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?
Normal/Inverse Fault on ZX80 (Grant Searle's project)
Re: Normal/Inverse Fault on ZX80 (Grant Searle's project) Fixed
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.
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.
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Re: Normal/Inverse Fault on ZX80 (Grant Searle's project)
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
Only in a Sinclair made ZX80, IC11 is a 74LS00.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
Re: Normal/Inverse Fault on ZX80 (Grant Searle's project)
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
but after my adjusts it's working very well... this is my final version with slow circuit and zx81 8k rom
Re: Normal/Inverse Fault on ZX80 (Grant Searle's project)
my screen looks like this ;
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
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
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
Re: Normal/Inverse Fault on ZX80 (Grant Searle's project)
@ abaffa ,
did you use a "spider"-modification for ic11 ??
picture ?
did you use a "spider"-modification for ic11 ??
picture ?
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
Re: Normal/Inverse Fault on ZX80 (Grant Searle's project)
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
Gino
Re: Normal/Inverse Fault on ZX80 (Grant Searle's project)
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
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
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .