HP 8560A Service Manual page 378

Note If the Transfer Roller Oscillator is locked, the address decoder is not at fault.
Address decoder U302 decodes a single address for both the Offset Oscillator
Pretune DAC and the Transfer Oscillator Pretune DAC.
9. If the pretune voltage is within the range listed in step 7, the Offset Oscillator is probably
at fault.
10. Measure the Offset Oscillator's frequency at A14TP302. If the frequency is not 200 MHz
Place the counter in the high-input impedance function.
11. Set the HP 8560A to the following settings:
C E N T E R F R E Q
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Hz
12. Check that UlOl pin 1 is high. This indicates that prescaler UlOl is in the divide by 128
mode. Refer to function block AC of Al4 Frequency Control Schematic (sheet 5 of 5).
13. The signal's frequency at UlOl pin 3 should be the Offset Oscillator's frequency divided
by 128. The frequency would be 1.562500 MHz if the Offset Oscillator was locked at
200 MHz.
14. The amplitude of the signal at UllO pin 3 should measure greater than 0.1 Vp-p.
Note
Integrated circuit UllO contains the reference and VCO dividers, phase
detector and modulus (pre-scaler) control circuitry.
15. Set the HP 8560A to the following settings:
C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..300MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Hz
16. Check that there are narrow (<l psec) pulses 400 psec apart at UllO pin 18 (reference
divider output) and UllO pin 15 (VCO divider output). Refer to function block AC of
Al4 Frequency Control Schematic (sheet 5 of 5).
17. Short A14AlOl pin 14 of the Offset Oscillator to A14AlOl pin 20. This sets the Offset
Oscillator's TUNE+ line at +lO V, setting the Offset Oscillator to its maximum
frequency.
18. The error voltage measured at Ulll pin 1 should be greater than f9 Vdc.
19. Short A14AlOl pin 14 of the Offset Oscillator to ground. The error voltage at Ulll pin 1
should measure less than -9 Vdc.
Note Under some unlocked conditions, the Offset Oscillator loop may appear to
become unstable. (The error voltage will oscillate wildly.) This is normal
behavior brought on by the conduction of zener diodes VR308 and VR309
which short out R336 to improve the lock time.
20. Remove the shorts from A14AlOl.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..300MHz