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Amp works, but heats up a lot under no load. Any diagnosis suggestions, please?

Discussion in 'Amps, Mics & Pickups [DB]' started by swingingoodtime, Oct 5, 2013.

  1. swingingoodtime

    swingingoodtime Supporting Member

    Mar 19, 2009
    Sydney, Australia
    Hi all,

    I'm currently using a Samson D412 powered PA speaker as my bass amp; sounds good and allows me to sing through it at the same time without having to lug an extra box. It's been working fine for the last couple of months, but yesterday it cut out on me a couple of times. I noticed that the rear metal panel was blazingly hot; you could have literally burnt yourself badly if you held your hand on it for several seconds. Even the metal flying mount insert molded into the top of the unit was very, very hot! It was a hot day, but we were under cover and weren't working the unit particularly hard at all. Each time, it came back on after about 30s, so I'm assuming that it was the thermal protection kicking in. It worked fine at my evening gig, but still ran super hot (no cutout, though; I'm assuming that this was because it the ambient temperature was a fair bit cooler.

    I pulled the amp unit out today and started it up with no load and the (internal) heat sink became almost too hot to hold within 15mins. The power supply looks to be a standard transformer/cap/rectifier set up (ie, not SMPS) and the tranny was running cool. The heat sink supports what appear to be 4 power amp transistors and a separate multi-leg 8 or 10) package (possibly a preamp IC) and it heats up at the end where the power amp transistor are, so I'm guessing that they're the ones dissipating the extra energy.

    The weird thing is that the unit works and appears to amplify signal like it's supposed to (although I can't verify that it's amplifying to the level that it's supposed to be).

    It's not under warranty so the return-for-a-free-fix/replacement is out. Do any of our resident techs have any suggestions as to what might be the problem and how I could go about confirming that? I have plenty of build experience in electronics, but less so in fault diagnosis, so any help would be most appreciated, please.

    Many thanks,

    Paul : )

    PS: turns out the flying mount is directly above the internal heat sink, hence it's heating up.
  2. swingingoodtime

    swingingoodtime Supporting Member

    Mar 19, 2009
    Sydney, Australia
    Oops, sorry mods: I just realised that I was in the wrong forum when I posted this (duh). Please delete this thread, as I'm reposting it in the Repair section.
  3. I believe this is the correct forum, actually. The repair section is for repairing basses themselves, not amps.

    As for your problem however, I'm afraid I'm rather clueless. Sorry. :/
  4. Don't worry, he solved his problem. See his other thread in the repai section.

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