14.Measure the loop-IF signal at the input to the IF Amp./Limiter (function block AK):

U401 pin 220 MHz (approximately -6 dBm)

15.Confirm the presence of a 20 MHz square-wave signal at TP405. The square wave should go from +0.6 V to +2.2 V.

16.Set HP 8560A to the following settings:

CENTER FREQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369.3 MHz SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz

17.Use an oscilloscope to confirm the presence of a 2.5 MHz TTL-level signal at TP406.

18.Confirm the presence of a 2.5 MHz TTL-level signal at TP407.

19.Force the loop’s phase/frequency detector to a lock condition by unsoldering one end of R436. Resistor R436 is located at the base of Q402 in function block AK of Al5 RF Schematic (sheet 3 of 3). This opens the path between the Sampling Oscillator and the phase/frequency detector.

20.If the dc power supply is connected to A15J200 pin 16, disconnect the power supply.

21.Connect A15TP406 to A15TP407. This forces both phase/frequency detector inputs to the same 2.5 MHz signal.

22.Observe the phase/frequency detector outputs, U406 pins 5 and 8, with an oscilloscope. Narrow TTL pulses should be present. Pin 5 is normally low, pulsing high, and pin 8 is normally high, pulsing low.

23.Check the cathode of CR401 with an oscilloscope. With the oscilloscope’s input ac-coupled, a sawtooth waveform approximately 5 mVp-p should be present.

24.Short C441 with a wire jumper. This changes loop integrator into a voltage follower. Refer to function block AB of Al5 RF Schematic (sheet 3 of 3).

25.The voltage at A15J200 pin 15 should measure +1.8 Vdc. Adjust A15R453 PHASE DET BIAS if necessary for +1.8 Vdc.

26.If the voltage at U408 pin 2 is not +1.8 Vdc, suspect U408.

27.Check the voltages at the following points:

A15U408 pin 6

+2

Vdc

(approximately)

A15J200 pin 16

+2

Vdc

(approximately)

28. Remove the jumpers.

Synthesizer Section lo-41