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ZX80 from NTSC to PAL?

Posted: Tue Jul 09, 2019 8:42 pm
by flaviosr
Dear All,

I have bought a ZX80 from USA supposing the mod from NTSC to PAL would be simple, like it is in the ZX81... but, since I am not a real technician and the motherboard does not have the easy welding point like the ZX81, I am stuck. :(
I am still waiting for the ZX80 so I am not able at the moment to make any tests but I would like to make some steps forward...
Is there a way to make the mod I need?

Thank you

Re: ZX80 from NTSC to PAL?

Posted: Wed Jul 10, 2019 10:08 am
by 1024MAK
On both the ZX80 and the ZX81, the display is generated by the Z80 CPU following the instructions from either the ROM code (the firmware, the normal BASIC system) or from RAM (for example, a loaded high resolution program).

In a ZX81, the ULA does have an input pin, whereby a resister (R30) is fitted/or not fitted (U.S.A./U.K. and Europe). However, the only thing this does at a hardware level, is to change the state of one bit in a I/O port (“register”), that the CPU can read. The firmware reads this bit during it’s operation to work out if the display should be 60Hz or 50Hz.

So on the ZX81, if you use software that ignores this hardware “register”, and which generates its own display (for example a high resolution display), it does not matter if R30 is fitted or not.

It would also be possible to hack the ROM code to fix it to work only as a 50Hz display, or a 60Hz display.

On the ZX80, there is no equivalent resistor, instead for use in the U.S.A. diode D11 should be fitted. Again, this only changes the value returned when the CPU reads the appropriate I/O port. The position of D11 on the PCB is between IC10 and IC9. Any silicon signal diode should do, so for example, use a 1N4148. Don’t use a normal rectifier diode.

It looks like the original ZX80 ROM code (the 4K ROM) will generate a 60Hz display, but I have not tried to fit a diode on my system so can’t confirm this. The “new” ROM (the 8K ROM) is the same as that used in ZX81’s, so that would support a 60Hz display system on a ZX80 machine.

Edited to update my comments after reading part of the disassembly of the ZX80 ROM code.

Mark

Re: ZX80 from NTSC to PAL?

Posted: Thu Jul 11, 2019 8:31 pm
by flaviosr
Hello,

if I understand well, if D11 is present, the ZX81 is ready for NTSC (USA) while, if D11 is NOT present, the ZX81 is ready for PAL (EU), right?
Only this mod?
D11 should be here, right:
Senza nome.jpg
Another thing is that probably a 4k ROM ZX80 could be able to handle a 60 Hz screen but this is not sure, right?
Anyway my ZX80 arrives from the US so it could solve this doubt! :)
I am going to open it and check if D11 is present or not (according to your message it should be!).

I also have to check if my TV can tune to NTSC, that would completely solve the issue... :)

Thank you

Re: ZX80 from NTSC to PAL?

Posted: Thu Jul 11, 2019 11:17 pm
by 1024MAK
flaviosr wrote:
Thu Jul 11, 2019 8:31 pm
if I understand well, if D11 is present, the ZX81 is ready for NTSC (USA) while, if D11 is NOT present, the ZX81 is ready for PAL (EU), right?
I presume you meant to say ZX80? But yes, if D11 is fitted, it is set up for use in the U.S.A., if D11 is not fitted, it is a U.K. / Europe model.
flaviosr wrote:
Thu Jul 11, 2019 8:31 pm
Only this mod?
D11 should be here, right:
Image
Yes, that is the position where a D11, a signal diode can be fitted.
flaviosr wrote:
Thu Jul 11, 2019 8:31 pm
Another thing is that probably a 4k ROM ZX80 could be able to handle a 60 Hz screen but this is not sure, right?
Yes. I’m not sure because I live in the U.K. and only have a single U.K. made ZX80 and have not tried changing it.
flaviosr wrote:
Thu Jul 11, 2019 8:31 pm
Anyway my ZX80 arrives from the US so it could solve this doubt! :)
I am going to open it and check if D11 is present or not (according to your message it should be!).
If the ZX80 you are getting was made for the U.K. market, or somewhere else that used 50 Hz TV, then D11 will not be fitted.
If however the ZX80 was made for the U.S.A. or another country where 60 Hz TV was used, then D11 should be already fitted.

Keep in mind that PAL and NTSC are colour encoding systems. They are added to the original monochrome TV signal. It is the standard of the underlying monochrome TV signal that is either 50 Hz or 60 Hz. I have to make this distinction because it is possible to have a 50 Hz TV using NTSC colour, or a 60 Hz TV signal using PAL colour.

Mark

Re: ZX80 from NTSC to PAL?

Posted: Sat Jul 20, 2019 9:00 pm
by flaviosr
Yesterday I received the ZX80!!! It is branded ZX80 USA so I suppose is prepared for 60 Hz.
I have only one doubt (not solved looking over internet...). The ZX80 is like ZX81, right? 9V central+ 0.65A, right?

Re: ZX80 from NTSC to PAL?

Posted: Sat Jul 20, 2019 9:42 pm
by Moggy
flaviosr wrote:
Sat Jul 20, 2019 9:00 pm
Yesterday I received the ZX80!!! It is branded ZX80 USA so I suppose is prepared for 60 Hz.
I have only one doubt (not solved looking over internet...). The ZX80 is like ZX81, right? 9V central+ 0.65A, right?
That's correct flaviosr I have both ZX80 and ZX81 supplies and can confirm the centre tip is positive. :D

Re: ZX80 from NTSC to PAL?

Posted: Sat Jul 20, 2019 9:43 pm
by 1024MAK
3.5mm mono “audio” jack plug (U.K. terminology) with the tip + positive, and the sleeve 0V/GND.
Required supply voltage 8V DC to 11V unregulated or 7.5V to 12V regulated. If the PSU does not say “regulated”, it’s an unregulated type.

A ZX81 or TS1000 PSU is okay to use.

Mark