Agilent Technologies 856290216 service manual Kick-start/Bias, Buck Regulator

Kick-start/Bias

1.Measure the voltage at TP108 to verify the output of the input rectifier. The voltage should be between t215 Vdc and +350 Vdc.

2.If it is not within this range, check the rear panel fuse, input rectifier, input filter, and the rear-panel line voltage selector switch.

3.Measure the voltage at TP206 to verify the output of the kick-start circuitry. The voltage should be approximately +14 Vdc.

4.If there is no voltage at TP206, check TP210 for pulses 200 ms wide with an amplitude of 14.7 V. If there are no pulses present, the kick-start circuitry is probably defective. If the pulses are low in amplitude (about 1 V), Q201 is probably shorted.

5.If there are pulses at TP206, or there are pulses at TP210, but not at TP206, the buck regulator control circuitry is probably faulty.

Buck Regulator

1.Disconnect the power cord from the spectrum analyzer.

2.Connect the positive output of a current-limited dc power supply to the cathode of A6CR201 (J7 pin 2) and the ground to J7 pin 1.

3.Set the current limit of the power supply to about 500 mA and the voltage to 12 Vdc.

4.Make sure a jumper is connected from AGTPlOl to A6TP301. This independently powers the buck regulator control circuitry.

5.Connect a jumper from the dc power supply +12 Vdc output to the positive end of C207, nearest C209.

6.Connect a jumper from i-12 Vdc output to the end of A6R202 physically nearest A6U211.

7.If the current draw exceeds approximately 50 mA, suspect a short in the buck regulator control circuitry, or a shorted CR201, or a shorted U207. (Shorted components may be hot .)

8.Check TP204 for an 80 kHz sawtooth (4 Vp-p). Suspect U203 if it is not present.

9.Check TP203 and TP207 for 40 kHz square (12 Vp-p). The waveform edges should be sharp. Compare the waveform to the inputs of U207, pins 6 and 7. If the waveforms at either TP203 or TP207 are bad, one of the FETs in the DC-DC converter is probably defective.

10.Check TP105 and TP106 for a 12 Vp-p sawtooth waveform that is flattened at the bottom. If the waveform is a squarewave, the FET to which the test point is connected has failed or shorted.

11.Check TP202 for 80 kHz pulses (12 Vp-p).

12.Short the leads of CR108. Check TP103 for 80 kHz pulses of about 11 V peak-to-peak.

13.Remove the short from CR108. The voltage of the lower side of the pulses should float up to about 4.5 V. If not, suspect that CR108 is shorted.

14.If the waveform at TP202 is correct but the waveform at TP103 is bad, suspect either Q102 or U132. (The waveform should have sharp edges.)

Display/Power Supply Section 13-13