sample from the output network. The oscillator sync is adjusted
with DIP switch S1 and inductor L4.
The Output Current sample from T6 at the combiner output is
brought into the Oscillator at J3-1. Resistor R37 provides a
50-Ohm input impedance, and zener diodes CR11 and CR12
protect Q4 from transient voltages. The signal phase is adjusted
by DIP-switch selected capacitors C30 through C33 and L4. The
signal is converted to TTL level by Q4 and fed to CMOS analog
switch U4-11.
During normal operation, the Oscillator signal is routed through
U4 to buffer-driver U3 and then to the Buffer Amplifier. During
VSWR protection a logic HIGH signal from the LED Board turns
on Q5 and switches U4 so that the output current sample is used
as the transmitter’s RF drive.
Because the air system does not operate until the Power Ampli-
fier stage is energized, the Oscillator output to the Buffer Ampli-
fier is muted to protect it from over-dissipation. The VSWR-H
input is held HIGH by the LED Board when the transmitter is
OFF.
A.4.9
Oscillator Output (Buffer-Driver)
The output of driver amplifier U3-7 is square wave at the carrier
frequency. The signal is sent to the Driver Combiner Mother-
board where it drives the input of Buffer Amplifier A16.
A.4.10
“RF Present” Output
The output from U3-7 is converted to positive and negative DC
voltages by peak detectors CR7-C18 and CR6-C17. These volt-
ages are used for the Oscillator RF Sense circuit on the LED
Board.
A.5
Troubleshooting
A.5.1
Symptom: Oscillator LED on ColorStat™
panel is Red, transmitter will not operate.
A.5.1.1
Possible Cause: Power Supplies
Check for +22 VDC at both sides of F1. The +22 VDC unregu-
lated voltage should be present when ac power is applied to the
transmitter and Low Voltage Supply circuit breaker CB2 is turned
on. The front panel multimeter will also indicate whether the low
voltage power supply voltages are present.
If the +22 VDC is present and F1 is good, check the voltages at
TP1, TP2, and TP3. If one voltage is missing, a zener diode may
be shorted or there may be a short in a circuit supplied by that
voltage.
A.5.1.2
Possible Cause: Oscillator Sync Circuit
Using an oscilloscope, check for RF voltage At TP5. A 4-4.5
Vp-p square wave at carrier frequency should be present.
If the RF voltage at TP5 is ok, check the output of CMOS switch
U4.
NOTE
Remember that the output to the Buffer Amplifier is held off by
CMOS switch U4 at the VSWR-H input J7-5 until the Power Am-
plifier stage is turned on. Use S4 on the LED Board to check the
output of U4.
A.5.1.2.1
CMOS switch U4
Observe the DC voltage at U4-6 while depressing S4 on the LED
Board. The voltage should change from LOW to HIGH.
•
If the DC voltage changes from LOW to HIGH but there is
no RF output at U4-2, replace U4.
•
If the voltage changes from LOW to HIGH and there is RF
output at U4-2, check U3.
•
If the voltage is LOW and does not change, check the
operation of Q5.
•
Measure the DC voltage at the junction of R45 and R34. If
the voltage changes from HIGH to LOW when S4 on the
LED Board is depressed, Q5 is leaky or shorted, or U4-6
input is shorted internally.
•
If the DC voltage at the junction of R45 and R34 does not
change when S4 on the LED Board is depressed, check the
operation of the VSWR circuitry on the LED Board.
A.5.1.2.2
U3 Defective
Using an oscilloscope, check for an RF signal at U3-2. If the RF
signal is present at U3-2 but not at U3-7, replace U3. If there is
RF present at U3-7, check the Oscillator output at J4-8.
A.5.1.2.3
Short on Output
Using an oscilloscope, check for RF output at J4-8 (a convenient
place to check is at the end of R31 closest to BNC connector J5).
A 4-4.5 Vp-p square wave at the transmitter carrier frequency
should be present.
If RF voltage is present at U3-7 but not at J4-8, there is a short
on the Oscillator output. This could be in the cable between the
Oscillator and the Driver/Combiner motherboard, the input of
the Buffer amplifier, or the RF detection circuitry.
A.5.1.3
Possible Cause: RF Not Present at TP5
A.5.1.3.1
P3 or P4 Installed Incorrectly
When the crystal oscillator on the Oscillator is used, the jumper
plugs must be installed from P3-1 to P3-3, and from P4-1 to P4-3.
When an external oscillator is used, the jumper plugs must be
installed from P3-1 to P3-2, and from P4-1 to P4-2.
If P3 and P4 are installed correctly, and the crystal oscillator is
used, continue with the list of possible causes below.
A.5.1.3.2
Crystal Failure
Move P1 to the other crystal position. If RF output returns, one
crystal (or associated circuitry) is defective. If you operate with
the alternate crystal, change the crystal oven plug P6 to the
alternate crystal.
A.5.1.3.3
Q1, Q2 faulty
Using an oscilloscope, check the signals at Q1 and Q2 collectors.
A sinewave of RF should be present at the output of Q1. The
output of Q2 output should be a square wave at 4-4.5 Vp-p. RF
frequency at these points should be at the crystal frequency.
A.5.1.3.4
U1, U2 faulty
Check the frequency divider outputs at P2-2 and P2-3 for a 4-4.5
Vp-p square wave. Each IC section should divide the frequency
by two. Faults in a divider IC section will normally cause the IC
output to go to ground or +Vcc supply voltage.
DX-25U
A-2
888-2297-002
WARNING: Disconnect primary power prior to servicing.
