Harris DX 25U, Техническое руководство

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Refer to SECTION M, DC Regulator, for a further description
of these voltage regulator circuits.
Refer to SECTION Q, LED Board, for a further description of
the fault circuits.
J.3
BCD (Binary Coded Decimal) Coding
The following information is included for reference, if missing
bits in the BCD Power Control signal are suspected as a problem.
For the two most significant bits, you should be able to use the
“raise” and “lower” controls to change power one “step” at a time
and measure logic level signals with a logic probe, voltmeter, or
oscilloscope.
The Binary Coded Decimal input to the Digitally Controlled
Attenuator in the transmitter consists of three decimal digits,
from 0.000 to 0.999. If this number is represented as “0.XYZ,”
“X” is the most significant digit, “Y” is the next most significant
digit, and “Z” is the least significant digit.
Each digit is represented in Binary form, as shown in Table J-1.
T h e c o m p l e t e “ B C D ” n u m be r i s re pr es en t ed as
“XXXX/XXXX/XXXX,” where each “X” (Binary “BIT”) can
be either “0” or ”1.” The binary BITS are also represented on the
schematic diagrams as D1 through D12, so that the BCD number
appears in the following order:
D1 D2 D3 D4/ D5 D6 D7 D8/ D9 D10 D11 D12
For reference, several transmitter power levels are represented
below in both decimal and BCD notation. (Maximum power is
set by the “Max Pwr Adj” control R27).
J.4
Troubleshooting
Observing waveforms and voltages at Test Points with an oscil-
loscope, with a sinewave at the audio input, will isolate most
faults to one stage of the Analog Input Board. (The sine wave
amplitude should be the level normally required for 100% modu-
lation; signal levels for this input are indicated on the schematic
diagram and given in Table J-3, “Analog Input Board Test
Points”).
Some specific symptoms and possible causes are described in the
following paragraphs.
J.4.1
Symptom: Normal Signal At TP4, No Signal
At TP7 (Digitally Controlled Potentiometer
Output).
J.4.1.1 Possible Causes
J.4.1.1.1
U8, U10, or U11 faulty
Check U10-8/4 output. If no signal is present, U10 is probably
faulty. Check U8 output at CR3 cathode; if (Audio + DC) signal
is present, U11 is probably faulty. If Audio + DC is present at U8
input but not at U8 output, refer to the following paragraphs.
J.4.1.1.2
Digital Control Signal at U8 is Zero
There are several possible causes, including:
a. BCD (Binary Coded Decimal) Control Signal from Con-
troller A38 is Zero. With the transmitter ON, use an oscil-
loscope or voltmeter to check Power Control Lines at
inputs to U17 and U18. If the inputs are all LOW, there is
no power control information from the controller. Refer to
SECTION P, Controller, for further troubleshooting infor-
mation.
b. DATA CLEAR Input (at J4-27) is LOW. Measure the
voltage at U13-8. If the voltage is less than +1 VDC, the
controller is instructing the data latches (U17-U18) to
CLEAR, that is, reset to zero. Refer to SECTION P,
Controller, for further troubleshooting information.
c. No Data Strobe Pulses (At J4-25). Measure the voltage at
TP15. A logic HIGH pulse from the Controller should
appear at this point when the LOW, MEDIUM or HIGH
button is depressed.
d. Defective U17 or U18. If the BCD data, Data Clear-L, and
Data Strobe-H signals from the controller are correct but
there is no output from one or both latches, the latches may
be defective.
e. Defective Logic Drivers (U14, U16). For each logic driver
section, the input and output logic should always be the
same. The TTL inputs will switch between zero and +5
VDC. The CMOS outputs will switch between zero and
+15 VDC.
J.4.2
Symptom: Power Increases or Decreases in
Steps, Not Continuously
J.4.2.1
Possible Cause:
J.4.2.1.1
BCD Information incorrect
This could indicate that some bits or digits in the BCD power
control signal are not changing or that one or more PA Modules
have failed. Check the logic level signals for the BCD bits at
different power levels, at J4, at U18 outputs, and at U14 outputs.
J.4.3
Symptom: +15 VDC or -15 VDC faults
J.4.3.1 Possible Cause:
J.4.3.1.1
Defective Component
If the unregulated voltages to the board are present, the first thing
to check when troubleshooting a voltage regulator problem is the
input fuse F2 or F3. If the fuse is open, try replacing the fuse. If
there is a problem with the regulator IC, pass transistor, or zener,
the fuse will open again.
If the fuse opens again, measure the resistance of the supply to
ground with the low voltage OFF. If there is a very low resistance
to ground, remove any IC associated with the faulty supply until
the resistance increases.
J.4.3.1.2
Defective fault logic
Measure voltage at TP12, TP13 or TP14 depending on which
supply is indicating a fault. If the voltage is normal, but the fault
is still indicated, measure the fault output on U1-10 or U2-10. If
DX-25U
J-4 888-2297-002
WARNING: Disconnect primary power prior to servicing.