HP 120A - Page 42

Sect. IV Page 2
current. DO NOT ATTEMPT TO OPERATE THIS
INSTRUMENT WITH A HIGHER VOLTAGE THAN 130
VOLTS IF CONNECTED FOR 115 VOLTS INPUT OR
ABOVE 260 VOLTS IF CONNECTED FOR 230 VOLT
OPERATION. Higher voltages will damage the trans 
istors.
4-4 TUBE REPLACEMENT
The heaters of VI and V2 are connected in series
from a regulated de voltage obtained from the Low-
Voltage Power Supply. If one of these tubes burns
out the other will go out also. Replace tubes one at
a time, ff replacing a tube does not cure the trouble,
replace the original tube as it is necessary to per 
form tests when replacing some tubes. See the tube
replacement chart.
4-5 ISOLATING TROUBLES TO MAJOR SECTIONS
If no trace can be found on the face of the cathode -ray
tube set all of the controls exactly as shown on the
two color fold-out diagram at the beginning of sec 
tion IL
IF THE INSTRUMENT DOESN ’ T OPERATE AT ALL,
check the ac circuit. First, check to see that the
power is supplied to the wall socket and that the plug
is securely in the socket. Next, check the fuse by
substitution, even though the original fuse appears
good. If the instrument still won ’ t turn on, remove
it from the cabinet and check continuity through the
switch, the transformer and the wiring.
IF THE INSTRUMENT TURNS ON BUT DOESN ’ T
PERFORM SOME FUNCTION, first be sure that the
cause isn ’ tmis-setting of the front panel controls .
Set TRIGGER LEVEL control to AUTO. If this isn ’ t
the trouble, try to localize the trouble to a particu 
lar main section by operating all functions of the
instrument and observing which it will perform and
which it will not perform. From these observations
it should be possible to determine in which section
the fault lies.
AFTER THE TROUBLE HAS BEEN LOCALIZED TO A
PARTICULAR SECTION, check visually for failures,
such as, charred resistors or tubes not lit. Two
common sources of trouble are tubes and mis-align 
ment of controls. Check the tubes before attempting
realignment of controls since it is a sure method of
finding the trouble while the results may be incon 
clusive if realignment is attempted first and the trou 
ble is a tube. If one of the tubes isn ’ t lit, check it
by substitution. Just because a tube isn't lit doesn ’ t
mean that it is defective. VI and V 2 tubes are in
series and a burn-out of any one of them will turn
off both tubes in this circuit. Check them one at a
time by substitution. Always replace the original
tube if no fault is found. Do not replace tubes in 
discriminately since in some cases it is necessary
to realign the circuits when replacing tubes (see tube
replacement chart). The tubes may also be checked
in a tube tester for burnout. However, when checking
tubes for other than filament continuity it is better to
check by substitution since tube characteristics other
than those checked by the tester may be used in the
instrument. After it has been determined that the
fault isn ’ t tubes, check alignment of the controls for
the faulty section by referring to the appropriate ad 
justment section. If the section won ’ t align, check
resistances and voltages with the aid of the resist 
ance-voltage chart. Start by measuring the voltages
of the main power leads from the power supply, then
measure voltages at the sockets of tubes in the cir 
cuits which are suspected. When trouble shooting
direct-coupled, push-pull circuits, the two sides of
the circuit are normally balanced and cause the spot
to be stationary in the center of the scope screen.
A fault in either side will usually unbalance the cir 
cuit and cause the spot to move off the screen. To
bring the spot back, short together the control grids
(or the plates) of the two sides of one stage. This
eliminates signals of all types, de unbalance, jitter,
etc. which originate prior to the shorted points. If
shorting the two halves of a stage together does not
bring the spot on the screen and hold it motionless,
a subsequent circuit is faulty. By continuing this pro 
cess through the amplifier the trouble can be isolated
to a small circuit area. If shorting the plates together
returns the spot, the trouble can be in the plate -load
resistors or in the grid or cathode circuits of the
following stage, etc.
After the faulty components have been located, re 
place them with good. These components may be ob 
tained at regular radio parts stores as long as the
ratings and tolerances, as shown in the parts list,
are observed. In a few instances, the components
have to be specially selected from regular stock. In
these cases the special tolerances are spelled out.
In general, these parts must be ordered from the
factory, unless the customer has access to a electri 
cal measuring bridge and a stock of parts from which
to select. Regular parts may also be ordered from the
factory if time is not important. We will send parts
by air, if so instructed, at additional cost. When
ordering parts, please include as much information
as possible, especially the parts description as it
appears in the parts list, the model number and the
serial number.
4-6 ADJUSTMENT PROCEDURE
The following is a complete adjustment procedure for
the Nfcdel 120A. Normally you will need to do only the
procedures associated with the circuits under repair.