8525B/8528 Technical Service Manual Technical Description 4-37
Publication No: 15-02036 Issue 6
4.14 Options
Options may be fitted to the transceivers as described in the sections which
follow.
4.14.1 F—Fan Assembly
Option F is a factory-fitted assembly containing an axial flow fan and its
control circuit. Mounted over the heatsink the fan enhances the heat
dissipation of the transmitter. When the option is fitted, heatsinks are added
to the driver and output transformers on the PA and Filter PCB.
The Fan Controller circuit (assembly 08-03334) switches on the fan if a
transmission lasts longer than 75 seconds, and keeps the fan switched on
even if short breaks (up to 600ms) are made during the transmission.
The circuit is powered from the transceiver A rail (12V) and the positive
side of this is taken through the PCB to the fan. Transistor V2 is in the
ground return line of the fan. The D rail (10V in transmit) is applied at the
PTT input. During reception, the 0V on the PTT input cuts off V1 allowing
C3 to charge via R3. The positive voltage applied to the MR input of IC1
holds the timer in the reset condition. In the reset condition the Q output of
IC1 is low so that V2 is cut off, breaking the return line of the fan supply.
When the PTT is operated the input goes high which switches on V1. This
immediately discharges C3 and the 0V on the MR input allows IC1 to start
timing. After 75 seconds the Q output of IC1 goes high to switch on V2
which completes the supply circuit to the fan.
If transmission stops, the PTT input goes low and V1 is switched off.
However, C3 must charge via R3 before the MR input to IC1 can go high
enough to reset the timer. This takes approximately 600ms and if
transmission is resumed in this period, C3 is immediately discharged again
and the timer is not affected. Consequently short breaks in transmission,
either during the initial timing period or after the fan has been switched on,
have no effect on the cooling.
4.14.2 LU—Switched LSB Operation
Individual channels can be programmed to operate on upper and lower
sidebands; the choice being made and indicated at the USB/LSB pad on the
front panel. For LSB operation, a 1647kHz crystal is fitted in the Z3 location
on the RF Mixer and Dual Synthesizer PCB 08-03740. Adjustment is
carried out as described in section 7.3.11 of this manual. Selection and
operation of the oscillator circuit is described in Section 4.4.5. The required
facilities are programmed into the EPROM.
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