Tektronix TYPE L, Instruction Manual, Page: 20 / 46

current. Shorted tubes w ill often overload
plate-load and cathode resistors.
Troubleshooting Procedures
It w ill be necessary to turn the oscilloscope
on its side (opposite the L Unit) and remove
the side plate and bottom plate to reveal the
circu itry of the plug-in. To trace the circuits
com pletely it w ill also be necessary to extend
the L Unit partially out of the plug-in compart­
ment with an EP 54 Plug-In Extension.
The firs t step in troubleshooting a Type L
Unit is to determine whether the trouble is
being produced in the main am plifier section
o r the X10 am plifier section. When the AC-DC
switch is in either the AC or the DC position,
only the main am plifier of the unit is function­
ing. When the switch is in the X10 GAIN AC
position , the X10 am plifier is connected in
s e rie s with the main am plifier section, and
both sections are functioning. When trouble
exists only in the X10 am plifier section, the
plug-in unit w ill function normally when the
AC-DC switch is set at either the AC or the
DC position. When trouble exists in the main
am plifier section, however, it w ill be apparent
in all three positions of the switch.
The troubleshooting procedures that follow-
are divided into sections according to trouble
symptoms. When a trouble occurs in the unit,
the proper troubleshooting section can be quickly
found.
1. Loss of Trace
If the sweep circuits within the oscilloscope
are functioning properly, a loss of the trace
can only be produced by improper vertical
positioning voltages. In other words, the trace
is positioned above or below the visible vertical
range of the crt.
To center the beam vertically on the crt,
the dc voltages at the output jacks of the Inter­
connecting Plug (pins 1 and 3) must be essentially
equal (within a fraction of a volt). As little
as a 0.2-volt difference between these two
points w ill position the beam above o r below
the range of visibility.
The dc voltages at the output jacks depend
upon the dc balance of the am plifier. Since
the am plifier is dc-coupled from the Phase
Inverter Stage (V6242-V6342) to the Output
Stage (V6632), a component failure anywhere
between these two points can unbalance the
am plifier and cause a loss of the trace. A
dc unbalance can be caused by defective tubes,
defective cathode, screen o r plate resistors,
shorted or leaky capacitors, open peaking c o ils ,
and incorrect ratios in the voltage dividers.
When the X10 am plifier is connected into
the circuit, the Second C.F. (V6132) is dc-
coupled to V6242 (the input side of the Phase
Inverter Stage). Due to the coupling capacitor
(C6112) between V6042 and V6132, the Second
C .F . is the only stage in the X10 am plifier
that can affect the dc balance of the main
am plifier. If the trace is present in the main-
am plifier positions of the AC-DC switch, but
absent when the switch is set at the X10
GAIN AC position, it is apparent that the
dc unbalance is produced by the Second C.F.
stage in the pream plifier. A possible cause
o f this condition is an im properly adjusted
X10 VARIABLE ATTEN. BAL. control, R6242.
If the trace is missing in all positions of
the AC-DC switch, the defect w ill probably be
somewhere within the m ain-am plifier section.
A step-by-step isolation procedure can then
be employed to establish the defective stage.
This procedure is accomplished by shorting
together
corresponding points on opposite
sides of the am plifier with a resistive
shorting strap, described under " Equipment
Required" in the Calibration Procedure. Start
at the output jacks and work back toward the
Phase Inverter Stage. F or example, when the
output jacks are shorted together the voltages
at these points w ill be equal and the trace w ill
appear at or near the center of the crt.
From this point, the shorting strap can be
moved back, in successive steps, to the cathodes
o f the Output Stage, the grids of the Output
Stage, the cathodes of the D river C .F ., the
grids of the D river C .F ., and finally to the
grids of the Phase Inverter Stage. Following
this procedure, a point w ill be reached where
the trace cannot be made to reappear when
corresponding sides of the am plifier are shorted
together. When this occurs, the defect has
been isolated in the stage in which these
points are located, and the stage can then
4-4
Maintenance - Type L Plug-In
AA