Here follows a walkthrough by a fellow aquarist.
Firstly, inside the light bar there is only a bunch of wires and a ballast. There are 8 wires from the 4 endcaps and a live and neutral. The ballast is long white metal box. If you have checked the tubes/switch/and plug fuse then your problem lays with this ballast.

Right, before you start you need to source a replacment ballast. I checked out eBay and found there was a ready supply of just about every sort. As there are no seperate starters in the modern Juwel light bar then you need an electronic or digital balast. Mine, for instance needed a 2 x 18w T8 ballast (being there are 2 tube at 18w and it's a T8 type light)
You will also have to source a piece of plastic slightly bigger than the piece you are going to cut out of the light bar

After making sure it's not pluged in! remove the tubes. You need to have a cutting tool. I used a small diamond cutter. A dremmel is a good tool too. I decided to cut in from the underneth side of the light bar so the repair would be hidden.

1st picture shows a cut out I made. (Do not cut too deep as you may cut wires, the plastic is onlly 1-2mm thick) Also, do not cut right to the edge as you will need a flange to stick the cover on later.



Remove the cut out to reveal the ballast



Next pull out the ballast, you may need to spread the light bar to allow it to come out



Then you need to disconnect the wires. I marked them with tape and corresponding numbers. To disconnect the wire press down on the little tags and pull on the wire gently. The live and neutral are at the other end.



Here's the clever part. Cut a small section from the removed piece of casing that the ballast holding screw goes into.



With these pieces screw them to the new ballast (for spacing then glue them to the new plastic 'patch' which has been made slightly bigger than the cut out. When dry remove the ballast from the patch leaving the mounting piece in place



Right now the wiring up. With the new ballast I got there were only 6 terminals but I had 8 wires from the tubes. If this happens to you then study the little diagram and it will show you that 2 of the wires need to be connected together.(see connector in picture) If your new ballast is the same as the old one then wire up using the numbers that you stuck on. The live and neutral go up the other end and unless marked you can put them either way around)
It would be a good idea at this stage to test the lights!



Next, fix the ballast to the plastic using the screws and your mounts that you stuck on earlier.



Now turn the assembly over and jiggle it into the light bar. You may have to spread it a bit. Now then, I decided not to glue it in place incase I ever had to remove it again. I opted to silicone it in place and added screw (stainless screws) for safety.




And finally the finished artical in working condition after being switch on and off several times then left on for 8 hours and again switch on and off several times......success :hyper:



Well I hope this helps someone but remember, I AM NOT AN ELECTRICIAN AND THEREFORE AM ONLY SHARING MY EXPERIENCES WITH YOU. IF YOU CHOSE TO COPY THEM THEN I TAKE NO RESPONSABILITY WHATSOEVER.