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Sect. IV Page 1
ECTION IV
MAINTENANCE
Adjustments and voltages referred to in this maintenance section are
intended for maintenance purposes only and are not intended to be in
terpreted as specifications.
4-1
INTRODUCTION
This section contains instructions for adjusting and
servicing the Model 120A
scilloscope, Components
of the Model 120A are readily accessible by remov
ing the instrument from the cabinet. All tubes and
transistors are removeable from above the chassis
while other individual components are easily reached
from below the chassis. Conventional point-to-point
wiring is used throughout so that the Model 120A may
be easily repaired with only the minimum of tools.
Whenever possible standard, readily obtainable, com
ponents have been used.
ther special components
may be obtained directly from the Hewlett-Packard
Company. When ordering directly from ($) be sure to
specify model and serial number of instrument and
description of component as given in table of re
placeable parts together with the stock number.
In general, sections in the Model 120A will be found
behind the controls for that section on the front panel.
The power supplies are in the rear; the low voltage
on top of the chassis and the high voltage in the
shielded box underneath and the transistor-regulated
filament supply on the upper chassis brace. Controls
and tubes are marked on the chassis. The material
in this section is divided according to circuit func
tions, each section having a complete set of adjust
ment instructions and, at the rear of the manual, a
schematic and a voltage-resistance diagram. The
material in this section is as follows:
4-2
Removing the Cabinet.
4-3
Connecting for 230-volt Power Lines.
4-4
Tube Replacement.
4-5
Isolating Troubles to Major Sections.
4-6
Adjustment Procedure.
4-7
Turn
n.
4-8
Power Supplies.
4-9
Trigger Generator.
4-10
Vertical Amplifier.
4-11
Horizontal Amplifier.
4-12
Sweep Generator.
4-2
REMOVING THE CABINET
First, remove the power cord from the wall recepta
cle. Remove the two large screws at the rear of the
cabinet. Do not remove any screws from the front
panel. Push the instrument from the cabinet by push
ing the rear of the chassis through the power cord
hatch at the rear of the instrument. Now slide the in
strument from the cabinet by pulling forward. Pull the
power cord through hole in the cabinet. The instrument
will now be completely free for servicing.
CAUTI N
When the cabinet is removed, dangerous voltages are
exposed. Take adequate safety precautions, especially
when working around the CRT terminals and power
supplies.
4-3
CONNECTING FOR 230-VOLT POWER LINE
The 120A is normally shipped from the factory with
the dual primary of the power transformer windings
connected in parallel for use on 115-volt ac lines, un
less otherwise specified on the order. The windings
can easily be reconnected in series for use on 230-
volt power, if desired. First find the power supply
input terminal strip located next to the power trans
former. Notice that the two outer terminals on each
side are jumpered together, connecting the windings
in parallel. Remove these jumpers and connect an in
sulated jumper between the second and fourth termi
nals, connecting the windings in series. Do not at
tempt to use this instrument on any voltage direct
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