Sinclair ZX80 / ZX81 / Z88 Forums

Sorry for no anwser... I have a lot of school work.

So I have a friend who can has an oscilloscope or maybe he can have a logic probe. If he have one I will let you know about!
The Z80 what I previously found is not available anymore...
I will search for an other one and I will buy a socket for this.
I tried to desolder the cpu but I can't... My friend can have tools for desoldering and I think he have some advantage with this.
"Let me know if you want to try using your multimeter to test the Z80."
If I can, I want to try to test it!


Gábor

Hello! I found a z80 from China on eBay.

https://www.ebay.com/itm/311081981813?h ... Sw7K1ep1kr

Its a DIP40 z80a. So, what did you think? Should I buy it? I don't want to cut the legs of my z80a before I didn't get another. This is good for the Spectrum?

Thanks,

Gábor

CsabaiGD wrote: ↑Mon Sep 13, 2021 6:23 pm Hello! I found a z80 from China on eBay.

https://www.ebay.com/itm/311081981813?h ... Sw7K1ep1kr

That’s not a CPU, it’s Zilog’s serial UART.

This site lists the various part numbers used by Zilog.

You can also use a NEC D780C-1 CPU (more details). This is an equivalent and was widely used by Sinclair.

Where about’s in the world are you?

Test the following pins with your multimeter (black/negative probe connected to a convenient 0V/GND point.

Z80 pins:

Pin Function
6 clock
16 /INT
17 /NMI (should be high, > 3V)
18 /HALT (should be high, > 3V)
19 /MREQ
20 /IORQ
21 /RD
22 /WR
24 /WAIT (should be high, > 3V)
25 /BUSRQ (should be high, > 3V)
26 /RESET (should be high, > 3V, except for a very short time after power is applied)
27 /M1
28 /RFSH

A diagram of the Z80 is here

Report back your results.

Mark

So, I found another z80(which is a cpu, I think...)

https://www.ebay.com/itm/162783576670?h ... SwzppaHRzF

Yes the Z84C00xx will work. It is a CMOS type of the Z80. It has lower power consumption than the Z80.

Although given the price and that it’s marked as a 20MHz part, it may well be a relabelled 4MHz NMOS type!

If you have a large electronic parts supplier in your country, it may be possible to get a brand new recent production part. Yes, CMOS Z80s are still in active production.

Look for a Z84C0006PEG, Z84C0008PEG or Z84C0010PEG. Zilog’s web page is here.

Mark

Hello!

https://www.ebay.com/itm/164801016320?h ... SwSddgbVrT

So, here is another z80. I will continue looking for a good cpu. I am in Hungary. Unfortunately, i didn't find a NEC D780C-1 cpu. i will tell you the result of testing the z80.

Hello! So, i have made a lot of things. First up, I measured the legs of the Z80 with my multimeter. Everything has worked, EXCEPT FOR THE PIN "M1". I was sure, the CPU is bad. So, the new challenge: find a working Z80. Well, after a little search I have found a Z84C0008PEG. I desoldered the old CPU with soldering iron and a tin sucker pump. I just wanted to try what happens if I power on the computer without the CPU. It's the same screen as previously(border yellow or black and colored blocks everywhere). I don't had any socket, so I wanted to try to simply put the new CPU to the old place, without soldering, but touching the pins on the board.
ANNDDD! The border gets white EVERY TIME when I powered the computer on(still without soldering the CPU in)! Still the same error, but with the border white! I think that's a good sign! After that, I bought some new sockets for the CPU. I soldered the socket in, and I put the CPU into the socket. But... After that, the border didn't want to get white. It get to white for about every 4th try, but not every time. Now, there is 5v on the M1 line. (And the border actually got very colorful, like light-blue, purple, green... )

M1 line is not used in ZX Spectrum. Many of ZX Spectrum left factory with faulty/non-active M1 line in the fitted Z80.

McKlaud wrote: ↑Wed Nov 10, 2021 8:58 pm M1 line is not used in ZX Spectrum.

Correct.

McKlaud wrote: ↑Wed Nov 10, 2021 8:58 pm Many of ZX Spectrum left factory with faulty/non-active M1 line in the fitted Z80.

But do we know that as fact, or is it just speculation? We do know that the Z80 and the ULA can be damaged by edge-connector abuse. So it is possible that the Z80’s /M1 line was damaged after the machine left the factory.


As to continuing the fault finding, before doing anything else, check your soldering using a good light and a magnifying glass. Look not only for poor solder joints, but also solder splashes (which can be as thin as a human hair) and any breaks in thePCB tracks.

Mark