Model 721A Sect. IV Page 5
*erminals in series with approximately 1,000 ohms. 4) Adjust R8 to obtain 230 ma.
The output VOLTAGE ADJUST control should be
idvanced until the standard meter indicates 10 ma. NOTE
rhe internal meter should be switched to the 10 ma
range. Adjust R25 until the internal meter reads This adjustment provides the best over-all cali-
*he same as the external meter. All other current bration of this circuit on all ranges. The circuit
ind voltage calibrations will then be determined by is slightly temperature sensitive. With the instru-
:he precision current and voltage multiplier re&- ment in its cabinet and hot, the maximum current
:or8 associated with the METER RANGE switch. available will be approximately 225 ma.
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5) Reseal the control with duco cement.
1-7 SETTING MAXIMUM OUTPUT VOLTAGE
rhe maximum output voltage should be set to be 4-9 REPLACING THE POWER TRANSISTOR
31 volts.
If you replace the power transistor (Ql) be careful
to note how the nylon bushings are installed in the
1) Turn the VOLTAGE ADJUST control fullclock-
transistor mounting holes. They must be reinstalled
wise.
properly since the case of the transistor is not at
chassis potential. You should also check that there
2) Measure the output voltage with either internal
are no burrs on the transistor case which can cut
voltmeter or an external standard voltmeter.
through the anodized surface of the mounting plate.
The anodized surface acts as a good electrical in-
3) Adjust R21 to obtain 31 volts.
sulator while allowing good heat transfer from the
transistor. If this surface is damaged the maximum
4) Reseal the control with duco cement.
voltage at which the power supply can be operated
off of ground potential may be reduced.
4-8 CALIBRATING THE SHORT CIRCUIT
4-10 REPLACING DIOD6 CR5, CR6 AND CR7
CURRENT CIRCUIT
The diodes CR5, CR6 and CR7 are manufactured
1) Rotate the SHORT CIRCUIT CURRENT switch by @. They have carefully controlled character-
to 225 ma. istics which result in a superior performing instru-
ment. Should replacement be necessary we rec-
2) Short circuit the output terminals. ommend that you use similar diodes. You may
obtain these from your local @Representative or
3) Rotate the VOLTAGE ADJUST control full clock- by ordering directly from the factory. The cathode
wise. end of the diode is marked with aspot of red paint. |
ACEC
