Figure 8-21. E-Ref [U20 (Pin 9)] - HP 5501B Operating And Service Manual

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Is U13 (Pin 6) < -10 Vdc (i.e., more negative)? If yes, go to step I. If no, go to step f.

Is U13 (Pin 3) approximately -0.1 Vdc? If yes, check R33 or U13. If no, check R49, R50,

R51, and U13.

While observing voltage at A3TP11, move POWER AMP jumper from DWN to UP. TP11

voltage should change from 0 Vdc to waveform of Figure 8-20. If OK, go to step h. If not,

check U20 (Pins 11, 12, 13, and 14) for activity (see A3 schematic for waveforms). If

waveform activity is not present, check U20 (Pins 3 and 7) for clock activity (see A3

schematic for waveforms). If waveforms and clock activity are OK, check A1 power amp

transistors for (A1Q5-9) proper operation. Check E-REF input to U20 (Pin 9). Waveform

should be as in Figure 8-21.

Figure 8-21 oscilloscope settings are as follows:

Set HEATER jumper to OFF. This applies +5V through A1R62 to U20 (Pin 10), via the

DISABLE line, disabling U20. If voltage at U20 (Pin 20) reads between 2 and 3 Vdc, go to

step i. If voltage is

approximately +5 Vdc, that input is open.

Set POWER AMP to UP, HEATER jumper to NRM. U13 (Pin 6) should read> +13 Vdc.

NOTE

Time/Division:

20 microseconds/division

Volts/Division:

1 Volt division

Coupling:

Figure 8-21. E-REF [U20 (Pin 9)]

2 Vdc, U20 (Pin 10) could be shorted. If U20 (pin 10) measures

HP5501B

Service

8-45

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