FT1000D Cooling Fan Page
This FAN Page details the required modifications needed to the FT1000D's cooling
fan to increase the cooling and cut down on the fan noise.
Note 7 details all of this.
NOTE 7This report details modifications to the FT1000D FAN circuitry to increase the radio's cooling while lowering the noise. This is accomplished by adding two (2) resistors.
The FT1000's fan is a source of noise for me due to the high gain of the audio processing equipment. This makes the mic very susceptible to noise pickup especially when the fan switches to HIGH cooling mode. The high frequency (400 - 700hz) whine seems to 'propagate' right into the output audio. When in LOW speed, it cools the complete radio as well as being almost noiseless. This modification keeps the fan on LOW speed continuously and changes the high speed rpm when it kicks in.
LOW Speed Operation:
The picture below shows the schematic (Rload simulates the fan current load) and circuit simulation results of the fan control circuit. A short theory of operation follows: thermistor TH7602, located on the HEATSINK's right side, controls the input to an op amp Q7611-2 such that when cold, the output is HIGH (+8V). The cold resistance is around 3.7K ohms and the voltage on pin 5 (+ in) is apx 5.5 volts. The voltage on pin 6 (- in) is fixed at 4.5 volts. Thus, the output of op amp Q7611-1 is LOW (-8V) which cuts off the driver transistor, Q7621.
As the radio warms, thermistor TH7602 heats up and drops in resistance, thus dropping the voltage on pin 5. When the voltage drops below the threshold point (4.5V) and the output switches LOW (-8V). This is inverted in op amp Q7611-1 to +8V which turns the FAN on LOW speed by turning on transistor Q7621 providing a path for current flow from J7607 FAN2 to FAN1. The fan is tied to +30V through R7640, 27 ohms. Transistor Q7612, that is AC coupled (220uf), provides a current 'kick' to the fan to insure that it starts.
HIGH Speed Operation:
The fans switches to high speed when J7608, pin 2 (TG standing for Thermal Guard) drops to ground potential. Referring to the FT1000 SERVICE MANUAL, page 7-125 shows the connector. This connects to the PA UNIT Board, page 7-87. In bottom corner, TG unit TS7001 applies a GROUND to effectively short around R7650 (120 ohm) resistor which allows the fan to go into high speed. At this speed it is really noisy! The sensor is manufactured by TOKIN (PN: OHD3-60M). It appears to be a temperature sensitive device with a contact closure that provides a 'ground'.
The fan will now run continuously on low speed when the radio is powered up. This keeps the radio remarkably cool. Even after 6 hours, the top, sides and back are very cool to the touch. If the PA output stage gets hot enough, the FAN will kick into high speed mode but at reduced rpm due to the 47 ohm resistor. This will take a little longer to cool down the PA since the rpm is about 60% of the original. However, the noise is quite low and airflow is adequate.
FAN Mod ResultsPostscript: [7/6/00]
This mod was made about 6 months ago during cool winter weather. Now that we are in summer's heat, the mod has really shown its medal. The fan has never gone into high speed mode even with three (3) hours of SSB 'ragchew' at the studio temperature at 83 degs! And the radio stays cool to the touch.
The results of the two added resistors are shown here. Note that soldering is required to do this enhancement.