Marshall DBS7200 Thermal Fan Control Circuit - Page 1
Forum section: electronics / repair

author: paul_g_787
author_rank: Frequent Contributor
timestamp: May 08, 2020, 03:30:59 pm
content: Hi All, I have just repaired my Marshall 7200 amp, but one thing has always bothered me and that is the fan. It is ALWAYS stuck on full speed regardless. Just like an old ealry 90s PC it is like a jet engine idling in the background. Very annoying.So I would like to put in a fan control circuit that can vary the fan based on temperature.See attached the part of the Marshall schematic concerning the fan.With the mains voltage currently at 247VAC I was getting 26.5VDC at the fan. So I assume this would nominally be approx 24V at 230VAC?So essentially I need a 24V thermal fan controller.Is there a simple solution to this? Preferably a passive solution involving a thermistor?

author: CaptDon
author_rank: Super Contributor
timestamp: May 08, 2020, 05:10:34 pm
content: It looks like you could lift the positive side of the fan from+LT where it is now and go to +MT which appears like itmay be a lower voltage of around +11. If you wantedautomatic speed increase you could take the +MT througha forward conducting diode to the fan, and then find a suitablethermal switch that closes at maybe 120f degrees and connectone side of the switch to +LT and the other side to the fan sideof the diode. The diode will block the +24 from back feedingthe +MT. I need to see a better view of the +MT diodes and supply.I would think +LT would be low tension, +MT would be mediumtension and the +HT is high tension. But in what I can see of thedrawing +LT is derived from 18 volt windings and +MT is derivedfrom 7.5 volt winding which would place +MT at a lower voltagethen +LT which doesn't make sense.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: May 08, 2020, 05:51:58 pm
content: Quote from: CaptDon on May 08, 2020, 05:10:34 pmThe -MT is derived from -47.5VAC and the -HT from -86VAC (where the primary is 230VAC). These are off of another secondary coil with a separate bridge rectifier that run the power amp, they are nothing to do with the split 18V winding which is used to power the pre-amp regulators and the fan. I don't think it wise to use those without causing other concerns such as the fan noise getting onto the power transistors.It looks like they have designed the amp with the two secondary windings, one for the power amp and one for the pre-amp, then they have thought about adding the fan after and just taken the 24VDC from before the 7815 regulator as to avoid the fan noise from going to the pre-amp.I think it best just to use the power available at the fan connector which is an unregulated 24VDC, and make a circuit to ramp this voltage to the fan down.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: May 08, 2020, 05:56:42 pm
content: This design is of interest to me:https://electronics.voltsandbits.com/projects/amplifier-fan-controller/He has made a 12V fan controller with two thermistors for a stereo power amp.I can see several stages in his design:First, a 24V zener diode as a simple linear regulatorThen an op-amp providing a reference voltageThen the thermistors, driving an op amp as a bufferThe buffer then drives a transistor which controls the fan.This seems over the top to me. Firstly my amp is single channel so I will only need one thermistor.I am wondering if I can simplify this design to have a single transistor controlled by a NTC thermistor / potentiometer. Perhaps with a 24V zener as a regulator?

author: paul_g_787
author_rank: Frequent Contributor
timestamp: May 08, 2020, 06:26:43 pm
content: Perhaps something along these lines would work?ZD1 is regulating to 24VR1 is a current limiting resistorTH1 is an NTC which makes a voltage divider with VR1 used as a trimmer.The voltage divider drives the NPN transistor which will allow the fan to switch on and ramp up?I am really only guessing at this so may have got something wrong.The Fan is marked as 24V 1.7W

author: CaptDon
author_rank: Super Contributor
timestamp: May 08, 2020, 08:50:33 pm
content: Hey there, very sorry, I did not see 47.5v, On this little screen I saw 7.5v.Yes, you are correct!!! Lets not hook them to the fan!!

author: CaptDon
author_rank: Super Contributor
timestamp: May 08, 2020, 09:10:28 pm
content: I don't like the resistor/zener as a voltage regulator, here's why. You would haveto calculate the max current draw of the fan and add a few milliamps to keep the zenerin regulation and go about calculating the resistor value. The problem is when the fanisn't drawing much current the 'surplus' current will end up in the zener. You wouldprobably need a 5 watt zener. It is do-able, but I think there are better options. If youdon't mind the generation of a bit of heat in a resistor, just use a dropping resistor tolower the voltage by about 10 volts going to the fan and find a thermal switch (open whencool, closed when hot) to short out the resistor. If you are playing loud and heating theamp you wont here the fan anyway, and if you go on break etc. the fan will slow againas soon as the amp cools down. My older Peavey stuff had 2 speed fans with a thermalswitch. The M4000 and M7000 worked that way. I had them on top of my cabinets andwhen we played loud I could tell the amps were getting hot. The fans would go to highspeed and I could feel the warm air on my back.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: May 19, 2020, 04:59:41 pm
content: Quote from: CaptDon on May 08, 2020, 09:10:28 pmPerhaps a thermal switch would be good, but I would prefer a more active solution if possible, most of the time the amp won't get mega-hot so I would prefer it didn't start just switching on-off-on-off to full power when it is right at the threshold of the switch.Perhaps I don't need the regulation at all? Any thoughts?

author: Renate
author_rank: Super Contributor
timestamp: August 06, 2020, 01:15:34 pm
content: I was getting annoyed by my power inverter (12 VDC to 120 VAC).You can never tell these things before you buy it but the the fan is on/off despite being run by a thermistor and a microcontroller.I guess that the real way to do it would be to reverse engineer the microcontroller and write the code for PWM.You already have a fan, but for something totally new you might consider a standard 12 V, 4 wire PWM fan for PC's.They have the advantage of already having the driver transistor in them and a tach output.You still need to control it, though.The cheapest part count might be some analog thermistor and transistor driver.I prefer to go with a microcontroller. I've got a fistfull of ATTiny85 8 pin DIPs.They can speak I²C. I'd use a sensor. (I'll have to lookup the number of the one I've got.)A thermistor would be cheaper but I can't be bothered to futz with the lookup tables.I'd power it with a 78L05 or something SMD. Yes, 24 to 5 is a big waste but the uC doesn't take hardly any current.(You also have to keep an eye on the 24 V if it's unregulated, it might exceed the regulator input voltage.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: August 12, 2020, 08:29:35 pm
content: Quote from: Renate on August 06, 2020, 01:15:34 pmI guess the first part to deal with is to regulate the 24V then?The supply voltage is around 24-30VDC when I have measured it, is there a regulator that can take such an input range and output 24VDC?

author: Renate
author_rank: Super Contributor
timestamp: August 12, 2020, 11:13:18 pm
content: A 7824 can go up to 40V. OTOH, those regulators are not LDO , they take a couple of volts.Does your 24V supply vary enough to worry about it?