Amplifier (fan) cooling circuit mod

I'm trying to figure out how to slow down the fan of an old PA amp (not just an amp, otherwise I wouldn't bother ;))
to make it more suitable for home use.
The easiest way would be replacing the fans but in this case we are talking about two 24VDC blowers from Rotron with cooling designed strictly around them (not enough room to use a Papst or even an axial fan).

The amplifier has temperature sensors and a LED panel showing the current temp between 40 and 80 degrees C but it never reaches 40*C.
Even at idle the fans are unbearable for home use, there is a slow/normal setting but it doesn't do anything with idle speed.

I'd like to modify the circuit to lower the base speed without butchering the PCB and if possible, make the mod reversible.

The schematic showing 1 channel's cooling, temp. sensors are not visible on this pic although probably not relevant since we are talking about the lowest speed setting.
Both channels are fully separated and have their own power supply.

[IMGDEAD]https://puu.sh/nTX3A/676ac142c9.png[/IMGDEAD]
 
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You would have to do some trial and error on resistors R836 and R837. Try replacing them with 120 ohm each as a starting point. 7 watt as original will be more than enough.

It would be worth checking if the fan is running full speed all the time first. Maybe due to a fault. Measure the voltage across it.
 
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Thanks for the reply, I was thinking about doubling the value on those 7W resistors too.

I already replaced all LM339's that control the slow speed and some other functions but that didn't make a difference. The fan(s) do slow down a little bit after startup.
Shame that this amplifier is such a PITA to work on - no access to measure anything when it is running.
 
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Fender Bass amplifiers use a very simple and reliable fully mechanical system.
Will cut and paste schematic here when I find it, but in general the idea is:

1) use a heatsink attached NO (Normal Open) bimetallic thermal switch, rated around 50C , in series with fan , so it does not turn a bit under 50C
Instant home peace.

2) fan has a series power resistor connected, so even when activated by first thermal switch, it runs at 1/2 to 1/4 speed, enough to move air around but not enough to make audible sound unless you apply your ear to it.

3) said low speed resistor has a second thermal switch, this one again NO, and rated, say, 65 or 70C .

Works like a charm and outperforms way more complex systems, at the same time being way more reliable.
 
After a more thorough inspection i decided to stick with the original circuit.
There is no room for any different circuit - everything is made perfectly to size in this amp.
(Yet big thanks for posting the schematics, I will surely keep those in mind for future).

The amp won't cook as it has multiple temperature sensors and a thermometer on the front panel. At around 80*C on the O/P devices it automatically shuts down.

This is what I mean with no room for mods:

[IMGDEAD]https://puu.sh/nXjB0/91522fbe0a.jpg[/IMGDEAD]
 
I'd use an nfet between those 60r resistor to -27. a 10k or 15 k ntc thermistor on the heatsink, one end to +27, other end to fet gate. Fet drain to 60r resistor, source to -27 v. A 10k resistor plus 10k pot between gate & -27. A 6 v or 7 v 1.3 W zener from gate to source. Fan totally off until hot. Fan totally under control of old board if hot. Set switching termperature with hair dryer and pot, before closing up. I salvage the nfets out of old PCAT supplies. Some have their own thermistors, use gate resistor and pot appropriate to that thermistor. Nfets on Vgatesource >6 v usually, off <2 v .
 
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