HEWLETT PACKARD (HP) 175A
Manual de Servicio

Section V Paragraphs 5-74 to 5-76 5-74. The cause of excessive output ripple on any of the supplies can be isolated in the following manner. Measure the ripple at the output of the rectifiers of the supply in question. Compare the measurement with the value given on the schematic diagram, figu r e 5-33. If the ripple is excessive, check the input filter capacitors and bleeder resistors. If the input ripple is normal, the cause is most likelylow gain in the amplifier transistors.

Model 175A 5-75. Table 5-5 gives troubleshooting aids for locating causes of loss of regulation in each supply. 5-76. The balance between the +6.3 volt and -6.3volt supply outputs and ground is determined by the supply load. The load is a series-parallel arrangement of tube heaters, diagrammed in figure 5-34. An unbalanced output voltage can usually be traced to an open o r shorted tube heater o r a wrong type tube.

Table 5-5. Troubleshooting the Low-Voltage Supply
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Symptom High output

Procedure

Effect

Trouble Q461 shorted Q461 good

Disconnect base lead of Q461 from Output remains high printed circuit board. This is the yellow lead which connects to r e a r Voltage drops edge of board adjacent to fan motor Reconnect yellow wire to printed circuit board and short collector of Q464 to emitter.

Voltage drops Output remains low Voltage rises
~

Q464 open Q461 open Q461 good CR465 shorted CR465 good Q463 open o r C466 shorted Q463 and C466 Q464 shorted Q464 good Trouble in +llOV

Low output

Short collector of Q462 to emitter

Remove short from Q462 and measu r e voltage across CR465 Measure voltage across R470

0 volt 6 volts
Within 10 volts of output L e s s than output

Measure voltage from collector of Q464 to emitter +llOV High output Check -100 volt supply output

0 volt approx 10 volts
Normal

Disconnect base lead of Q442 from printed circuit board. This is the white lead which connects to r e a r edge of board adjacent to fan motor Voltage drops Reconnect white lead and measure voltage from collector of Q443 to emitter Measure voltage from collector of Q444 to emitter
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0 volt

Q443 shorted Q443 good Q444 shorted Q444 good Trouble in + l l O V

Greater than 2 volts

0 volt Greater than 2 volts
Normal Abnormal

Low output

Check -100 volt supply output

Short collector of Q443 to emiiter

Output remains low Voltage r i s e s

I
I

Q441 o r Q442 open Q441 and Q442 good

I

5-14

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