NEWBIE - Composite Mod planned, and related question

[USAJoe]

I'm planning to do a composite mod on my Timex Sinclair 1000 by using a "TS1000 and ZX81 Composite Video Adaptor Kit" I purchased via ETSY. I'm waiting for it to be delivered. In the meantime, I have been viewing some composite mod videos on YouTube. One that I found particularly interesting is this one https://www.youtube.com/watch?v=7SFuLd1Tjy4 titled "How to add composite out to a Sinclair ZX81". I like this video because the person in the video connects the new composite mod board to the existing connections in the Sinclair, thus preserving the RF video output while also adding the ability to use composite by merely changing what type of monitor/TV is connected. This is something that I would like to do as well.

My Sinclair unit has a modulator that has three solid wires coming out through the three exit holes. Those wires (ground, power, and video) are soldered directly onto connections on the board just below each corresponding exit hole. There are no components between the ULA chip and the modulator, so I'm thankful for that. For reference, My ULA is a 2C184E.

In the video I provided a link to, the user connects his mod board (same kind I ordered) to the unit in a variety of ways. One being directly onto the back of the motherboard, and another to the modulator and other parts of the system. He finally solders all of the connections and places the mod board on the exterior top of the modulator housing via double sided tape.

So here's my question: After I solder 5 wires to the mod board (2 ground, 2 video, and 1 power), can I use wires with alligator clips as temporary jumpers to connect the wires from the mod board to the existing three solid wires (ground, power, video), the existing RCA connector (video), and the remaining ground to the modulator chassis to test the mod board while not disconnecting any existing RF connections? It's just a thought right now, because the clips might end up being too close to each other and might touch (which I don't want to happen).

I'm practicing my soldering before I attempt anything. I won't move forward until I feel very confident in my soldering skills.

Joe

[1024MAK]

For the benefit of others reading this, the video board that is being discussed is a ZX8-CCB.

My Sinclair unit has a modulator that has three solid wires coming out through the three exit holes. Those wires (ground, power, and video) are soldered directly onto connections on the board just below each corresponding exit hole.

Small correction here. A TS1000 has a modulator that can output on one of two different frequencies (or TV channels). The switch is mounted on the main board. Hence the three wires going into the VHF modulator are switch input (can be 0V/GND/ground or +5V/power to select the different frequencies), +5V/power and the video input signal. The connection to 0V/GND/ground is via the solder connections that hold the modulator to the board.

In the video I provided a link to, the user connects his mod board (same kind I ordered) to the unit in a variety of ways. One being directly onto the back of the motherboard, and another to the modulator and other parts of the system. He finally solders all of the connections and places the mod board on the exterior top of the modulator housing via double sided tape.

So here's my question: After I solder 5 wires to the mod board (2 ground, 2 video, and 1 power), can I use wires with alligator clips as temporary jumpers to connect the wires from the mod board to the existing three solid wires (ground, power, video), the existing RCA connector (video), and the remaining ground to the modulator chassis to test the mod board while not disconnecting any existing RF connections? It's just a thought right now, because the clips might end up being too close to each other and might touch (which I don't want to happen).

Taking into account my comments above, yes, you can use alligator clips (we call them croc clips, short for crocodile). I do recommend small ones though. I myself use miniature probes that hook on to wires or component wires.

To attach the black/0V/GND/ground to the modulator, you can either try to get it to hold on to the “fingers” of the lid of the modulator. Or pop the lid off and then clip to the case of the modulator. Or cheat and clip to the metal heatsink of the 7805 voltage regulator, or to one of the “earthing”/‘grounding” spring metal that press against the case.

I’ll post some photos later…

Mark

[Lardo Boffin]

Interesting video thanks Joe!

I didn’t know it was possible to have both RF and composite video from the same socket using a ZX8-CCB.

Does anyone know if doing this is likely to cause any issues?

[Paul]

Does anyone know if doing this is likely to cause any issues?

It is recommended to desolder the resistor at the RF OUT and reconnect it via a 33pF ceramic capacitor for decoupleing purpose.
This way both RF and video will be fine.

[Lardo Boffin]

Does anyone know if doing this is likely to cause any issues?

It is recommended to desolder the resistor at the RF OUT and reconnect it via a 33pF ceramic capacitor for decoupleing purpose.
This way both RF and video will be fine.

Thanks Paul.

I assume you mean this resistor?

[1024MAK]

I assume you mean this resistor?

That’s the one, yes.

Mark

[1024MAK]

Here’s an old photo showing my “test” ZX8-CCB connected to a ZX80:

(From this post)

ZX8-CCB connection points for a TS1000 or USA ZX81:

ZX8-CCB connection points for a TS1000 or USA ZX81

(Click the photo for a bigger image)

In a TS1000 or USA ZX81, the video output from the ULA does not directly go to the modulator video input. Instead it passes through diode D9 and a resistor. Hence it’s better to pick the signal up at one end of diode D9 as shown.

Mark

[Lardo Boffin]

I assume you mean this resistor?

That’s the one, yes.

Mark

Thanks Mark.

[USAJoe]

Thanks to all that replied! I really appreciate it.

Here are some photos of my 1000. As you can see it's different from the photos posted here. You can see that there's no components between the modulator and the ULA chip. The RCA connector is also at the opposite end, and the modulator components look a little different (at least to me).

Joe

[1024MAK]

The photo of the modulator connections is that of a Sinclair ZX81 issue 3 board, as used in both U.K./European ZX81 computers and some Timex TS1000 computers.

There are also ZX81 issue one boards, used in U.K./European ZX81 computers. It’s possible that some may have made it to America in Sinclair ZX81 machines.

No one (that we know of) has ever seen an issue two board.

And there are Sinclair/Timex TS1000 issue boards marked “Sinclair ZX81 USA Issue One” which are noticeably different to the boards used in the U.K./European ZX81 computers. Here is an image of of one of these:


Is this board like yours (ignore the modifications that have been made since it was originally manufactured).

Mark