Disclaimer: I in no way take responsibility for you doing something stupid like electrocuting yourself, your dog, your cat or your grandmother. I also don't take responsibility for you burning your house down. This is a simple mod and can be done by anyone who has a little bit of experience with a soldering iron, and can follow directions. DON'T HURT YOURSELF!
Now that's out of the way, on to the fun stuff. I want to thank Saltfish35 for the inspiration for this mod. Without him poking around inside the ballast, we wouldn't have a fix for this. Again, thanks Saltfish35.
The Odyssea 250W ballast is our victim today. Many people know that these ballast underdrive the bulb considerably, and as a result affect the total light output. The low cost of these fixtures attracts a lot of people, but having to upgrade the ballasts later is a hassle just to get it to do what it should have done in the first place.
I would suggest leaving the ballast unplugged for a few hours to let the capacitors inside discharge. They are large and can give you a very nasty shock if you short the outputs. Luckily, we don't mess with them during this mod, so just watch where you start putting screwdrivers/soldering irons.
First thing, remove the four screws holding the end cap on that looks like a fan grill, ie. not the end with the cords. Remove the cover and put it aside.
Next, remove the four screws holding the end cap with the cables. This end doesn't come off, but just gently pull it out.
Slide the top cover off and put it aside.
Pay attention to the corner on the end with the cords. There is a ground wire that grounds the case that is pinched between the case body and the end cap. Be careful with this, as it can be delicate and needs to be put back into place when you reassemble the ballast.
Next, remove the screw holding this block in place. Keep an eye out for washers under the nut. You don't want this falling onto the pcb and shorting something out. This ballast didn't have one, but the last one I modded did. Your mileage may vary.
There is a white glue holding the pcb in place. Sometimes they put it on both ends. Just slide a knife down to cut most of it. A good push will unseat the rest of it.
When you take the pcb out of the case, remove the insulating layer (cardboard) from underneath and put it aside. Don't forget to put this back in when you reassemble.
The two heatinks on the end are what we will be dealing with. They are attached to two MOSFETS that are causing the trouble.
If you turn the pcb over, you will see the two groups of three large solder points for the MOSFETS towards the end.
Other than the obvious soldering iron and solder, you will need solder-wick. This can be obtained from Radioshack or online if you don't have one close. It is a copper braid with flux in it that will pull the solder away when you heat it over a solder joint. This will make life much easier. A solder sucker (vacuum pump) will also work if you have one.
For those of you that haven't used solder-wick before, just lay it on top of the solder joint, and apply the soldering iron. Careful though as the solder wick will get hot quickly. Once the braid is full of solder, move up the braid a little to a fresh spot until all the solder is gone from the joint.
Once all the solder is removed, you should just be able to gently wiggle the heatsink to pull it out. If it does not move, go back again with the solder-wick.
Hey presto, they're out!
Remove the screw holding the MOSFET onto the heatsink. You will notice that there is no thermal paste between the two parts. This is not good.
These are the new parts. They are Fairchild FDH44N50 MOSFETS. They can be found at Mouser Electronics for $5.59 each, plus a little for shipping. You will need two per ballast. They work the same as the original parts, but won't limit the current at the same temperatures like the old parts did.
I highly recommend using a thermal paste when you put the new parts on the heatsinks. I like using Arctic Silver, but any thermal compound from Radioshack will work. This stuff will stain your clothes, so be careful.
You only need to apply a thin coat, but cover all the metal on the back side of the MOSFET. Here you can see a before and after.
Screw the MOSFETS back on the heatsinks, metal side down, plastic side up.
Inset the pins through the holes, making sure that the heatsink has the fins facing the end of the pcb. Getting this backwards will almost certainly destroy the part and could set something on fire in the right circumstances.
Reapply solder to the pins. Make sure you have a good solder joint. It should be shiny when cool. If it is dull, you need to redo the solder joint (needs more heat).
Once everything is soldered, clip the pins as short as possible, but don't cut into the solder.
It should end up looking like this
After that, reassemble everything in the reverse order you took it apart. Remember to put the insulating layer back under the pcb, and put the ground wire back around the front of the case. If the pcb is loose in the case, a little silicone can help to hold it still. You don't want it moving around.
Once you are sure that everything is put back together correctly, go ahead and fire it back up. I found that the ballast now pulls an average of 247W measured on a Kill-a-Watt meter. It still takes a lot longer to warm up the bulb than an Icecap ballast, but produces the same lux on the same bulb.
I hope that some of you find this useful and we can finally get the most out of an inexpensive, but fairly well built fixture. Please ask any questions if anything I have described is unclear.
If you need a soldering tutorial, there is a decent one here. There are many more on the net if you need. If you are new to soldering, I suggest you buy some perf board and some resistors from Radioshack to practice on. It's not hard.