ts440s.38

The fan is controlled by two sensing circuits on the FINAL UNIT.  On the
schematic, Q9(1/2) is the failsafe trigger that activates the powerdown
circuit in case the final reaches 80 deg C.  It won't normally do this,
even under continuous keydown conditions, unless the fan has failed.
Q9(2/2) is the stage that controls operation of the fan.  It is a simple
voltage comparator and therefore can be made to trigger wherever you want.
TH1 is a thermistor with a negative coefficient.  That is, when the temp
rises, the resistance goes down.  This pulls the - input to Q9(2/2) lower
and lower until it is at or below the 2.26v reference that is present on
the + input of the same stage.  At that point, the output flips high and
turns on Q8, thus turning on the fan.  By raising the reference voltage
at the + input, the TH1 voltage will fall to the reference voltage sooner
and turn on the fan at a cooler temperature.  By clipping the top loop of
R27 on the final unit (2.2k res. that provides reference voltage) and
temporarily inserting a 5k precision multi-turn pot in series with it, I
determined that I liked the way it operated with about 800 ohms extra
resistance in the circuit.  
continued in file ts440s.39