PLX 2402 thermal shutdown problem

Moderators: Cameron Shoffner, Christian Cook, Kirk Fyvie, Martin Barbour, Gary_QSC

Mon Mar 17, 2008 12:38 pm

  • Hi:

    After reading the great support on Talkbass with Bob Lee I now find myself needing to ask a technical question on my beloved PLX 2402

    Did a big outdoor show last Sat night, and my trusty rig went into shutdown about 5 times. I run a PLX 2402 into an Alembic F2B with a SansAmp Para DI driver in front of the Alembic. Only drove one speaker (Epifani T212 at 4 ohms) using just one channel of the PLX. This was a big stage with a bunch of power amps driving the PA, so I don't think the power level was dropping (especially since it went into shutdown 5 times.
    I checked the clip lights while playing and I was running it conservatively (maybe two green lights on, definitely not close to clipping). It's rack mounted, and cables and speaker cable are OK.

    I've used this rig for many years now without a problem, but at the gig all of a sudden I would lose power, and then about two minutes later it returned. I noticed that the front of the PLX seemed warm, although I've never checked that.

    Could it be a fan? Will the other channel work if one is going into thermal shutdown?

    Can I ship the amp and have QSC do a full power load and try to duplicate the problem? I really don't want to gig with this rig if it is not reliable

    Thanks!

    Phil Anderson
    Tucson, AZ
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  • Hi Phil,

    When the amp thermals, both channels shut down.

    Did the clip LEDs come on when the amp shut down? It would be really unusual for an amp to thermal with a 4Ω on only one channel.

    What was the power source for the show?
    Bob Lee
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  • Hi Bob:

    Thanks man for the fast reply.

    No, I was only using one channel and very
    conservatively at that. I didn't notice
    if clip lights came on as I was busy playing
    and facing the crowd, but I did check when things
    were going OK and I only saw two green LED lights
    on Channel A flashing with my hardest notes, no yellow or red. I run the amp mono parallel so
    if I want to add another set of 4 ohm speakers
    I just use the other channel and my one signal
    from the Alembic F2b works on both channels

    This was a "Downtown
    Saturday Night" show with power coming from the
    Main Tucson City Library. I noticed a stack
    of about 5 power amps to the side of the stage
    (for Mains and Monitors). Their amps were not
    affected. No stage lights when we played.

    I thought it might be thermal shutdown because
    all of a sudden power was lost, then toward the
    end of a song it came back on as before

    Thanks!!!

    Phil
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  • Hi Phil,

    A thermal shutdown usually takes about 30 seconds to a minute to clear, which is how long it takes the amp to cool down enough.

    But the only ways I can picture an amp operated lightly on just one channel driving a 4Ω load and overheating are these:
    </font>
    • <font size="2" face="Helvetica, Arial">The ambient temperature is blisteringly hot. But it would have to be unbearingly hot.</font></li>
    • <font size="2" face="Helvetica, Arial">Air can't get into the back of the amp and/or out the front. Check that air flow is not impeded.</font></li>
    • <font size="2" face="Helvetica, Arial">The fan isn't working. Check that the fan is working; you should feel some slight air flow through the front slots any time the amp is turned on.</font></li>
    • <font size="2" face="Helvetica, Arial">The heat sinks are clogged with dust. Open the amp and clean them out with gentle compressed air or vacuum.</font></li>
    <font size="2" face="Helvetica, Arial"> If none of these describe the situation, then the possibilities remain that the amp has a defect or that there was some other outside cause, like abnormally low AC voltage, AC was shut off, et al.
    Bob Lee
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  • Hi Bob:

    Just checked the fan, it is fine, turns on with the amp,
    and blows air forward through the front vents.

