Problem resurrecting my ZX81, please help :)

[asbesto]

Hi there,

I'm new and this is my first message here, so I'm very happy to have found this community

My name is Gabriele and I founded "Museo dell'Informatica Funzionante" Computer Museum here in Palazzolo Acreide, Italy (http://museo.freaknet.org)

I just decided to restore my first computer ever - a ZX81 of course I remember being a young idiot playing with wires into the expansion port... with the computer on, obviously, generating casual data in the ram... until my computer died :/ It was about... 1984? I don't remember.

Now this computer is at our Museum and time has come to restore it!

I supposed the ULA was burned. So I had here a pair of ZX81, they seem working - well, one is a TIMEX so it goes to USA channel, and the other is a Sinclair, but it came from USA, so it goes to USA channels. They are socketed, so I just desoldered any chip from my ZX81 and I started swapping them.

So on my little boy I have found:L

1) ULA is burned. the 2 ULA on the other two ZX81 are OK, they are old and new version.
2) CPU is OK!
3) ROM is OK!

I changed the ULA, but zeddy refuse to cooperate - white screen, no K on screen :/

So I socketed the two 2114 RAM. Here in the lab we have plenty of them, brand "NOS" new, and here comes the trouble.
I always have this white screen. I tried any pair of ram - nothing.

And here's first question: is a matter of timing? Some 2114 are 450ns (ceramic), maybe they're too slow? Some are -2, -3, some don't have any indication of speed. Brands are different (tomorrow I will do a list, if needed).
Any hint?

I changed the capacitors. Clock is OK on CPU and from the ULA. Swapped the two ULA's, nothing.

The other two zeddies have a single 4801 RAM chip - what I tried also was to socket a single chip into mine, and put there a 6118 ram(I saw it's compatible, but I'm not sure!) - but nothing, same white screen without K. Next try tomorrow: I will desolder a working RAM chip from one of the ZX81 and put it into mine.

Also tried two different 16K expansion - one from sinclair, untested, and other is a MEMOTEK Memopack 16, seem brand new. Nothing, same white screen.

I can't spot any broken trace at all.

Any hint?

Thank you all

[1024MAK]

Hello and Welcome Gabriele

The external 16k byte RAM expansions disable the internal RAM chip(s) anyway. Indeed, I have used a ZX81 issue 1 board with the internal RAM chips missing (I de-soldered them) with a 16k byte Memopack plugged in.

If you have a working ZX81 / Timex 1000, you can test both the16k byte RAM expansions (the Sinclair and the Memotech) on it. Keep in mind that there is a pause after switch on where you will get just a white screen, as the Z80 CPU carries out a test to see how much RAM is present. With the 16k byte expansion fitted, this take rather longer than with only the internal RAM.

The Sinclair RAM packs appear to be less reliable than the Memotech ones, but the Memotech RAM needs the Sinclair PSU to produce a voltage of between 10.8V and 13V (test on the board while connected to the ZXX81 / Timex 1000). You will get a much higher voltage if the PSU is not connected to the computer - this is normal.

Yes, the speed rating of the 2114 RAM is important. Also you do know that the 2114 chip is only 4 bits wide, hence you always need two chips fitted.
I forget which is the minimum speed needed. I will have a look tomorrow. But I think the ones marked " -2" should be fast enough (the markings actually relate to the access time, lower is faster).

More tomorrow...

Mark

[asbesto]

OK, this morning I socketed one of the working zed, and desoldered his TMM2016P-1 ram chip.

So I tested the TMM2016 in my zed: nothing, white screen.

I reinserted it in the working zed, now socketed: K was there, waiting for keypress!

And so I tester my other RAM chip, the MHS 61161-5: K on the screen! So my big ram chip are working.

At this point, I have changed everything on my zed, I'm going to check continuity between traces, otherwise I don't know what to do :/

[1024MAK]

Have you confirmed that the output from the 7805 voltage regulator is at 5V (4.75V to 5.25V)?

Also check the /RESET pin. It should be high. See the pin-outs, link in my sig.

Mark

[asbesto]

Regulator push out 5.05V, it's on every chip. I changed every electrolytic capacitor.

-reset on cpu, pin 26, is high, as has to be. If I put down (wire jumper to gnd) screen goes blank, when I disconnect screen goes weird (spinning pattern etc.)

I also changed the 47pF capacitor in C1 to be sure.

Every resistor is OK, I checked them all.

I think the modulator itself can't be source of problem, isn't it? It's the only part I can't swap easily.

Now I'm checking for solder bridges and all the traces

[asbesto]

I did a check out on every single connection. No problems, they all go where they're supposed to.

Now really I have no idea about how to fix this



Next step: desolder all the sockets again, check, and use other / new sockets.

[PokeMon]

Could be the modulator as well.
Do you have an oscilloscope ?

[1024MAK]

It's possibly the modulator, but may be something else. What test gear do you have apart from a multimeter?

Can you post a picture please of the display.

Or try a composite modification. More info in this thread.
You don't have to remove the modulator, just build the circuit on a piece of stripboard or matrix board.

Mark

[asbesto]

The museum Lab is well equiped, we have a dual channel 50 MHz digital oscilloscope, and other tools.

About the modulator: I think it's working, because I get a white picture with the computer area almost visible - like the working z's, but without the "K". Sometimes, when I turn it on, it goes white (no K), but after a while seem to print some characters in a weird out-of-sync pattern.

Can the modulator affect other parts of the circuit? For what I see in the schematic, this is not possible :/

Will try the composite mod anyway

[1024MAK]

No, the modulator should not affect the logic circuits, but it may affect the tape-out (mic) circuit.

If you have a 'scope, test to see what signals you get on the pins of the Z80 CPU.

Even if the ROM and/or RAM are faulty and the CPU is processing garbage instructions, the CPU should still be doing an instruction fetch cycle followed by a memory refresh cycle. With a 'scope, you can't tell what instructions it is executing, but you should see all the important signals switching between logic levels.

Mark