generation is very closely tied between both the Z80 and the ULA. For the K (or indeed all text and graphics) to be shown, the CPU must execute the "shadow" image of the display data in the RAM that appears in the upper part of the memory map.
Does your ICE show this?
If the Z80 is doing this correctly, then I would suspect that the faulty part is the ULA, but you say you have already tried another ULA.
So check the ULA socket and all the tracks/traces for the digital signals going to the ULA including the resistors.

Do let us know how you get on, and good luck

Oh and welcome to our forum

Mark

Edit: tidy up and better explain what I am saying...

[inaki]

I have checked the NMI.
It is alive for a couple of seconds and then vanishes.
The edge an frequency of the NMI pulse matches the start video signal. Is this right? The pulse is 5us wide.
I can figure out the NMI should last forever, unless fast mode is used, right ?

[gozzo]

Now check (or just replace, they're cheap enough) those 2 transistors... I would think failure is unlikley, as they are not passing much current, but still possible.. not much else to check/replace!!! What is this ICE thing you have 'checked' it with??

[iturbez]

Hi, Inaki (o debería decir: Hola, Iñaki)

I've recently fixed a "dead ZX81 (see viewtopic.php?f=7&t=1236), and I had to change both CPU and RAM chips. With old CPU, I had no video signal (black screen). As I changed CPU, I had a white screen with no cursor, the same as yours. You say you've changed RAM chip... Perhaps there is some sort of bad contact between sockets and chip pins (in case chips are socketed)... See the thread below to see some samples of scopes signals you have to see in your Zeddy... PokeMon is a source of knowledge. If you are Spanish-speacking (as I think by your username), you can seZDP forums in Spanish. http://www.zonadepruebas.com/

I hope this help you.
Greetings.

PS: "Hola, Iñaki" is the same as "Hi, Inaki", but in Spanish.

[inaki]

gozzo, I have replaced both TR133 with new ones. No way. Same white display!

iturbez, I will check RAM connections and will take a look at these scope signals too. Thanks!

[inaki]

I have taken a look at the thread where pokemon explains the shape I should get on A15.
I get a similar pattern to the one in the pictures for a couple of seconds. Then the signals starts changing. Some seconds later the signal is flat.
So it seems clear to me that some component is heating up and then fails.