ment which can introduce noise and cause errors in measure-
ment.
K.3.8.4
Voltage Regulators (U2, U16, U18, U19, U20, U21, Q1)
Four regulated voltages are provided by on-board regulators. U2
is a 7815 which converts 22 V to 15 V. U18 is a 7915 which
converts -22 V to -15 V. U21 is a 7905 which converts -15 V to
- 5 V which is used by A/D chip U1.
U16 is a LT1123 and Q1 is a MJE1123 transistor. These two parts
combine to form a +5 V low dropout regulator. The input voltage
to this regulator can get as low as +5.5 V and it will still maintain
an output of +5 V. It can also supply 4 A of current.
U19 is a 78L05 which co22 V or + 8 V to +5 V. Two
voltages drive this regulator in case one of them fails. This +5 V
supply is used by U20, an LM339 quad comparator chip that
monitors the regulated voltages. If the +15 V fails the 15
FAULT-L goes from +22 V to 0 V. If the -15V fails the signal -15
FAULT-L goes from -7 V to -20 V. If the +5 V fails the 5
FAULT-L goes from +5 V to 0 V.
K.4
Maintenance
K.4.1
Printed Circuit board Maintenance
Refer to section 5, Maintenance, in this technical manual for
general printed circuit board maintenance procedures.
K.4.2
Adjustments
K.4.2.1
Sync Sample Phasing (S1)
Adjustment of sync sample phasing is described in the Tun-
ing/Frequency Change procedure in section 5, Maintenance, in
this technical manual.
K.4.2.2
Clock Pulse Width Adjustment (R78)
Adjustment of his control is described in the Tuning/Frequency
Change procedure in Section 5, Maintenance, in this technical
manual
K.4.2.3
Digital to Analog Converter Bit Selection (S2)
Switch S2 determines the number of bits that go into the D/A.
Section A of S2 is between pins 1 and 4. Section B of S2 is
between pins 2 and 3. Section A and B are open for DX25
operation so the 6 MSB’s go to the D/A. Section A of S2 is closed
in the DX50 operation so the 7 MSB’s go to the D/A.
K.5
Troubleshooting
Refer to Schematic 839-7855-177, in the Drawing Package. Test
Points and waveforms are provided at various signal points on
the board.
NOTE
Analog signal amplitudes (including reconstructed analog signal
amplitudes) given are for 50 kilowatt transmitter output with
100% modulation. At lower power levels and/or lower modula-
tion levels, these analog signal amplitudes will be smaller.
K.5.1
Symptom: ColorStat™ panel CONVERSION
ERROR Indicator is RED, transmitter oper-
ates normally.
If the transmitter operates normally, but there is a CONVER-
SION ERROR on the LED Board, the problem is in the indicator
circuits. Check DS1 on the Analog to Digital Converter. If the
ColorStat™ panel “Conversion Error” indicator is RED but
A34DS1 is GREEN, measure the output of U12-4.
1. If you measure a logic LOW, the problem is in the
indicator circuits on the LED Board. Refer to SEC-
TION Q, LED Board, Troubleshooting.
2. If you measure a logic HIGH, replace U12.
NOTE
The “Conversion Error” indicator DS1 on the Analog to Digital
Converter will indicate RED whenever there is no RF drive, for
example, when the transmitter is “OFF.” The ColorStat™ panel
CONVERSION ERROR LED will still indicate GREEN because
LED Board logic inhibits the conversion error fault indicator
when the transmitter is turned OFF.
K.5.2
Symptom: ColorStat™ panel CONVERSION
ERROR indicator is RED, transmitter can be
turned ON. No RF out.
K.5.2.1
Check Logic Level at TP8.
a. If TP8 measures logic HIGH, measure U15-5. If it is logic
LOW, U14 is faulty.
b. If TP8 measures logic LOW, the problem is the EOC-L,
the CLK ERROR-L, or the POWER RESET-L signal. To
isolate the cause to a circuit on the Analog to Digital
Converter, check logic levels at U15-1 and U15-2, then
refer to the appropriate paragraph. If pin 1 is LOW, an
“EOC-L Fault” is present; if pin 2 is LOW, a “CLK
ERROR-L Fault” is present; if both pins are LOW, a
“POWER RESET-L Fault” is present.
K.5.2.2
CLK ERROR-L: No signal at TP6
If no TTL pulses are present at TP6, make certain that sample
frequency input is present at J3-1. A loose connector is the most
likely cause of no sample frequency input because no RF drive
would also cause an Underdrive Fault on the ColorStat™ panel.
If the sample frequency input is present at J3-1, check the Schmitt
Trigger input U12-5 and output U12-6. If there is no signal, check
for shorted CR13 or CR14, or defective Schmitt trigger U12. If
signal is present at U12-6 output but not at TP6, U29 or other
sections of U12 are defective.
K.5.2.3
CLK ERROR-L: Signal present at TP6
Check the Frequency Determined Components chart for the
proper position of P10, and calculate the frequency of the logic
signal at TP6 for your operating frequency. The frequency of the
logic signal at TP6 should be between 410 kHz and 820 kHz,
depending on transmitter frequency. Check the factory test data
sheet for the transmitter, or the Frequency Determined Compo-
nents Chart, for the proper position of P10 (and therefore whether
the divider divides by 1 or 2). If the frequency at TP6 is wrong,
P10 is in the wrong position or U29 is faulty.
DX-25U
K-4
888-2297-002
Rev. X: 05-13-98
WARNING: Disconnect primary power prior to servicing.
