HP 8560A Service Manual page 382

13. If the Transfer Oscillator drive signal is not present, check the XFER AMP BIAS at J303
pin 3. The voltage should measure +2 Vdc to +4 Vdc.
14. Check that the voltage at U424B pin 7 is nominally -14 Vdc. Diodes CR302 and CR303
form a PIN diode switch that turns off the Transfer Oscillator drive to the mixer in Main
Roller Spans (100 kHz <LO Span 51 MHz). Refer to function block AI of Al4 Frequency
Control Schematic (sheet 5 of 5).
15. Check that control signal XFEROSCSW is near 0 V (U424B pin 5). If XFEROSCSW is
not near 0 V, check the Roller Interface Latch U401 and its address decoder U303. Refer
to function block AG of Al4 Frequency Control Schematic (sheet 5 of 5) and function
block B of Al4 Frequency Control Schematic (sheet 2 of 5).
16. If the signal amplitude at U109 pin 1 (refer to step 11) is within range and near 2 MHz
in frequency, measure the signal with an oscilloscope at U109 pin 7. The voltage should
nominally be 100 mVp-p.
17. The waveform at U106 pin 7 should be a square wave with a lower limit between 0 V and
Frequency Control Schematic (sheet 5 of 5).
18. The signal at U105 pin 1 should have a similar waveform as U106 pin 7, but may be
different in amplitude from the Offset Oscillator Dividers U103 and U104.
19. The Offset Oscillator Divider, function block AJ, is probably at fault if the signal at U105
pin 1 is not present and the Offset Roller is locked.
20. If the input signals to the Main Roller Phase Detector U105 (measured in steps 17 and
18) are in range, check for narrow (<O.l ps) pulses 0.5 ps apart on U105 pins 2 and 13.
21. Short U115B pin 5 to ground and check that U115B pin 7 is nominally -9.1 Vdc.
22. Short U115B pin 6 to ground and check that U115B pin 7 is nominally +9.1 Vdc.
23. If ERR 304 is displayed and the Main Oscillator PLL is locked, troubleshoot Roller MUX
AG in Al4 Frequency Control Schematic (sheet 5 of 5).
24. If U305, U328, and U326B are working properly, suspect a malfunction of the A3 Interface
Assembly or the W2 Control Cable.
25. Remove the jumpers.