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Amp works, but gets really hot under no load from startup. Any diagnoses, please?

Discussion in 'Setup & Repair [DB]' started by swingingoodtime, Oct 6, 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
    Ok, sorted the problem out. I suspected that the bias current was too high, so I got the correct specs from the distributor and reset it. It's running much cooler now. Hope this helps someone else if they ever have the same problem.
  3. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    Bias current is one of the things that flew in and out of my mind when reading your post. I dismissed it (and shouldn't have) because it just seemed so unlikely that it would change dramatically on a solid-state amp. I wonder if there is not still something wrong. Just as when you blow a fuse or trip a breaker, there is a reason these things happen. Perhaps there is a bad component that caused (and might continue to cause) the bias current to be unstable. I suggest that you keep a close eye on it.
  4. swingingoodtime

    swingingoodtime Supporting Member

    Mar 19, 2009
    Sydney, Australia
    Hi Les, nice to hear from you, hope you've been well! Thanks for the input; yeah, I'll be monitoring it. I found that the bias trimpot was not held into position by anything (ie, threadlock or similar), so I'm hoping that it'll be something as simple as mechanical vibration causing a physical shift and hence the current drift. I've thread locked it into place now, so if it changes, I'll know it's component related.

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