Problem resurrecting my ZX81, please help :)

[PokeMon]

Well - the first test I check if the ZX81 is working is measuring A15 (pin5 of cpu).
With 5us/div or 10us/div you should see A15 toggling regularly for about 42us with a pause (low) of about 22us, giving a cycle of 64us (horizontal frequency).

When turning horizontal resolution to 5ms/div or 10ms/div you should see a package of 12ms pulses, a pause (low) of 3.5ms, a small package of about 0.5ms and a second pause of 3.5ms - repeating. This is the vertical frequency, showing display generation (viewable picture) NMI periods above and below the picture and the small package keyboard check/vertical sync.

Whenever you see these signals, you can expect the system working. A15 is a directly indicator of the video signal, so if it is generated then CPU, ROM and RAM are working correctly. It maybe the ULA video output which does not give a black level (only white and sync) but you told you did already change it to a working one. This maybe a misstuned modulator as well. If no picture some people begin to turn the screws inside the modulator.

[asbesto]

Yes, and I saw signals. ("I saw signals",seem a movie title! hahahhahaha)

They seem coherent, TTL levels are OK, not only on the CPU but also on data / address lines. I will check better on Monday

[PokeMon]

If you have this signal at A15 (check for the described packages and length and regular signal) you don't have to check any other address or data line or signal. If you have this, the ZX81 is running, producing the video picture. I only measure other signals if I don't have this signal on A15. If you have it all is okay. If not, you may check further signals. This saves a lot of time by the way.

[asbesto]

I do have stuff on A15 - tomorrow I will check if the signal is coherent; I had difficulties to trigger it, maybe it's time to test the memory / sample feature of our oscilloscope (a RIGOL DS1000E )

[PokeMon]

Here is a scope picture.

http://sinclairzxworld.com/viewtopic.ph ... ideo#p5363

Important is to use the correct horizontal resolution to see some senseful data.
Here it is 10us/div in the picture above, triggering this signal should not be too hard.

The other one with 5ms/div or 10ms/div is quite harder depending on your scope and your experience.
If you scope offers triggering options for tv signals ("TV field" on my scope) it will be easier to catch the slow vertical signal.

[asbesto]

Well, this morning I thought that my Zed need some real LOVE to come back alive.

So, I desoldered all sockets - they were brand new sockets, normal ones, cheap but good. After that, I scavenged in the box of spare boards, from which we save components from the same era of what we're restoring.

I choosed some nice blue sockets from a phone pbx box dated about 1983 - same era! I carefully desoldered them, checked, cleaned with alchool, and soldered on the ZX81. I went back to the 1K config with two 2114 ram chip.

I was prepared to check the A15 signal. Power on...

... and IT WORKED!

https://www.instagram.com/p/BEVqet5E_33/

Well, it's alive for the first time in about... 30 years or some!

I also managed to fix the keyboard: I cut out the broken part, added some cardboard on the back to insert the new connection in the molex connectors, and the keyboard is back working again, with a 3 wires patch I made 30 years ago because I tried to hack the keyboard with a knife back then

Every key is working, maybe I need a new membrane ASAP but for now it's working again!

Thank you all

Now I need a keyboard membrane, IDK if I want to do the Composite video mod, maybe yes, it's reversible so it's ok for our museal procedures (we always avoid invasive modifications on what we restore)

[PokeMon]

Maybe it was just a bad solder joint somewhere.
Congratulations, you made it.

[sirmorris]

Woohoo! Another one comes back to life

Also - great TV! So retro

[1024MAK]

Hey, great that you got it going

Do stick around

Mark

[Zxkim81]

Hi Gabriel

Now I'm no expert but sometimes an incompatible TV would display a white screen with nothing else, or makes the k cursor do weird things
I've had this sort of problem myself.

Hope this helps
Regards
Kim