Timex Sinclair 1000 Part 1: Multi Region Composite Mod

I recently picked up a Timex Sinclair 1000. I first tried connecting it to my LCD TV which does have an analog tuner, but I couldn’t get the signal on it. I then dug out my VCR from the storage area and connected to that. That worked just fine. I am not sure why the TV wouldn’t take it directly. The signal looked pretty good too. I really didn’t care to use the 1000 on the TV all the time though, and not with the VCR all the time either. I went on and looked into a Composite Video modification for it. The modification is done with the same little transistor circuit as the Atari 2600, which I have done before. This is only when the system has the later ULA 2C210E chip such as mine, the earlier ULA 2C184E does not generate quite a standard video signal, so that requires a different circuit. I don’t know if any of the Timex models have the early ULA chip or not. The Timex Sinclair 1000 is a rebranded Sinclair ZX81. The RF Modulator is different for the US Timex model (and the US ZX81 which they did sell some of), there are a few other differences. The Timex 1000 came with 2k of ram rather than 1k like the ZX81 came with and outputs to 60hz NTSC video while the UK model does 50hz PAL video output.

I used Tynemouth Software instructions on the Composite Mod. Found here: http://blog.tynemouthsoftware.co.uk/2016/11/ts1000-multiregion-composite-video-mod.html

I used the stripboard circuit from TheFutureWas8bit Atari 2600 mod:


I also used info from GadgetUK164’s video: https://youtu.be/5OQuJ-GMwF4

Although in his video he had the OLD ULA chip, and the simple Transistor circuit didn’t work properly for him, he went on to make the more complex circuit later on. His initial bit was using the transistor circuit such as I ended up with.

I found good schematics for the ZX81 here with other useful info. This is where I ended up getting information as to what the various extra Modulator connection labels were.


I would post the schematics and other bits from the sources, but that is their material, this is how I used their material, you would need to build the transistor circuit from TFW8bit, or you can buy the board they sell which would simplify matters.

I went with a little different approach than I initially intended. I was going to leave the RF Modulator in but disconnected, that didn’t work out. There wasn’t much room, and I couldn’t remove the capacitor and resistor(?) and wire from the RCA jack easily, and I couldn’t get the strip board to fit reasonably with the Modulator still in the box. In the end I removed the internals of the RF Modulator and installed the new circuit inside.

I want to note that with the ZX81 and the US Timex 1000 (and I guess the ZX81 US kit), the pins used for the RF Modulator are different. It uses the “USA” marked pins, it uses 3 of them.

For the US model, it uses 3 wires instead of 2 for the UK versions. USA3 (Not FR3) is the far left wire, that is apparently the Video in signal although I don’t know where it comes from exactly, as that is not the direct pin from the ULA video output. The next USA2 is actually +5Volts. Then USA1 is the last and it goes down to the Channel 2/3 switch. That switch actually switches between Ground and +5Volts ( I use that later as Tynemouth did).

For the Composite signals, I actually removed the RF Modulator board etc:

Modulator bits, the board, with the 3 wires, and the Capacitor and resistor, and the output wire on the right.

Here is the board installed, it is built as TFW8bit’s stripboard, although on a larger piece of board so it would stay solidly in the modulator box.

Composite Video circuit.

Above you can see the wires used. Originally the modulator of course used USA3, USA2, and USA1. In this case I am only using USA2, which is now the third wire not the middle one going in. The first is going to FR3, which is actually Ground, yes the modulator chassis is ground as well, but I didn’t care to solder to it, and had the opening for the third wire. The middle wire goes to UK2 which is the video directly from the ULA chip pin, and not the round about way that USA3 apparently gets it.

First I tested this out and it worked great. Here it is via the Composite input on my little LCD monitor I keep on my bench.


There is a second modification though. The channel switch is no longer needed, and it can be re-purposed for something else. Tynemouth used it as a NTSC/PAL switch, which is a simple change. This involves R30, which goes to the one pin on the ULA, and to ground. It is a 10 ohm resistor that when pulling that pin to ground causes the system to operate in NTSC 60hz video mode. When it is disconnected from ground the ULA sets the system to run in PAL 50hz mode. To do this I reused “USA1” which goes to the switch center pin already.

USA1 ready to use.
The Grounded side of R30 lifted from the board.

Next the Switch needs a bit of a change. As I mentioned earlier the switch changes USA1 from Ground and +5Volts. We don’t want to wire R30 to +5Volts. To do this you could remove pad from the circuit board, or you could trim the wire from the switch and ensure it was insulated. I didn’t want to alter the board, so I altered the switch.

Next I installed the switch and tested it. Be aware, I made a mistake installing the switch..

So, some may notice I installed the switch to the wrong side of the board. So this won’t work in the case now. I realized this shortly after putting all of the tools away, but before I was going to reinstall the board.. So I desoldered the switch again and installed it to the bottom of the board like it should be. When I put the switch on, I reinstalled the old paper label they used to insulate it, and to beef it up a bit, and cover the hole from the old pin that went into the +5Volt pad, I put a little piece of Kapton Tape, you can’t see the tape in the photos though as it is under the original paper label.

Switch installed properly..

I then retested it. I have another bit I am looking to do with this board and that is to put in a 16k ram upgrade on board based on Tynemouth’s post on it, and that again GadgetUK has done a video on. It really is not going to be anything new, while the Composite mod, I did a little differently.

I haven’t tested the keyboard on this unit, I did order in a replacement keyboard for it, and if I don’t need it I will keep the original on it for now. I won’t be reassembling the system though until the ram upgrade is finished. The computer did come with the 16k ram expansion, but the expansion doesn’t work. I had looked at repairing it, and the one capacitor was bad on it, but replacing that did not correct the problem. They are rather complex boards, and while I wouldn’t mind repairing it, I am not sure where to start. I thought of stripping out the 74 logic chips to test and such, but that is about all I could do. The internal 16k ram upgrade is quite easy though with the right ram chip and a few wires. I don’t have to damage the board or anything, the Timex came with the socket already installed with the 2k ram chip in it. I just have to pull that and do the few wires onto the new chip.


4 thoughts on “Timex Sinclair 1000 Part 1: Multi Region Composite Mod

  1. This is really awesome. I’d love to do this mod, but worry my attempting to learn on the device could do more harm than good. Would you ever consider doing this mod as a service for someone? I have a nice working S1000, but while the RF works fine, it would be so much cleaner if it were a composite out. I’m located just south of Boston.


    1. It has been awhile, I don’t check in often if I am not posting a new project. I haven’t done work for someone else. I could but there is always risk shipping equipment. The S1000/Z81 keyboard membranes are old, opening it and flexing it around may cause it to fail. They do make replacement keyboards, but they will be the Z81 button labels not the slightly different S1000 ones if you need one at some point. Before I worked on mine, I purchased a replacement incase it ended up damaged. It still worked fine afterward though. If you just want to do the RF to Composite Mod, they make little Composite mod boards typically used on systems such as the Atari 2600 that can be used in place of the homemade board I used that would make it easier for someone starting out.


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