TM 11-6625-2837-14&P-7 TECHNICAL MANUAL OPERATOR'S ...

SERVICE SHEET 2 (Cont’d)
TM 11-6625-2837-14 & P-7
Assembly related components. Refer to Service Sheet 3 if the input is incorrect. If necessary refer to Section V and perform the RF
Output Level and 1 dB Step Attenuator Adjustment procedures. If the Adjustments cannot be done or do not correct the tracking
across the VERNIER range, check the Meter Driver and meter circuitry, and the AM Gain circuits. Refer to Service Sheets 6 and 7
respectively. Also check the circuits in the A4 Assembly which are influenced by the 10H input.
Test 4. Proceed to Test 5 if the RF output level is higher than normal. The RF outputs listed in each step of this test (4) are lower
than normal. The voltages enclosed in parenthesis are Modulator Bias Signal ranges. They indicate that the ALC loop is (1) holding
the RF output low, (2) is trying to increase the RF output or (3) that a quiescent level, although incorrect, has been reached. Refer to
the block diagram for the normal range of Modulator Bias Signal levels.
a. The RF output is low but the ALC loop is trying to increase the level (>-3 Vdc). Check the RF outputs of FL1,
A7, and A6 to isolate the problem to Service Sheets 4 (for other than option 002 instruments), Service Sheets 4 or 5 (option 002
instruments only), or Service Sheet 6 respectively.
If the output of FL1 is correct and the output of A7 is incorrect, the problem may be on either Service Sheets 4 or 5 in option 002
instruments. In this case, refer to the LO Signal Circuits Repair procedure and the Troubleshooting Block Diagram to isolate the
problem to an assembly or cable.
On other than option 002 instruments, if the output of A7 is defective, refer to Service Sheet 4.
Each of these assemblies and circuits, if defective, must be replaced as a unit with the exception of A7. If A7 is defective, refer to
Service Sheet 4 for further troubleshooting information.
b. The RF output is low and the ALC loop is holding the modulator Bias Signal level low (>+10 Vdc). First, check
the A10 reference Assembly output with the VERNIER control set to the pw and ccw position with A4S1 in the Normal position. If the
output is abnormal, refer to the troubleshooting information on Service Sheet 7. A normal output indicates the defect is either on the
A3 ALC Assembly, or the A4 Detector Amplifier Assembly.
Set the A4S1 switch to the Test position. If the Modulator Bias Signal exhibits the same response as shown in the following table, the
problem is probably in the A4 Detector Amplifier Assembly. (Check the Detector Signal input at A4 pin 11.)
System Troubleshooting Block Diagram
SERVICE SHEET 1
8-18B