SWR SM500S running way hot

Discussion in 'Amps and Cabs [BG]' started by edwinhurwitz, Jun 20, 2021.


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  1. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    I brought out my SM500S last night after issues with another rig. It sounded great but ran really, really hot and during the second set, shut off. I moved to a different preamp/power amp, and later it turned back on, presumably after sufficient cooling.
    I could not feel any airflow from the side vent, which is where the hot air is supposed to go.

    Does anyone know if the fan is supposed to run all the time or if not, how it is switched on?

    The amp was made in 2000.

    Thanks.
     
    Last edited: Jun 20, 2021
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I believe the fan runs all the time at a slow speed and increases speed proportional to temperature. If the fan is not spinning, it cold be a defective fan but it could also be the fan control circuit (which is fairly complex). Because of the proportional speed controller, the fan needs to be replaced with one that has the same properties.
     
  3. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    Thanks for the reply. The fan is not running at all right now. Do you have any idea what voltage the fan might run at? I'm thinking of unsoldering the wires that go from the fan to the left power amp module and attaching a 9v across the wires to see if the fan seems to be functioning. I've got an SM500 schematic, which shows a pretty simple circuit where the fan wires hit the board. Maybe it changed with the 500S. Remember that this is a pre-Fender amp.

    Thanks again!
     
  4. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    One strange thing is that there is a space on the left power amp module marked SW1, which would correspond to the thermal switch on the schematic, but it is unpopulated.
     
  5. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    South Shore MA
    Make sure the power amp bias hasn’t drifted, that can lead to overheating as well.
     
    MuthaFunk, Al Kraft and edwinhurwitz like this.
  6. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    Thanks, I was just thinking about that!

    I think that for my gigs next weekend, I'll just use the preamp section to an external power amp and then take it to a tech to get the bias checked and to have the fan hard wired on all the time. My gigs tend to be not very quiet.
     
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    The fan in the circuit I am looking at appears to be a 24V fan with a range of up to 16-17V at top speed. This would have been somewhere around the transition to Fender.

    The early SM400/500 used a normally open thermal switch that would turn the fan on and off. Just a simple 1/2-wave DC supply off of a shared transformer tap. One common modification was to arrange for a power resistor across the switch so that the fan ran at about 1/2 - 2/3 rated voltage all the time and when the switch closed it would increase to the full voltage. The thermal switch is mounted on the heatsink. There may be 2 thermal switches, the second one shuts the AC down in the event of overheating (normally closed) .
     
    Rip Van Dan and edwinhurwitz like this.
  8. Jim C

    Jim C I believe in the trilogy; Fender, Stingray, + G&L Supporting Member

    Nov 29, 2008
    Bethesda, MD
    For the record my SM500 only comes on when it's hot and the fan is reasonably loud.
    Ironically, once the fan comes on, and you stop playing, it does not shut off quickly. I've waited at least 15 minutes for it to shut off with no signal, then turned it off, then back on and the fan stays off until it gets played hard again.
     
  9. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    Update:
    Having found the switch (right in front of my eyes, duh!), I shorted it and the fan works well. Not much of a breeze coming out the other side of the heatsinks, which I cleaned, but I'm thinking of doing the resistor mod. Any suggestions for value/watt rating?

    Or I might just permanently short it and see if that helps.
     
    dralionux likes this.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I will have to look and see what info I have. Do you have the specs on the fan that’s installed in your amp?
     
    dralionux likes this.
  11. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    I don't. Despite looking carefully, there are no markings on it that I can find.

    Again, I really appreciate your help!
     
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Then what you will need to do is measure the voltage when the fan is running full speed, and make an assumption that the fan is 100mA, then calculate about 2/3 of this voltage as the desired voltage and calculate the resistance needed to drop the voltage. Say the voltage is 24V, that means that the fan has an effective resistance of 24V/.1A = 240 ohms.

    When you increase the resistance, both voltage and current will fall according to the voltage divider rule. In this case, I would look at a resistance of about 150 ohms which will drop the current to 24V/390 ohms = .06A and the voltage across the 240 ohm fan will be .06A x 240 ohms or 14.4 volts. In this case, the voltage approximation may be a little low and perhaps 120 ohms would be more appropriate with a resulting voltage of about 16 volts.

    These calculations won't be exact because the fan is not a true resistor (and in fact uses a complicated driver circuit to create a rotating magnetic field from a static DC voltage).
     
  13. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    Thanks, that's very helpful. Gives me a great starting place.
     
    agedhorse likes this.
  14. Ken R

    Ken R Supporting Member

    Jan 14, 2007
    Greater Knoxville TN
    I continue to stand in awe of @agedhorse. The man always offers to help and truly knows so much. Thanks for always helping!
     
  15. Eric Swaim

    Eric Swaim GOD, U.S. MIlitary, Country Supporting Member

    Sep 21, 2004
    Murfreesboro, TN
    I use the exact same amp with no problems. I have used SWR since 1995 (SM400, SM400S, Basic 350, Studio 220, SM900, Red head, Baby blue and Electric Blue). Never had a single issue but I do not they run hot. Only thermal shutdown I ever experienced was from a SWR Working Pro 400 I borrowed for a gig. Real piece of crap that particular amp was. Hopefully its just your fan and thats an easy fix.
     
  16. skyline_01

    skyline_01 Endorsing artist: Nordstrand Audio, Tsunami Cables Supporting Member

    Nov 14, 2006
    Lebanon, OH
    I had a pre-Fender SM500 from 1999 that always ran the fan. I can't recall it ever shutting down but it could get fairly hot.

    Sadly, I ended up selling it after it started making a constant 60 cycle-type hum and I unsuccessfully attempted to repair it. Little did I know that years later it would be a sought-after piece of bass history.
     
  17. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    Seriously!
     
  18. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    I'm going to short out the switch to start with so the fan always runs. I'll see if that helps (I suppose I could do further testing and run it hot to see if the fan comes on, but I don't think I or my neighbors can stand that kind of SPL!).

    At the gig I swapped over an Eden WP100, which has a very different sound. And it has its own issues (it keeps losing the switching FETs that result in losing output).
     
    skyline_01 likes this.
  19. bigdaddybass12

    bigdaddybass12

    Feb 26, 2021
    this is not quite a comparison, I had a SWR combo had the big heat sinks on the back. When I played it loud at a outdoor gig, they would get so hot you almost could not touch them.
     
  20. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    Off-topic I know, but I've done a similar resistor mod to an old MacBook when the fan tacho logic circuit failed. Much easier and cheaper than replacing a motherboard.
     
    edwinhurwitz, Jim C and agedhorse like this.
  21. Primary

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    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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