Tektronix 535A, Instruction Manual

Page: 57 / 156

Maintenance—Type 535A
No Intensity
If the low-voltage power supply is operating normally, but
no spot or trace is visible on the crt, the trouble could be a
defective crt, a defect in the crt cathode circuit including the
supply, or an unbalanced dc condition in either or both of
the deflection amplifiers. In the latter case the dc unbalance
is producing improper positioning voltages and the beam
is being deflected off the screen.
To determine which circuit is at fault, turn the INTENSITY
control full right (cw). If a flare is observed on the crt
screen (it may be necessary to darken the room), one of the
deflection amplifiers is probably at fault; the procedure for
troubleshooting these circuits follows a bit later in this sec­
tion. If no flare is observed with the INTENSITY control
turned full right, the trouble will either be due to a de­
fective crt or to an inoperative cathode supply circuit. The
cathode supply can be checked by measuring the voltage
at the H.V. Adj. Test Pt. In the majority of instruments
the test point is located at the top left rear of the instru­
ment. Fig. 5-14 shows the location of typical high-
voltage test points. Refer to the schematic diagrams for
your instrument to determine the correct voltage. If the
voltage reading you obtain is in the vicinity of the correct
voltage noted on the schematic, turn the instrument off, and
measure the value of the 27 K resistor attached to the test
point (see Fig. 4-10). If this resistor is not open a defective
crt is indicated.
If the voltage at the H.V. Adj. Test Pt. is zero or abnorm­
ally low replace the Oscillator tube and the Error-Signal
Amplifier tube. If this does not restore operation the Oscilla­
tor circuit should be checked.
A quick check on the operation of the Oscillator circuit
can be made by observing the heater glow in the 5642
rectifier tubes, located under the shield at the upper right
rear of the instrument. These tubes are visible through the
opening in the side of the shield. If no heater glow is
visible the Oscillator circuit is inoperative. This could be
due to an open high-voltage transformer, or to a defec­
tive component in the oscillator or error-signal amplifier
circuits.
If heater glow is visible in the rectifier tubes, the Oscillator
circuit is operating. If the heater glow appears to be dim,
however, the output of the Oscillator may be insufficient
for proper operation. A more accurate check on the Oscil­
lator may be made by removing the shield covering the
high-voltage transformer and measuring the bias at the
grid of the oscillator tube. This can be measured at the
junction of the 100 k resistor and the 0.01µfd. capacitor.
See
the simplified schematic, Fig. 4-10 for the normal circuit
configuration in the high-voltage oscillator. The voltage at
this point should measure about —65 volts.
CAUTION
Do not let your hand or body touch the chassis
when making this check. Secondary reactions to
an otherwise harmless shock might result in a
painful injury.
If the Oscillator circuit is operating properly, but the volt­
age at the H.V. Adj. Test Pt. does not measure in the vic­
inity of that indicated on the schematic diagram for the
instrument, the 5642 rectifier tube, V822, shown in the simpli­
fied schematic of Fig. 4-10, is most likely defective.
Abnormal Intensify
If a trace is visible on the crt, the relative intensity of the
trace may be used to identify trouble in either the negative
bias supply or the positive anode supply.
If the trace is excessively brilliant, and if the brilliancy
does not change as the INTENSITY control is adjusted, check
the negative bias supply including the lead to the first grid
of the crt. Check for a defective rectifier tube V822, (shown
in the simplified schematic diagram of Fig. 4-10), an open
supply winding or filament winding, an open resistor in­
cluding the INTENSITY control, or a shorted or leaky capaci­
tor. If trouble is not found in any of these components, a
defective crt is indicated.
If the intensity of the trace is extremely low, check for an
inoperative positive supply. Also check the anode connec­
tion to the crt, including R836 and C836.
If the accelerating potentials appear to be too high, as
evidenced by decreased deflection sensitivity, check the
Error-Signal Amplifier circuit.
If a badly distorted trace or spot is visible on the crt,
check the Geom. Adj. control and its connection to the neck
pin on the crt, and the ASTIGMATISM control and its con­
nection to the crt base socket. If the FOCUS control has no
effect on the trace, check this control and its connection
to the crt base socket.
NOTE
If any components in the Oscillator, Error-Signal
Amplifier or cathode supply circuit are changed,
or if the setting of the H.V. Adj. Control is
changed it will be necessary to recalibrate the
instrument.
5-13