Question about 48K composite mod

[akeley]

Hi guys, could you help me out here? I just wonder if this machine has or hasn't the composite mod done?

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If these pics aren't good enough I can try to take better ones.

[1024MAK]

Yes, it has definitely been modified. The (phono) output connector on the modulator (what was the UHF TV output) will now output a composite video signal.

I can't tell which version (there are more than four...) of the composite video modification has been done. If you want to know, you need to carefully prise/lever the lid off the modulator with a flat bladed screwdriver and take a photo of the inside of it.

Mark

[akeley]

Thanks for the speedy (7 min!) reply @1024MAK.

Reason I'm asking is that I took one of my 48k machines to another side of the world, where I mostly live, and am trying to connect it to a PVM monitor (via Composite) - and it's not working. So I thought maybe I took the RF model by mistake.

[akeley]

Thinking about it now, could it be the socket voltage problem? I'm in Taiwan, which has 110V – 120V / 60Hz standard. Do I need some sort of step-up converter? And, have I possibly damaged my ZX by plugging it in? (this was done using just a simple travel plug adapter).

This seems blindingly obvious now and I'm kicking myslef for not checking it out earlier. I guess I was fooled by the fact my laptop and assorted other chargers work with just the adapter.

[1024MAK]

I assume you were using an original Sinclair power supply unit (model UK1400 9V 1.4A). This is not a universal mains voltage type, as it requires a nominal 240V AC 50Hz mains input voltage. It will not produce enough output voltage if connected to a 110V – 120V 60Hz mains supply. The good news is that it is unlikely that you have damaged it or the Spectrum.

Note, if you have a Sinclair power supply unit designed for the European market (model Euro 1400 or EU1400 9V 1.4A) its mains input voltage is a nominal 220V AC 50Hz, hence would have the same problem.

So, how to solve the problem.

There are two answers.

1) Yes, a step up transformer can be used. The rated input (primary) voltage needs to match your local mains supply. The output (secondary) voltage needs to be 230V or 240V. The power rating should be 25W (25VA) or greater. The slight difference in frequency (60Hz rather than 50Hz) is normally not a problem, as the computer does not use this for any timing functions.

2) Obtain a modern DC power supply unit (PSU) suited to your local mains supply. The output voltage needs to be 8V DC or 9V DC or a voltage between these values. The maximum output current rating should be between 1.4A and 3A. If you are only going to be using the ZX Spectrum (16K/48K/+ models only) on its own without any expansions, a power supply unit with a 1A rated output will work.
Now the most important point, you HAVE to ensure that the polarity of the 2.1mm/5.5mm "barrel" plug is correct. Otherwise you will damage the ZX Spectrum. The outer contact has to be the positive (+) and the inner contact has to be the negative (-). Today, this polarity for 2.1mm/5.5mm "barrel" plugs is rather uncommon, the other polarity is far more common. If you can't find a suitable PSU with the correct polarity, someone who can solder can cut off the existing plug and fit a new plug with the correct polarity.

The correct polarity is centre negative as per this diagram:


If you have a multimeter, it’s simple to test for the correct polarity.

This shows the correct polarity for a ZX Spectrum:
(click for a larger picture)
The red croc clip and lead is positive, the black test probe and lead is negative. Hence NO minus sign showing on the display of the multimeter

The voltage shown here is the ‘no load’ voltage from a Sinclair 9V power supply unit (PSU). As these are unregulated, the ‘no load’ voltage is higher than the nominal on load voltage. When connected to the ZX Spectrum the voltage will fall to between 9V and 10.8V.

With more modern regulated PSUs or power adapters it’s good practice to use one set to, or rated at 8V to 9V. 9V is a commonly available output voltage and the heatsink in the computer will run a bit cooler than when running from an original Sinclair PSU.

Mark

[akeley]

Thank you for the as always exhaustive reply. I will try to source a charger with the parameters you describe locally.