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SWR SM-400S fan always on

Discussion in 'Amps and Cabs [BG]' started by FireGloRIC, Mar 10, 2010.

  1. FireGloRIC


    Jan 27, 2010
    I picked up a used SM-400S recently and it sounds great. I did notice that the fan appears to be running all the time. It's not real loud but you can hear it. Is this normal or could it need a new thermal sensor?
  2. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    it's not normal, but i had the fan in my old sm-500 hardwired to always on because sometimes it wouldn't kick on when the amp got hot. i wouldn't worry about it. if it's too noisy for you, get a quiet fan.
  3. marc40a


    Mar 20, 2002
    Boston MA
    It may need to be biased. That is a specific symptom that's mentioned in the manual or the service manual.

    I think the instructions for biasing it are somewhere on the internet.

    From what I understand, bumping it around over the years can throw it out of whack a bit.
  4. Lapicide


    Feb 19, 2008
    The fan on the 400s does pretty much run all the time, when it reaches 50c or 122f it turns on then off again when it reaches 40c or 106f, that's pretty much all the time. It runs slow when cool (not playing loud) and speeds up when running hot (playing loud). There is a thermistor that controls the speed of the fan. Note if you only use one channel, use the left. The left channel has the thermistor on the heat sink of the power amp, that controls the fan speed. If it is running full speed, then the fan may have been replaced with a fan that does not have variable speed. Currently the replacement fan you can get supplied by Fender is a one speed fan and not the variable fan. I went through this about a year ago. Ordered the fan to find out it is not variable. The variable fan that was in the 400 groaned a bit, in the end, I rebuilt the fan to keep the variable fan. Playing quietly with the fan at high speed was annoying to me......Lapicide
  5. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    if you all ever used an early svt, you wouldn't complain about noisy fans at all. holy crap is that a noisy fan! but i keep it because the last thing anyone hears on a gig is the fan.
  6. FireGloRIC


    Jan 27, 2010
    That sounds about right. The fan is on but at a low speed. Low hum, not jet about to take off.
    The noise doesn't bother me at all. I just want to make sure that it shouldn't be addressed.
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