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rect digit, displaying the appropriate number. The
segment outputs of U1 are connected to the cur-
rent driver U3 and RN2 which delivers approxi-
mately 40 mA to each segment. The displays are
common cathode variety so that the current is
supplied to the anode and pulled to ground at the
common cathode connection by the scanned open
collector driver U2. This current level is required
to ensure sufficient brightness at 1:8 multiplex-
ing. The SiC blue LEDs, D9-11, require over
100 mA peak multiplexed current to achieve use-
ful brightness. Transistors Q2-4 are driven by U2
to boost its sink current for these devices.
3.6.2 Controls
Control pushbuttons are part of the A1 control
panel and are connected to the Display PWB
through connector J3. There is also a ground con-
nection from the front panel electrostatic shield
which is incorporated into the membrane switch-
es. This ground strap is connected directly to the
chassis and prevents electromagnetic interference
and electrostatic discharge from being conducted
into the logic circuitry via the membrane traces.
Each pushbutton is normally open and is com-
mon to circuit ground at J3-1.
Output power adjustment is done via 36 step/rev-
olution rotary encoders, ENC1 and ENC2.
When rotated from one step to the next, only one
of the two encoder contacts will close or open.
The microcontroller reads the state of each
encoder signal A2U1 RL0-3 every 4 ms. The
firmware detects each step change and its direc-
tion for further processing.
3.7 POWER SUPPLIES
Refer to Figure 5.4 for a schematic detailing
the chassis-mounted power supply components.
The Sabre 180 is designed to operate from 100,
115 or 230 volt, 50-60 Hz, single phase AC
mains power. The detachable power cord is con-
nected via a standard CEE-22 6A inlet on the rear
panel. A safety ground in the power cord bonds
the chassis and circuit common to earth. This
cord should always be connected to a properly
grounded AC receptacle to insure that an internal
mains-to-chassis short does not create a serious
shock hazard to the user.
Mains overcurrent protection is provided by a
pair of 5 x 20 mm 250V externally replaceable
slow-blow fuses, also on the rear panel.
CAUTION: Fuses should be replaced only
with the type and rating marked for the
strapped voltage range.
3.7.1 Mains Isolation Transformer
Transformer A6T1 steps the mains voltage down
to separate 40 VAC and 22 V CT AC secon-
daries. The 40 V secondary is full-wave rectified
to supply bulk unfiltered dc to the RF amplifier
switching power supply. The other secondary is
rectified and filtered to provide the +28U and
+14U unregulated low voltage sources.
The dual primary is tapped and dressed out to a
6-pole screw terminal block, A6TB1, which pro-
vides easy access for strapping the mains for any
of the specified nominal mains voltages. The same
transformer is used for all mains voltages, but
mains fuse ratings are different for the series and
parallel primary strap configurations. See Section
4.4.1 for strapping changes.
Leads 3 and 6 contain internal thermal fuses to
guard against insulation failure in the event of a
mild overload which does not open the mains
fuses. If either of these devices operates, the
power transformer must be replaced.
3.7.2 Low Voltage Supplies
Refer to Figure 5.7a for schematic details of the
Low Voltage Supplies.
Rectifier A6BR2 acts as a full wave bridge to
source the +28U supply which is filtered by
electrolytic capacitor A4C11. The voltage on this
supply varies with mains voltage and RF output
power, but is typically around +28 VDC at nom-
inal mains and idle.
+28U is delivered to the A3 Controller PWB
(See Fig 5.6) via A6W6 to supply the VBASE
pass transistor A8Q1, the +12V regulator
A3VR1, the Tone Generator and the Relay
Enable switch A3Q1. Note that the +28U supply
is denoted as +16V in Fig. 5.7; this reflects the
approximate value of this supply in another appli-
cation.
3-21
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Troubleshooting
