HP 6268B Operating And Service Manual

d. Turn CURRENT controls fully clockwise, turn on
supply, and adjust VOLTAGE controls for maximum
rated output voltage.
e. Adjust oscilloscope to observe 120Hz sawtooth
waveform. Peak amplitudes of adjacent sawtooth peaks
should be within 25% of each other.
f. tf amplitude difference is greater than 25%, turn off
supply and replace R82 with decade resistance.
g. Turn on supply and adjust decade resistance to
reduce imbalance to within 25%.
h. Vary input line voltage from 208 to 254Vac and
insure that excessive imbalance does not exist anywhere
within this range. Replace decade box with equivalent
resistor.
NOTE
If imbalance cannot be reduced to within
25%, check capacitors C70 and C72, and
diodes CR79 through CR84. If these
components test satisfactorily, the problem
may be due to distortion present on the
ac po wer line.
5-102 Preregulator Tracking Adjustment
5-103 To adjust the voltage drop across the series regu¬
lator, proceed as follows:
a. Connect load resistor specified in Figure 5-1 across
rear output terminals of supply.
b. Connect variable autotransformer between input
power source and power supply input and adjust it for
a 230Vac input to the supply.
c. Connect dc voltmeter across series regulator
(between TP102 and TP103).
d. Turn CURRENT controls fully clockwise.
e. To check voltage drop across regulator at low output
voltage, short circuit the load resistor and adjust VOLTAGE
controls for maximum rated output current on front panel
ammeter.
f. Adjust R70 (RAMP ADJ.) until voltmeter reads
3.5 ±0.3Vdc.
g. To check the voltage drop at high output voltage,
remove short circuit from across load resistor and adjust
VOLTAGE controls for maximum rated output current.
Voltmeter reading should again be 3.5 ±0.3Vdc.
h. Vary input line voltage from 208 to 254Vac.
Voltmeter reading should vary between 3.2 and 3.8Vdc.
If reading exceeds this range, proceed to step (i).
i. Replace R77 with decade box. Vary input line
voltage between 208 and 254Vac and adjust decade box
until voltmeter reading variation is minimal and within
range of 3.2 to 3.8Vdc. Replace decade box with equivalent
resistor.
5-104 Crowbar Trip Voltage Adjustment
5-105 To adjust the voltage at which the crowbar fires,
proceed as follows:
a. Turn front panel OVERVOLTAGE ADJUST
potentiometer A5R1 25 fully clockwise.
b. Turn on supply.
c. Set output voltage to desired trip voltage.
d. Turn A5R125 slowly counterclockwise until the
crowbar fires (amber OVERVOLTAGE lamp lights and
voltmeter indication falls to zero).
e. Turn off supply and turn down output voltage controls.
f. Turn on supply and set desired output voltage.
NOTE
It is recommended that the crowbar trip volt¬
age be set higher than the normal output volt¬
age by no less than 5% of the output voltage
plus 2 volts for the Models 6259B, 6260B,
or 6261B, or 5% of the output voltage plus
one volt for the Models 6268B or 6269B. If an
occasional tripping of the crowbar can be
tolerated as a load is being disconnected, the
crowbar trip point can be set much closer to
the operating voltage of the supply.
5-106 Maximum Crowbar Trip Voltage Adjustment
5-107 To adjust the maximum voltage at which the
crowbar fires, proceed as follows:
a. Rotate A5R125 (OVERVOLTAGE ADJUST) and
CURRENT controls fully clockwise.
b. Disconnect one end of R72 (at TP/0 or TP71) to
temporarily disable the overvoltage limit circuit.
c. Connect decade resistance box in place of R95
(mounted on standoffs on main circuit board).
d. Turn on supply and adjust VOLTAGE controls for
output voltage shown below:
Model
Value
6259B
12Vdc
6260B
12Vdc
6261 B
23Vdc
6268B
45Vdc
6269B
45Vdc
e. Adjust decade resistance box until crowbar fires.
f. Replace decade resistance with appropriate value
resistor in R95 position and reconnect resistor R72. Maxi¬
mum crowbar trip voltage is now set at voltage given in step (d).
5-108 Disabling the Crowbar
5-109 To disable the crowbar completely, disconnect
either end of R98 atTP96 or TP97 on the main circuit board.
5-20

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English