11.If there are pulses at TP206, or there are pulses at TP210, but not at TP206, the Buck Regulator Control circuitry is probably faulty.

12.Disconnect the power cord from the HP 8560A.

13.Connect the positive output of a current-limited dc power supply to the cathode of A6CR201 and the ground to A6TP201.

14.Set the current limit to about 500 mA and the voltage to 12 Vdc.

15.Connect a jumper from the power supply’s +12 V output to the end of A6R202 physically nearest A6U211.

16.Connect a jumper from AGTPlOl to A6TP301. This independently powers the Buck Regulator Control circuitry.

17.Connect a jumper from +12 Vdc to the end of C207 nearest C209.

18.If the current draw exceeds approximately 50 mA, suspect a short in the Buck Regulator Control circuitry or a shorted CR201.

19.Check TP204 for an 80 kHz sawtooth (4 Vp-p).

20.Check TP203 and TP207 for 40 kHz square (12 Vp-p). If the waveforms at either TP203 or TP207 are bad, one of the FETs in the DC-DC Converter is probably defective.

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

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

23.Short TP401 to TP102. Check TP103 for a waveform similar to that in Figure 12-4.

24.If the waveform at TP202 is correct but the waveform at TP103 is bad, suspect either QlO2 or CR106.

Figure 12-4. Buck Regulator Waveform

12-12 Display/Power Supply Section