HP 6268B Operating And Service Manual

from chassis until it is resting on table (sufficient lead
length is provided). Gentle leverage with a thin screw¬
driver may be necessary to allow heat sink assembly to
clear upper chassis flange. Access is now provided to all
components mounted on heat sink except resistors A4R1 50
through A4R165, and A4R123.
5-67 A4 Heat Sink Disassembly. To gain access to
resistors A4R123 and A4R1 50 through A4R165, it is
necessary to disassemble the heat sink assembly as
follows:
a. Remove heat sink assembly as described in paragraph
5-66 above.
b. Turn supply upside down and place heat sink
assembly partially into chassis so fan (A4B1) is protruding
above chassis.
c. Remove four screws and four shoulder washers
attaching fan mounting plate t o heat sink. Do not remove
fan from mounting plate. When reassembling heat sink,
do not overtighten these screws. Too much tension will
damage the insulating rods.
d. Remove two screws holding current sampling
resistor A4R123 to heat sink. If necessary, the resistor
may be unsoldered at this point.
e. Remove mounting nuts from A4CR106 (and
A4CR107) on left side of heat sink, and from A4CR108
on right side of heat sink. Remove mounting nuts, bolts
and shoulder washers from transistor A4Q102 on right
side of heat sink (see Figure 7-5).
f. Slide this section of heat sink forward and off
insulating rods.
g. Remove four screws holding emitter resistor circuit
board to adjoining heat sink section. Access is now provided
to series regulator emitter resistors A4R150 through
A4R165 (see Figure 7-9).
h. To remove emitter resistor circuit board completely,
unsolder connections to board, marking wires to permit
proper replacement.
5-68 A3 Interconnection Circuit Board Removal. To
replace capacitor A3C3 or transformer A3T2, (shown in
Figure 7-2), it is necessary to remove the interconnection
circuit board by following the following procedure:
a. Remove main circuit board, RFI assembly, and heat
sink assembly as described in paragraphs 5-62, 5-65, and
5-66.
b. Remove six screws holding back panel to chassis
frame.
c. Stand supply on left side, and remove two screws
holding main circuit board support tray to back panel.
Move panel away from frame.
d. Remove two screws holding main circuit board
support tray to internal chassis divider.
e. Working from top rear of supply, interconnection
circuit board (still attached to main circuit board support
tray) can be angled up enough to allow access.
f. I f necessary to completely remove interconnection
circuit board, remove two screws holding board to support
tray, one screw holding A3C3 capacitor clamp to support
tray, and two screws holding bias transformer A3T2 to
support tray. Unsolder connections to board, marking wires
to enable correct replacement, and remove board.
5-69 REPAIR AND REPLACEMENT
5-70 Section VI of this manual contains a list of
replaceable parts. If the part to be replaced does not have
a standard manufacturers' part number, it is a special part
and must be obtained directly from Hewlett-Packard.
After replacing a semiconductor device, refer to Table
5-8 for checks and adjustments that may be necessary. All
components listed in Table 5-8 without A-designators are
on the A1 main printed circuit board.
5-71 ADJUSTMENT AND CALIBRATION
5-72 Adjustment and calibration may be required after
performance testing, troubleshooting, or repair and replace¬
ment. Perform only those adjustments that affect the
operation of the faulty circuit.
5-73 Meter Zero Adjustment
5-74 The meter pointer must rest on the zero calibration
mark on the meter scale when the instrument is at normal
operating temperature, resting in its norma! operating
position, and turned off, To zero the meter proceed as
follows:
a. Connect load resistor of value shown in Figure 5-1,
turn on instrument, and allow it to come up to normal
operating temperature (about 30 minutes).
b. Turn instrument off and wait two minutes for power
supply capacitors to discharge completely.
c. Insert pointed object (pen point or awl) into small
indentation near top of round black plastic disc located
directly below meter face.
d. Rotate plastic disc clockwise until meter reads zero,
then rotate counterclockwise slightly in order to free
adjustment screw from meter suspension. Pointer should
not move during latter part of adjustment.
5-75 Voltmeter Calibration
5-76 To calibrate the voltmeter, proceed as follows:
a. Connect dgital voltmeter across plus and minus
output terminals of supply, observing correct polarity.
b. Turn on supply and adjust VOLTAGE controls until
digital voltmeter reads exactly the maximum rated output
voltage.
c. Adjust R106 until front panel voltmeter also
5-16

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English