    Understand extreme heat, I've used this amp setup
    in Tucson during the summer at 105 + degrees with
    no problem. Amp is rack mounted in a SKB deep case
    with a space above the amp for ventilation, nothing
    is blocked

    So, it sounds like your description: amp turns off,
    takes about a minute to cool down, then works until it shuts off again

    If my fan is OK would bad power create the same symptoms?
    I thought bad power would just not allow the amp to
    function rather than shutting it off.

    One thing, I did feel the front of the amp, and towards the
    right side, above the potentiometers the ventilation slits
    seemed hot (I've never checked before, might be normal
    as I've never had any problems.)

    In some ways I wish it were a bad fan, fix it, done.
    It's the uncertainty . . . . . . . .

    Thanks!

    Phil
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  • If the AC voltage drops below about 90 volts (which is a very drastic drop from the normal 120V) for longer than a few AC cycles, the amp's power supply will shut down and stay off until the voltage gets back up above 90V. Then it goes through the normal startup sequence that takes about 5 to 6 seconds.

    Usually, AC voltage drops (as opposed to complete interruptions) that are severe enough to cause an amp to shut down are very short in duration, so that the amp is off mostly for the start-up sequence.

    If the amp is near the point of overheating, the air exiting the front slots will be very warm, and the fan will be running at full speed. Imagine a hair dryer set on "low." If the amp thermals, the audio will mute and the clip indicator LEDs will light up, bright and steady.

    Since the cause and circumstances of the problem are still undetermined, I'd suggest trying to make the problem reoccur.
    Bob Lee
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  • Hi Bob:

    Thanks for all your help, much appreciated.

    Doesn't sound like the power dropped, so I guess
    I'll have to put my Eden WT400 on top of my rig
    as insurance if this problem persists.

    I have two Amperex Bugle Boy 12AX7 tubes in
    my Alembic F2B preamp. Could a bad tube mimic
    the symptoms of a thermal shutdown?

    The amp *was* warm, but with no clip lights so I'm
    starting to think there might be a component inside the 2402 that is failing

    Thanks again,

    Phil
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Wed Mar 19, 2008 10:22 am

  • A bad tube could manifest itself in various ways. It would not be likely to make the amp go into protect.

    There could be a failing or intermittant component in your system, including the bass, cables, preamp, power amp, etc. It's hard to pin down without symptoms.

    How old is the amp, and do you have its serial number at hand?
    Bob Lee
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  • Hi Bob:

    Amp is I think about 5-6 years old (might be older), and the serial
    number is 110150466.

    I took all the cables and connectors out of the
    rack and will test them Thurs at work where I have
    a good Fluke test machine. I use a heavy speaker cable'
    with banana plugs and a thick power cord.

    Much thanks!

    Phil
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  • Okay, since your amp was made in November 2001, we can rule out a problem with intermittant ribbon cables.

    Have you been able to duplicate the problem?
    Bob Lee
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  • Hi Bob:

    My normal gigs are med-lg sized clubs,
    where I can get by with an Eden WT400
    and a Bergantino HT-112 cabinet

    The QSC2402/F2B rig is for outdoors
    and gets a 4 ohm Epi T212. My next outdoors
    gig is May 3-4, so no chance to fire it up
    until then under gig conditions

    I did replace my speaker wire with a new
    Speaktron cable. I might fire up the rig
    in my garage this weekend, but it can't
    duplicate live situations.

    Anything I can do to put a big load on the
    system without getting the local law enforcement
    out to my quiet neighborhood?

    Thanks!

    Phil
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  • Update:

    Used my SWR Baby Blue and my Sansamp
    ParaDI stomp box, different bass, and lo and behold
    in the middle of a song the whole system shut off
    for one song at a big street fair with my other
    band last weekend (bluegrass trio)

    QSC not to blame for this one, looking very much
    like the Sansamp ParaBDDI is the culprit. New battery,
    but possible oxidized and/or loose 1/4" jacks

    Looks like I'll use another DI box next time out
    and put this little drama to rest

    Thanks Bob Lee for hanging with us, great board,
    excellent service!!!!

    Phil
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