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Little Dot MKII keeps blowing fuse

  1. jrnb123
    My friend recently purchased a LD MKII from the recent massdrop. However, he purchased the 220v version instead. After researching it, we learned its possible to wire the primary windings of the transformer in parallel in order to accept 110v. After doing this the blue LED would light up upon flipping the power switch but then after a few seconds the fuse at the AC line blows. We took the unit apart to make sure everything was grounded properly and then put it back together. When doing this, we broke a wire off the 2W10 rectifier on the board (we did not know this at the time). The amp no longer blew a fuse, but the sound was very distorted and quiet (as expected from an issue with the rectifier). After reattaching the wire, the fuse continues to blow. The fuse also blows without any tubes attached, and we tried using slow blow fuses as well. Any help is appreciated. Thanks!
  2. jrnb123
    Not sure if I'm allowed to bump this. I've tried posting for help on this on like 4 different forums and no one is responding :/
  3. tomb
    If it was set up for 220V, then the fuse at 110V needs to have twice the rating. Power draw from the amp is the same, regardless of voltage. However at the lower line voltage, it will draw twice the current. If you are using the same rated fuse that was used at 220V, then it will probably blow because it's exposed to twice the current. I can't speak for the Little Dot, but a 250ma slo-blo would be fine for many single/dual tube headphone amps at 110V line voltage (at 220V line voltage, the same fuse should be 125ma).
  4. jrnb123
    Ahhh that was a theory of mine but was kinda spooked to put a higher rated fuse in. I will try that thank you! The fuse that came with it was a 1A fast blow. I will try a 2A.
  5. jrnb123
    I tried the 2A fuse, it sounded great for like 5-7 minutes and was the first time I actually got it working and then the fuse blew. The transformer is decently hot and theres a sort of burning smell as well unfortunately :/. New fuses do nothing, no more LED. Chances I blew the transformer?
  6. tomb
    Maybe not, but the result of all of this proves that you somehow wired it incorrectly. Go over what you thought you found with regard to wiring it for 110V. Double check all of that information, even differences among model types. Something is obviously wrong. If you can figure out the mistake, you may be able to get it working again. Unless the windings are obviously burned or melted, just smelling a burn doesn't necessarily mean it's fried.

    One thing you can do is disconnect the transformer outputs from the amp and measure them. The best way is to use some sort of clamping probes and an AC-RMS-voltage capable DMM. Give the transformer power, note the reading on the DMM, then cut it off. That way, you are measuring it without letting it heat up and possibly burn again. If you're not getting 110V, you have it wired incorrectly (or it's really not capable of wiring in this fashion). The clamping probes are also for safety. You don't want to be anywhere close to touching anything when you give it power.

    Since you've gone this far, you can always find another transformer that meets the ratings at 110V and replace it yourself, if the existing one turns out to be burned up. Just be safe - this voltage can kill you.

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