Composite Mod help needed.

[PhilD]

So guys, I was about to carry out the composite mod on my new acquired ZX Spec, having researched what to do. However, I am a little confused what to do, as there is a single wire (transparent plastic) coming from a transistor to the RF modulator thingy, that is not described in the mod tutorials. Can anyone help with this? Many thanks.

[1024MAK]

That’s because someone has already done some kind of modification. Can you get some better photos? Also take the lid off the modulator and take a photo of the insides.

If using a camera, if it has a macro mode, use this.

Mark

[PhilD]

Hi Mark, hope this is clearer, thanks for your help. I have tried connecting to an old(ish) CRT telly with composite input but no joy even after a fine manual tune.

[1024MAK]

That’s not a standard modulator, normal U.K. production ZX Spectrum computers use a UM1233 E36 type modulator (UHF output, with no sound).

Can you please post up a photo of the label on the lid of the modulator please.

Also where in the world are you? And do you know where in the world this ZX Spectrum was originally sold? If not, which country did you buy it in/from?

Mark

[PhilD]

I am currently in New Zealand, the ZX in question was almost certainly sold here in NZ at the time (but not %100 on that). It was picked up at an estate clearance and has not been used for some time. Quick search suggests its NTSC, but not really sure, since I have little knowledge on these PC's. We use PAL here, so if correct might explain why I m not getting a signal, lol

[1024MAK]

I can’t find any information on the UM1622 modulator apart from a couple of comments where people think it may have been used in the the Acorn NTSC USA version of the BBC Micro.

TS1000 (USA version of the ZX81) uses a UM1082 LA 2/3 which had an additional input so that a switch could select channel 2 VHF or channel 3 VHF output.

However ASTEC made many different types of modulators, some did include a sound/audio input. On versions with a sound/audio input, that’s what the extra input wire could be for.

If you post details of the ULA markings (the 40 pin chip that is not the Z80 CPU) or just a photo of it, we can tell if it is a NTSC version.

It would also be helpful if you can post the PCB issue number (normally printed on the top front edge) and post a photo of the entire PCB.

Mark

[PhilD]

Does this help?

[Lardo Boffin]

My USA beeb has a UM1233 E36 modulator. It was supplied by Retroclinic so could have been changed. I can ask if it is of interest?

[bola_dor]

I am not sure what kind of video mod is done with your ZX but video output should be connected to the center pin output. At your picture it looks like wire is not soldered or its been cut.. or may be picture is not good enough..
The other wires to the modulator should be cutted.. leaving modulator without power.
It doesn't matter if you are PAL or NTSC .. you should see something any way with no colour and distorted but an image at least.. NTSC Spectrum are a rarity.. Some were made for Chile but not much.. if your country had PAL-x TV the easiest way should be use a different modulator according to your PAL variation as it was in my country but always keeping 50Hz ..
In short.. make sure video out is connected to the center pin. That modulator has no power. And that the tin case has good GND continuity.

[bola_dor]

In issue 2 video comes from the transistor originally.. the resistor seems to be the mod there. Can you measure its value? Or remove it and use instead a 100uF electrolytic capacitor (-) lead to the video output..