A' 721A Sect. IV Page 3
'ABLE 4-1. TROUBLE LOCALIZATION 20 v x .003 = 60 mv maximum change
CHART 10 v x .003= 30 mv maximum change
At voltages less than 10 volts, change fromno load
I Trouble Symptom Probable Cause to full load will result in less than 30 mv change
in voltage.
Output voltage unstable Defective reference
Poor regulation diode CR7.
Wandering voltage 10) Repeat test with line voltage at 103 and 127
volts.
Poor regulation (line Poor contact at end
or load) clips on CR1, CR2, 11) Adjust line voltage to 115 volts.
CR3 or CR4.
Defective Q1, 42, 43 12) Adjust output voltage to 30 volts and load to
449 200 ohms (for rated current). .
High noise or ripple Open C6 or possibly 13) Change line voltage tlO% to 103 and 127 volts.
on dc output (ap- c10. Output will change less than .3% (.09 v). This is
proximately 30 mv worst condition. At outputs below 5 v dc, regulation
50 to 150 kc) is within *15 mv.
Microphonics Noisy R19. 14) Measure ripple with Model 400D, use 1 mv full
scale range. Ripple + noise will be less than
Voltage control not Defective R19. 15Opv (0.15 mv).
smooth
4-5 MEASURING AC INTERNAL IMPEDANCE
The internal impedance of the supply is affectedby
4-4 CHECKING VOLTAGE REGULATION the ac gain of the regulator circuit. If the supply
AND RIPPLE has good dc regulation and low ripple ac in the out-
1 put the supply should also have low ac impedance.
1) Set Model 410B VTVM to 30 volt range. Figure 4-3 shows a suitable set-up for checking the
internal ac impedance if desired. The set-up shown
2) Adjust Model 721A to 30 volt output. should be followed faithfully if meaningful results
are to be obtained. The level of signal to be mea-
3) Adjust line voltage to 115 volts. sured is very low and ground loops in the system
can easily give very large errors. The measure-
4) Adjust Battery-Potentiometer to obtain 30 volts. ment is made by driving a constant 10ma alternat-
VTVM will then read 0 volt. ing current through the power supply and measuring
the IZ drop across the output terminals. The inter-
5) Switch VTVM to 1 volt range. nal impedance can be easily calculated by ohm's law.
6) Adjust bucking voltage so VTVM pointer sits
at about 1/2 scale.
4-6 METER CALIBRATION
7) Switch from full load to no load. The meter mechanical zero should be accurately
Load = 200 ohms. set before calibration. The correct way to do this
is to rotate the adjust screw clockwise until the
pointer swings up scale and then starts to swing
8) Voltage will not change more than 0.3% (0.09 v)
at 30 volts output, which is worst condition. down scale toward zero. Continue rotating the ad-
just screw clockwise until the pointer is exactly
over zero. If you overshoot, continue turning the
9) Repeat at 20 volts and 10 volts if desired. screw clockwise until the pointer is again approach=
(Short out one or two 66.7 ohm resistors to get ing zero from the up scale side.
150 ma rated load, depending on voltage selected).
Readjust bucking voltage so VTVM is again at The internal meter is calibrated by connecting an
mid-scale. external standard milliammeter across the output
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ACEC
HEWLETT-PACKARD
