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SWR SM500S running way hot

edwinhurwitz

edwinhurwitz

Supporting Member
May 13, 2003
4,053
4,756
Boulder, CO
www.indra.com
Disclosures
Endorsing Artist: DR Strings, SMS
#1
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.
 
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#2
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.
 
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#3
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!
 
#6
ihixulu said:
Make sure the power amp bias hasn’t drifted, that can lead to overheating as well.
Click to expand...
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
edwinhurwitz said:
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!
Click to expand...
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) .
 
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#8
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
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.
 
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#10
edwinhurwitz said:
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.
Click to expand...
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?
 
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#12
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).
 
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#15
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
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.
 
#18
skyline_01 said:
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.
Click to expand...

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).
 
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