View Full Version : ZX81 video modes help

August 11th, 2009, 07:39 PM
When you build a Sinclair ZX81, you get the option to set the tuner to output either an NTSC or PAL standard signal by soldering two/three different wires into one of three sets of three holes on the PCB. For some odd reason I will never know, my uncle set his ZX81 to PAL which requires only two of the wires to be soldered and then he clipped off the third so you can't set it to NTSC.
I could switch it back to NTSC if I added the wire again but looking inside the tuner box I don't see where the wire specifically goes to.

Anyone know what to do?
I finally found a cassette of games that will load in the 1K of onboard ram. All the other cassette software I own requires I have the 16K memory module and I don't have that (but if you want to part with a Sinclair branded 16K cartridge, please PM me ;) )

August 11th, 2009, 10:26 PM
I don't know about the ZX-81, but the spare hole in the RF modulator is what me and other people use to route a composite video signal on the ZX Spectrum, effectively bypassing the RF circuit. More or less the same RF modulator assembly seems to be present in most home computers and video games, although incoming signal strength may vary.

August 12th, 2009, 01:22 AM
I'll dig one out and check the wiring inside the modulator, unless someone beats me to it.

You may also need a few extra components (diode, resistors) to get it running in NTSC (the "USA only" parts labeled here (http://www.zx81kit.com/circ1.jpg)).

August 12th, 2009, 06:49 AM
Need more parts?
Hmm, I read through the owners manual and it didn't say anything about additional parts being needed.

August 12th, 2009, 10:45 AM
I'll have to dig through the schematics... I thought for switching between PAL and NTSC there was a wire you hooked to either ground or +5v.

I'll check sometime tonight if I get a chance.

August 12th, 2009, 11:28 AM
Had a look inside another ZX81 here and it looks almost identical to yours. The only difference is that the middle lead of the modulator pokes back up through the right-most of your two red circles on that second photo.

However, I also found a copied set of the assembly instructions for the US version. On the very last page was:

Sinclair UHF KITS Card Enclosure
for 60Hz U.S.A. video

This package contains an extra component necessary for the assembly of this kit. It is R30, a 0.5 in. jumper wire. Insert this component at the position marked R30 on the PCB. Be very careful in the placement of this jumper. There are three holes to the left of the box marked R30. Insert the jumper into the middle one of these. This jumper selects 60Hz video as opposed to the normal U.K. 50Hz.

Insert the UHF modulator following the instructions given in the Assembly Instructions leaflet. Be sure to insert the two wire leads into the holes marked "Fr/UK1" and "Fr/UK2". These are in effect the UHF locations for the modulator. (Disregard the USA openings on the PCB.)


So, long story short: ground pin 22 of the ULA to enable NTSC video. The other "USA only" components on the schematic are apparently not required; they are marked "Not used" even in the US version of the assembly instructions.

August 12th, 2009, 03:56 PM
So just locate R30 and on whatever side of the resistor I prefer, just ground it?
Sorry, I'm looking at this PCB and I'm totally confused.

August 12th, 2009, 11:45 PM
No, R30 is the jumper - you need to fit it. The position is marked with a silkscreen box, between the ULA and the voltage regulator. As the instructions say, make sure you get the correct hole on the left:


August 13th, 2009, 07:00 AM
There's already a ten ohm resistor (brown, black, black gold) right there. :?

August 13th, 2009, 07:30 AM
Then it would appear it's already set to NTSC. According to the manual, that jumper is the only difference and 10 Ohms should be close enough to zero to work. The "USA" markings on the PCB are a red herring. I suppose you could measure the actual resistance from pin 22 of the ULA to ground and replace R30 if necessary, but I really doubt that's your problem.

Going back to square one: what makes you think it's not set to NTSC? Be aware that modern TVs with digital tuners can have a hard time tuning to the ZX81's signal. An old TV with analog fine-tuning is best. Alternatively, you could look up how to get composite video out of it and bypass the modulator entirely.

August 13th, 2009, 09:02 PM
I'm assuming it's in PAL mode as A: It's Sinclair and this unit is normally known as the Timex 1000 on this side of the Atlantic and short of a different pattern of static, I can't get any thing on the tv (and yes I'm tuned to channel fifty something).
I have tried my mom's trusty old Electrohome (http://i11.photobucket.com/albums/a166/ballsandy/p5242750.jpg) and I got nothing at all short of above mentioned different pattern of static when the thing was plugged in.
If there is a mod to give me direct composite, I guess it would be an interesting mod but I need to know what to do and what I need.

August 17th, 2009, 07:20 AM
Anything? I really wanna get this thing running.

August 26th, 2009, 04:23 PM
Picked up a NOS 16K menory upgrade but it's useless unless I can get pointers on how to get video output on this computer.
Please, someone help!

September 28th, 2009, 03:34 AM

This took about 10 seconds of Googling.


November 6th, 2009, 05:57 AM
Yes, this is very simple as in the link above. I've actually wired up directly to the ULA's video output. The transistor mod would be nice as the output is a little dark straight through though.


Changing the scan rate isn't strictly necessary sometimes. Some TVs can sync on the 50Hz signal, and some older composite monitors will as well, after fiddling with the V-hold control.