Table of Contents
Advertisement
Model 721A
Sect. IV Page 3
TABLE 4-1.
TROUBLE LOCALIZATION
CHART
Trouble Symptom
Probable Cause
No output, fuse blows
Shorted Cl or T1
No output, fuse does not
blow
Shorted C8
Output voltage unstable
Poor regulation
Wandering voltage
Defective reference
diode CR7.
Poor regulation (line
or load)
Defective Q1,Q2,Q3,
Q4 or CR1, CR2, CR3,
CR4.
High noise or ripple
on dc output (approxi¬
mately 30 mv 50 to
150 kc)
Open C6 or possibly
CIO.
Microphonics
Noisy R19.
Voltage control not
smooth
Defective R19.
1) Set Model 410B VTVM to 30 volt range.
2) Adjust Model 7 21A to 30 volt output.
3) Adjust line voltage to 115 volts.
4) Adjust Battery-Potentiometer to obtain 30 volts.
VTVM will then read 0 volt.
5) Switch VTVM to 1 volt range.
6) Adjust bucking voltage so VTVM pointer sits
at about 1/2 scale.
7) Switch from full load to no load.
Load « 200 ohms.
8) Voltage will not change more than 0.3% (0.09 v)
at
30
volts
output,
which
is worst condition.
9) Repeat at 20 volts and 10 volts if desired.
(Short out one or two 66.7 ohm resistors to get
150 ma rated load, depending on voltage selected).
Readjust bucking voltage so VTVM is again at
mid-scale.
20 v x .003 = 60 mv maximum change
10 v x .003 = 30 mv maximum change
At voltages less than 10 volts, change from no load
to full load will result in less than 30 mv change
in voltage.
10) Repeat test with line voltage at 103 and 127
volts.
11) Adjust line voltage to 115 volts.
12) Adjust output voltage to 30 volts and load to
200 ohms (for rated current).
13) Change line voltage
+10% to 103and 127 volts.
Output will change less than .3% (.09 v). This is
worst condition. At outputs below 5 v dc, regulation
is within ±15 mv.
14) Measure ripple with Model 400D, use 1 mv full
scale range.
Ripple +
noise will be less than
150 /±v (0.15 mv).
4-5
MEASURING AC INTERNAL IMPEDANCE
The internal impedance of the supply is affected by
the ac gain of the regulator circuit. If the supply
has good dc regulation and low ripple ac in the out¬
put the supply should also have low ac impedance.
Figure 4-3 shows a suitable set-up for checking the
internal ac impedance if desired. The set-up shown
should be followed faithfully if meaningful results
are to be obtained. The level of signal to be mea¬
sured is very low and ground loops in the system
can easily give very large errors. The measure¬
ment is made by driving a constant 10 ma alternat¬
ing current through the power supply and measuring
the IZ drop across the output terminals. The inter¬
nal impedance can be easily calculated by ohm,s law.
4-6
METER CALIBRATION
The meter mechanical zero should be accurately
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
down scale toward zero. Continue rotating the ad¬
just screw clockwise until the pointer is exactly
over zero.
If you overshoot, continue turning the
screw clockwise until the pointer is again approach¬
ing zero from the up scale side.
The internal meter is calibrated by connecting an
external standard milliammeter across the output
Advertisement
Table of Contents
