Samsug are the best. Read the most. Most reliable. Thomsons are the worst. I've went through three. All have Samsung drives now. I purchsed a few things from The Lamma to beef up an xbox. I had a good experience.
I've repaired literally hundreds of these things. Thomson was the worst DVD drive by far, and Samsung was the best. Unfortunately, virtually all used Xboxes on the market right now have busted DVD drives. They were just built very poorly.
(As a side-note, I rarely find a 3DO system with a broken drive, despite the greater age of that system. This speaks volumes for how quality has changed over the years.)
Installing a new laser assembly isn't terribly difficult. But it *is* expensive compared to the cost of a new unit. If replacing the laser assembly on a Thomson, you will need a soldering iron. A new assembly will cost you about $20 or so after shipping. Considering the fact that you can buy a used unit from EB/GameStop for $40, it's just not worth paying 50% of the value on the unit to get a new laser assembly.
On the other hand, if you've made substantial modifications to the system, then maybe it is worth it.
But with that being said, it's pretty damn easy to mod an Xbox these days. With TSOP flashing techniques or software mods, it's pretty painless to modify a Xbox all on your own.
-RS.
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