BK Precision 2120C, Instruction Manual, Page: 22 / 30

MAINTENANCE
WARNING
The following instructions are for use by
qualified service personnel only. To avoid
electrical shock, do not perform any serv-
icing other than contained in the operat-
ing instructions unless you are qualified
to do so.
High voltage up to 2000 V is present when
covers are removed and the unit is oper-
ating. Remember that high voltage may
be retained indefinitely on high voltage
capacitors. Also remember that ac line
voltage is present on line voltage input
circuits any time the instrument is
plugged into an ac outlet, even if turned
off. Unplug the oscilloscope and dis-
charge high voltage capacitors before
performing service procedures.
FUSE REPLACEMENT
If the fuse blows, the “ON” indicator will not light and the
oscilloscope will not operate. The fuse should not normally
open unless a problem has developed in the unit. Try to
determine and correct the cause of the blown fuse, then
replace only with the correct value fuse. For 110/125 V line
voltage operation, use an 800 mA, 250 V fuse. For 220/240
V line voltage operation, use a 600 mA, 250 V fuse. The fuse
is located on the rear panel adjacent to the power cord
receptacle.
Remove the fuseholder assembly as follows:
1. Unplug the power cord from rear of scope.
2. Insert a small screwdriver in fuseholder slot (located
between fuseholder and receptacle). Pry fuseholder
away from receptacle.
3. When reinstalling fuseholder, be sure that the fuse is
installed so that the correct line voltage is selected (see
LINE VOLTAGE SELECTION).
LINE VOLTAGE SELECTION
To select the desired line voltage, simply insert the fuse
and fuse holder so that the appropriate voltage is pointed to
by the arrow. Be sure to use the proper value fuse (see label
on rear panel).
PERIODIC ADJUSTMENTS
Probe compensation and trace rotation adjustments
should be checked periodically and adjusted if required.
These procedures are given below.
Probe Compensation
1. Connect probes to CH 1 and CH 2 input jacks. Per-
form procedure for each probe, one probe at a time.
2. Set the probe to X10 (compensation adjustment is not
possible in the X1 position).
3. Touch tip of probe to CAL terminal.
4. Adjust oscilloscope controls to display 3 or 4 cycles of
CAL square wave at 5 or 6 divisions amplitude.
5. Adjust compensation trimmer on probe for optimum
square wave (minimum overshoot, rounding off, and
tilt). Refer to Fig. 11.
Trace Rotation Adjustment
1. Set oscilloscope controls for a single trace display in
CH 1 mode, and with the channel 1 AC-GND-DC
switch set to GND .
2. Use the channel 1 POSition control to position the
trace over the center horizontal line on the graticule
scale. The trace should be exactly parallel with the
horizontal line.
3. Use the TRACE ROTATION adjustment on the front
panel to eliminate any trace tilt.
Correct
Compensation
Over
Compensation
Insufficient
Compensation
Fig. 11. Probe Compensation Adjustment.
20
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