3 3 . If the voltage monitored in step 23 is correct with a 0 dBm output but not with -15 dBm output, suspect A14U201 or A14U202. If the voltage does not change, check the YTO PLL Loop’s divide-by-two circuits as follows:
Set the signal generator for an 80 MHz output.
Measure A14U203 pin 2 with an active probe and a spectrum analyzer. The signal should be approximately 0 dBm (ECL levels) and 47.35 MHz (94.7 MHz Main Roller Frequency divided by two).
The signal at A14U205 pin 2 should measure 40 MHz at approximately 0 dBm.
Note | A14U205 is turned off during YTO FM coil sweeps (LO spans between 1.01 |
| and 20 MHz). |
| |
34.Place jumper A14J23 in the NORMAL position and connect W32 to A14J501.
35.Steps 27 through 31 verify that the YTO-loop error voltage is reaching the FM coil. Refer to function blocks N and 0 of Al4 Frequency Control Schematic (sheet 2 of 5) in the Component-Level Information binder. To troubleshoot the YTO FM Coil Driver, refer to step 6 of “First LO Span Problems (1 MHz to 20 MHz).”
36.Remove jumper A14J23 and connect a dc power supply to A14J23 pin 2. Set the dc power supply to t7.5 Vdc.
37.Verify the nominal test-point voltages listed in Table 10-13.
38.Change the input voltage to -7.5 volts and re-verify that the voltages listed in Table lo-13 are the same except for a change in polarity.
39.The Main-Loop Error-Voltage Driver has a gain of either 1.5 or 15; the analyzer’s firmware controls the gain during the locking process. The error voltage is read by the ADC on the A3 Interface assembly. U324D calibrates out any offsets from true ground. A14U326A inverts the sense of the YTO loop to lock the YTO on lower sampler-sidebands (YTO frequency < (sampler frequency xsampler harmonic)). The Main Roller frequency indicated in the FREQ DIAGNOSE menu will be negative in this situation. Changing the CENTER FREQ to 678.8 MHz with a SPAN of 0 Hz will change the switch setting of U326A and invert the voltages listed in Table 10-13.
Table 10-13. Voltages in FM Coil and Main Loop Drivers
Measurement Points | Voltages |
A14U405 pin 6 | +2.8 Vdc |
A14U322 pin 2 | 0 Vdc |
A14J17 pin 4 | >+12 Vdc |
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40. Place jumper A14J23 in the TEST position.
lo-30 Synthesizer Section