Sinclair ZX80 / ZX81 / Z88 Forums

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

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

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: 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

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:

Yes, that is the position where a D11, a signal diode can be fitted.

flaviosr wrote: ↑Thu Jul 11, 2019 8:31 pmAnother 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

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?

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.

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

Ok. Thank you. The ZX80 is working.
Sadly I remembered badly and my TV does not sync @ 60 Hz... but I have been able to see something.
The most important thing is that the ZX80 seems to works and all keys are ok ("obviously" I have not been able to run anything). I could modify the ZX80... but it is branded ZX80 USA: and I want it original. Probably if it were only branded ZX80 only I would have modified it but not in this case.

Having discovered that B&W is only a matter of frequency and not of PAL/NTSC & friends... I hope I can find a small gadget that can simply convert the picture from 60 Hz to 50 Hz! I am very optimistic...

It's probably not even interlaced so at 50Hz display you're only looking at ~312 lines progressive scan.

Like a lot of 1980s computers, the ZX80 just repeats the same (non-standard) field over and over again at 50Hz or 60Hz. So no, it’s not interlaced. It is actually 50 or 60 frames a second.

Mark