Tektronix 502A, Instruction Manual

Page: 39 / 110

Maintenance — Type 502A
If the output voltage is high with excessive ripple,
check:
1. For high line voltage
2. The amplifier tubes and transistors (V746, V666, V636,
Q644, and Q634)
3. For insufficient loading
If the output voltage is high with normal ripple,
check:
1. For proper resistance values in the dividers (R751 and
R753; R671 and R673; R621, R622, and R623; and R648 and
R649)
If the output voltage is low with excessive ripple,
check:
1. For low line voltage
2. The series regulator tubes and transistor (V747, V757,
V677, V637, or Q647)
3. For excessive loading
4. Open or leaky filter capacitors
5. Rectifiers (V722, V652, V602, or D642A,B)
If the output voltage is low with normal ripple, check:
1. The resistance values in the dividers
2. The capacitors across the dividers
If the output voltage is normal with excessive ripple,
check:
1. Filter capacitors at the output of the rectifiers and at
the regulated output
2. AC bypass capacitors in the grid circuits of the regulator
amplifiers
3. Regulator amplifiers (screen grid circuits)
TROUBLESHOOTING THE CRT CIRCUIT
For troubles which affect both beams
The intensity, calibration, focus, and geometry of the
crt display depend on the proper operation of the high-
voltage power supply in the CRT Circuit. In general,
troubles occurring in the high-voltage power supply will
produce similar effects on both beams.
If no high voltage (or insufficient high voltage) is
available from either the grid or cathode supplies, a de­
fective oscillator circuit or excessive loading is probably
the cause. The oscillator can be quickly checked by placing
a
neon bulb against V800, the oscillator tube. If the bulb
glows, the oscillator is operating and the trouble is likely
to be in the secondary windings of T801 if there is no high
voltage, or in the regulator circuit (V814), if the high voltage
is abnormally low. It is unlikely that all three rectifier tubes
would simultaneously be defective.
If the neon bulb does not glow, the oscillator is not
operating. If replacement of the oscillator tube V800 and
the regulator tube V814 does not clear up the trouble, check
Fig. 4-9. Checking operation of the high voltage oscillator circuit
by means of a neon bulb.
the components of the oscillator circuit
including the primary
and secondary windings of transformer T801.
If the proper output voltage is obtained from at least
one of the high voltage supplies, the oscillator circuit need
not be checked. In this case, you can check the rectifier and
components associated with the inoperative supply. If the
proper output voltages are obtained from all the H.V. sup­
plies and the circuit is still not operating correctly, check the
voltage dividers which control the intensity and focus of the
beams. If these are also normal, the trouble will likely
be the crt itself or its cathode circuitry. Badly misfocused and
distorted displays can be caused by an open Geom Control
or a loose connection at the neck of the crt.
If the high voltage appears to be abnormal, as evidenced
by decreased or increased horizontal and vertical deflection
sensitivity, the regulator circuit (V814) should be checked.
If this tube or any component of this circuit is changed, the
setting of the HV
Control (R826) should be checked and ad­
justed if necessary according to the procedure given in the
Calibration Procedure section of this manual.
For troubles
which affect only one beam
Troubles which affect only one beam will generally be
caused by defects in the intensity and focus voltage dividers,
by the Astigmatism Controls, or by the crt. These parts
(except for the crt) can be checked by voltage and resistance
measurements. If the circuit checks out satisfactorily, re­
place the crt.
TROUBLESHOOTING THE VERTICAL
AMPLIFIERS
For no spot or trace
If a trace (or spot) is visible when the vertical deflection
plates are externally shorted together, but disappears when
4-5