HEWLETT PACKARD (HP) 721A
Service Manual

Model 721A *erminals in series with approximately 1,000ohms. The output VOLTAGE ADJUST control should be idvanced until the standard m e t e r indicates 10 ma. rhe internal m e t e r should be switched to the 10 ma range. Adjust R25 until the internal m e t e r reads *hes a m e as the external meter. All other current ind voltage calibrations w i l l then be determined by :he precision current and voltage multiplier re&:or8 associated with the METER RANGE switch.
4) Adjust R8 to obtain 230 ma.

Sect. IV Page 5

NOTE
This adjustment provides the best over-all calibration of this circuit on all ranges. T h e circuit is slightly temperature sensitive. With the instrument in its cabinet and hot, the maximum current available will be approximately 225 ma.

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5) Reseal the control with duco cement.
1-7
SETTING M A X I M U M OUTPUT VOLTAGE

rhe maximum output voltage should be set to be 3 1 volts.
1) T u r n the VOLTAGE ADJUST control fullclock-

4-9 REPLACING THE POWER TRANSISTOR If you replace the power transistor (Ql) be careful

wise. 2) Measure the output voltage with either internal voltmeter or a n external standard voltmeter. 3) Adjust R21 to obtain 31 volts.
4) Reseal the control with duco cement.

to note how the nylon bushings are installed in the transistor mounting holes. They must be reinstalled case of the transistor is not at properly since the chassis potential. You should also check that there are no b u r r s on the transistor case which can cut through the anodized surface of the mounting plate. The anodized surface acts as a good electrical insulator while allowing good heat transfer from the transistor. If this surface is damaged the maximum voltage at which the power supply can be operated off of ground potential may be reduced.
4-10 REPLACING DIOD6 CR5, CR6 A N D CR7

4-8

C A L I B R A T I N G THE S H O R T CIRCUIT CURRENT CIRCUIT

1) Rotate the SHORT CIRCUIT CURRENT switch to 225 ma. 2) Short circuit the output terminals. 3) Rotate the VOLTAGE ADJUST control full clockwise.

The diodes CR5, CR6 and CR7 are manufactured by @. They have carefully controlled characteristics which result in a superior performing instrument. Should replacement be necessary we recommend that you use s i m i l a r diodes. You may obtain these from your local @Representative or by ordering directly from the factory. T h e cathode end of the diode is marked with aspot of red paint.