Finally, Watching TV !!!

Works!  Here we are watching TV in the back seat of a Range Rover using the hacked DTV converter box running off the 12V car battery.   I'm only using a paper clip for an antenna and I still get lots of channels.  I recommend getting a real antenna though.

Setting up for a test

Now for the proof of concept test.  This is not meant to be a clean install.  We're just checking to see if it will work.  I simply plugged into the AV input of the rear seat entertainment.  The Range Rover has both the 12V plug and the AV inputs conveniently close to each other:

Finished box

With the adaptor cable in place and secured by the original grommet you can put back all the screws an install the cover.  Finished modified unit photos below:

Almost looks like it came from the factory this way!

Modifying the 12V Adapter

Now it's time to modify the small end of the 12V adaptor.   Making sure the adaptor is not connected to anything electrical (like the car), simply cut off the small end and remove the insulation as shown below.  I put on some shrink tubing for later.

The tip of the large end of the cigarette lighter is 12V and the contacts on the side are ground.  Simply solder the wire going to the tip of the cigarette lighter plug to the two center pins of the 4-pin header and the other wire to the outer pins.  I can't recall how to identify the two wires from the markings alone.  One has a solid gray line and the other has a dashed gray line.  You'll just have to buzz it out with a voltmeter to find out.

When you're done soldering the wires and covering it with some electrical tape or shrink tubing you can connect it to the board.  It will look like:

Make sure that nothing's shorted by checking the impedance (ohms) between the tip and side contacts of the cigarette lighter end of the adapter.  If it reads close to zero ohms you've got a problem.

You'll definitely want to secure the cable so it doesn't get yanked out.  I decided to reuse the grommet that was used to secure the original power cord.  You can use a screwdriver to unlatch the grommet from the original power cord.  There are two pieces that sandwich the cord that will come undone when you pry it with a screwdriver.  When it comes apart it will look like:

The 12V adaptor cable is much thinner than the original 120V AC power cord.  In order to have the cable held securely in place by the grommet you'll want to wrap it with some material to give it a larger diameter.  You can use some electrical tape or some of the shrink wrap tubing material.  Once the grommet is snapped into place over the thickened cable, it can be reinserted into the original opening on the back of the unit:

I decided to forgo the extra credit assignment and left the cut away notch open.  For a cleaner setup you can come up with a way to fill it.  

Gathering the parts

The parts you'll need to modify the converter box include:

• 4-pin header (standard 100-mil spaced)
• 12V cigarette lighter jack with cable
• various sizes of heat-shrink tubing

The 4-pin header should be bent so the outer two pins are touching and the inner two pins are touching as shown.  

For a 12V cigarette lighter jack I chose the MPD ZA5077 available from Digi-Key for $3.74/each.  The Digi-Key part number is ZA5077-ND.  

Removing the power supply

Removing the power supply is quite easy.  First, be sure the unit is unplugged.  Both the motherboard and the power supply have about four screws holding them to the plastic chassis.  Once these screws are removed, the boards can be lifted and separated from each other. 

The power cord going to the power supply can be removed by squeezing the white tab the locks the connector in place.  Here's what it looks like when you're done taking it apart.  

Now we're ready to remove the power cord.  While there may be a more elegant way of doing this, I chose to take a more brute force approach.  I simply took a pair of wire cutters and cut a notch in the plastic chassis to remove the power cord without destroying the cord.  The image below shows the notch having been cut out.

With the notch in place simply slide out the power cord and keep it in case you want to undo the modifications.

The Power Connector

The next step was to figure out the power connector pinout.  Using a voltmeter, it the connector pinout quickly became apparent.  Nothing could be easier.  Power is delivered using a standard 100-mil 4-pin header.  The outer two pins are ground and the center two pins carry 12V DC as shown below.