Normal/Inverse Fault on ZX80 (Grant Searle's project)

[abaffa]

Hi folks,

I am building a ZX80 following the ZX80 project on the Grant Searle website and it is working, but the screen is inverted.

If I remove the R25(470) I get a "normal" inverse image but the characters are before the expected position and also with some glitches.

I've checked all pins and I can't find where is the problem:

IC11, IC12 74LS00
IC16 74LS10
IC9 74165 (not LS but I read from Wilco that it works http://wilco2009.blogspot.com/2017/07/p ... ading.html)

IC11 PIN 2 has clock (about 0-1.0v)
IC18 PIN 9 has clock (about 0-3.8v)
IC11 PIN3 <> IC12 PIN2 always 1

I changed the 74LS74 (IC18) and 74LS00 (IC11) and got the same result.

When I remove R25 IC11 PIN 3 transmits /MREQ but I guess IC16 gets crazy
Without R25 gets same clock from IC18 PIN 9 (about 0-3.8v)


Any suggestions?

[abaffa]

I've found an issue in the project.

There is an high pass RC filter after IC8 PIN9 that creates a pulse signal to IC11 PIN2 (LS7400 NAND) and IC16 PIN9 (74LS10 3-NAND).

That differential RC Pulse circuit works correctly to IC 16 feeds IC9 S/L PIN 1 but is not working on IC 11 to activate the normal/inverse latch'd circuit.

Fortunately I was able to find a solution. yey \o/

What I found is that I kept the RC filter using (47pf + 470r) to IC16 but created another to IC11 using (100pf + 3k7R using serialized resistors - I've checked from 2k7 and 4k7 it has worked).

I'm not sure if it's caused by the IC18 74LS74 voltage 'cos Q pin is creating a 3v signal (when it was supposed to has about 4v at least) or it's the 74LS00 tolerance, or it's quality.

--

What I can't understand is that other zx80 projects like Searle's, ministrel, zx80 core, etc apparently use this same schematics without the changes I made. And many people built Searle's project and did'nt mention this problem.

[1024MAK]

Are you sure that IC11 (74LS00) is actually a 74LS part and not another type that has been remarked?

Only in a Sinclair made ZX80, IC11 is a 74LS00.

Mark

[abaffa]

Well... I've tested the logic and I bought it from a trusted store but I'm not entirely sure

but after my adjusts it's working very well... this is my final version with slow circuit and zx81 8k rom

[jojo]

my screen looks like this ;

which is rather unconfortable , is there any adjustment possible to get it right ?

secondly i noticed that i can make it display this in zx80 mode , but nothing will be displayed in zx81 mode ....puzzled

any help highly appreciated

[jojo]

@ abaffa ,

did you use a "spider"-modification for ic11 ??

picture ?

[Gino]

Oh my God, I have the same problem: problems with letters in ZX80 mode and black video in Zx81 mode. Do you solve in the meantime? Let me know!
Gino

[jojo]

regarding the picture in my post above with the double K symbols on screen ,

using a trim pot of 5 K allows for the adjustment of that problem .

So now i have a single K .


albeit a NON inverted one , so that hurdle must still be taken .

Any ideas ?? please let me know .


jojo