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8-84. OSCILLATOR TROUBLESHOOTING
8-85.
The oscillator circuits are relatively simple and straightforward. The following paragraphs will
briefly describe the major circuit areas, a troubleshooting outline, and some helpful suggestions to
make the troubleshooting process easier. The oscillator consists of four sections. They are:
1.
Oscillator Q1, Q2, and associated circuitry.
2.
AGC Q3, CR4, CR5, and R6.
3.
Output circuit Q5, Q9.
4.
5.7V power supply CR2, CR3, and Q4.
The oscillator is the signal source. Its output level is controlled by the AGC. The 5.7V power
supply provides an extra-stable clean voltage source for the oscillator circuits. The output circuits
provide a high level signal capable of driving a 50 to 1K ohm load.
8-86. NORMAL OPERATION
8-87.
The output of the oscillator circuit at Q2(C) is a 10 MHz undistorted sine wave; with an amplitude
of approximately 2.8V p-p. The AGC voltage (measured at CR5-C13 junction) is approximately -
1.5V. The 10 MHz signal passes through Q5 to Q9(B) at about the same level. The voltage gain
of amplifier Q5 (base to collector) is approximately 2 with a 50-ohm load on the output. The
output of transformer T1 is approximately 1.5V p-p. All10 MHz signals found in the 10811A/B will
be undistorted sine waves unless otherwise noted in
8-88. OSCILLATOR TROUBLESHOOTING TECHNIQUES
8-89.
When troubleshooting the oscillator section, remove the oven mass from the housing and the
covers from the oven mass as described in
use special connector described in
oscillator circuit common in 10811B. Set the power supply current limit to 60 mA. Do not apply
power to the oven circuits!
8-90.
Initial troubleshooting and probing should be done on the backside of the boards (trace side)
while they are secured to the oven mass (see
more easily handled. When the fault is isolated to a few components, the unit may then be
disassembled for final troubleshooting and repair.
8-91.
Helpful Hints
1.
Most points in the oscillator circuits cannot be measured with a dc voltmeter. The reactance
of the voltmeter probe and leads will load the circuit and give false readings. Instead, use an
oscilloscope with a high input impedance probe for these measurements.
section normal voltages) indicates when a dc voltmeter can be used.
2.
Before reinstalling the oven mass into the housing, adjust the output amplitude (with R6) to
0.53V into a 50-ohm load. Although this value is slightly below normal, the output will
increase slightly when the unit is reassembled and is at normal operating temperatures.
3.
If a tear in the flex circuit occurs, go to
58
Table 8-4
paragraph
8-59. Connect 12V to the oscillator section;
paragraph 8-61
for 10811A, connect power to T1(1) and
paragraph 8-59, step
paragraph 8-82
for repair instructions.
Model 10811A/B
(next to the schematic diagram).
10). This way the circuits are
Table 8-4
(oscillator
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Troubleshooting
