Troubleshooting Information - HP 10811A Operating & Service Manual

8-92. TROUBLESHOOTING INFORMATION

8-93. Symptoms of failures in the oscillator sections will generally fall into one of the following
categories:
1. No output.
2. Output amplitude is low or high.
3. Output is distorted (contain excessive harmonics).
4. Excessive drift of output frequency.
5. Time domain frequency stability (short-term stability) does not meet specifications.
8-94. Troubleshooting of these faults will be discussed in the following paragraphs.
8-95. NO OUTPUT. This is usually easy to repair by simple signal tracing. Localized faultfinding (to
actual defective component) can be somewhat more difficult if the problem is in the main
oscillator circuit (Q1, Q2, and AGC). If the fault appears to be in the oscillator section and does
not yield to normal troubleshooting techniques, measure the AGC voltage at the junction of CR5-
C13 (see Note 7 on
problem may be a defective quartz crystal (Y1). To verify this possibility, obtain a 10 µH (HP Part
No. 9100-2265) and a 12 µH inductor (9100-2242). Use the HP numbered parts as these have
been tested in the circuit. On the oscillator board, remove the red and blue wires connecting the
crystal to the board. Place the 12 µH inductor in place of these wires. With 12V applied to the
circuit, adjust the FREQ ADJUST (C1), and amplitude control (R6) for a good sine wave signal.
At some settings of C1 and/or R6, intermittent oscillations may appear.
Some minor adjustment of C1 and/or R6 should clear this. If this fails,
replace the 12 μH inductor with the 10 μH inductor and repeat the C1/R6
adjustment.
If replacing the crystal with an inductor produces oscillation, this is a very good indication of a
defective crystal. When replacing crystal Y1, read
ations. If the circuit will still not oscillate, the problem is most likely one of the oscillator circuit
elements.
8-96. OUTPUT AMPLITUDE HIGH OR LOW. Many times this can be cured by the adjustment of R6 as
described in paragraph
monitor the signal at Q6(C) with an oscilloscope and set R6 to obtain an amplitude of 2.8V p-p.
Then check Q5 and Q9 stages. If the R6 adjustment isn't effective, you should suspect the AGC
circuitry (Q3, CR4, CR5, C5, C6, R5, R6, R7, or Q1).
8-97. OUTPUT DISTORTION. Check the distortion with a spectrum analyzer (see
procedure). If the distortion products are harmonically related to 10 MHz, trace the signal to the
distorting stage. If the distortion is not harmonically related to the 10 MHz output:
1. Check the mode suppression components of L2/C5 and/or L3/C6. These components
suppress oscillations at all frequencies other than 10 MHz.
2. Check for spurious oscillations from the amplifier stages and oscillator transistors.
8-98. EXCESSIVE DRIFT OF OUTPUT FREQUENCY. When a quartz crystal oscillator has not been
operated for a long period of time, or if it has been subjected to severe thermal or mechanical
shock, the oscillator may take some time to stabilize. In most cases, the crystal will drift and then
stabilize at or below the specified rate within a few days after being turned on. In isolated cases,
depending on the amount of time the oscillator has been off and the environmental conditions it
has experienced, the 10811A/B may take up to 1 week to reach the specified aging rate. This
should be taken into consideration if the drift rate of the unit is out of specifications. If the unit has
had sufficient time to stabilize but is still out of specification, the most likely cause of excessive
drift is a defective crystal (Y1). If Y1 is to be replaced, read
causes are unstable C3 and/or C8.
8-4, Oscillator Normal Voltages). If this voltage appears normal, the
Table
NOTE
5-14. If the correct amplitude cannot be obtained with this adjustment,
paragraph 8-68(b), Special Parts Consider-
paragraph 8-68(b). Other possible
Model 10811A/B
Table 4-2 step 2. for
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