Tektronix 516 series, Instruction Manual

Page: 35 / 132

Maintenance — Type 5 1 6
If the graticule lamps are not lighted and the fan is not
going, check the power cord connections, the fuse, and the
source of power. If your instrument is wired for 220-, 234-,
or 248-volt operation, also check the Thermal Cut-Out Switch
by checking the continuity through it with an ohmmeter. If
your instrument is wired for 110-, 117-, or 125-volt operation
and the fan is running but the graticule lamps are not lighted,
check the Thermal Cut-Out Switch. If none of the above
checks reveals the trouble, then the primary of T601 is prob­
ably open.
Failure to Regulate a t the Correct Voltage. If any of
the supplies fail to regulate, the first thing to check is the
line voltage. The supplies are designed to regulate at line
voltages between 105 and 125 volts at a nominal line volt­
age of 117 volts and within proportionate limits at the other
nominal line voltage for which the instrument can be wired.
Improper line voltages may cause the supply voltages to
be off.
If the line voltage is the correct value, the next step is
to turn the instrument off and measure the resistance be­
tween the Power Supply test points, shown in Fig. 6-4, and
ground. The —150-volt test point should measure approxi­
mately 4500 ohms to ground, the + 100-volt test point should
measure approximately 4000 ohms to ground, and the
+300-volt test point should measure approximately 12,000
ohms to ground.
If these values check out, the next step is to check the
tubes. Then check the rms voltage across each secondary
winding of the transformer. The voltage between terminals
8 and 9 should be about 195 volts ac; between terminals 15
and 16, about 125 volts ac; and between terminals 5 and 6,
about 180 volts, ac. If these voltages are correct, check the
rectified voltage at the input to each regulator. The voltage
at the plate of V667 should be about +410 volts; at the
plate of V667A, about +175 volts; and at the plate of V627,
about +72 volts. If these voltages are all correct, check for
open or leaky capacitors and improper resistance values,
especially in the dividers. If it becomes necessary to adjust
the —150 ADJ. potentiometer, it will be necessary to recali­
brate the instrument.
The material that follows may be used as a quick index
to troubleshooting the low voltage Power Supply.
If the output voltage is high with excessive ripple, check:
1. For high line voltage.
2. The amplifier tubes (V634, V654, V674).
3. For insufficient loading.
(Proper loading of the supplies can be checked by mea­
suring the voltage drop across the 10-ohm resistors, R640,
R642, and R644, in each supply. With the AMPLITUDE CALI­
BRATOR control set to OFF and the STABILITY control set
so that no sweep is being produced (counterclockwise but
not to PRESET), the drop across the resistors should be as
follows: R640, 1.65 volts ±0.1 volt; R642, 1.4 volts ±0.1
volt; R 644,1.75 volts ± 0 .1 volt.)
If the output voltage is high with normal ripple, check
for proper resistance values in the dividers (R615, R616, and
R617; R650 and R651; R670 and R671).
If the output voltage is low with excessive ripple, check:
1. For low line voltage.
2. The series tubes (V667, V627).
3. For excessive loading.
4. Open or leaky filter capacitors.
5. Bad diode rectifiers.
If the output voltage is low with normal ripple, check:
1. The resistance values in the dividers.
2. The capacitors across the dividers.
Troubleshooting the CRT Circuit
If the trouble has been isolated to the Crt Circuit, first
check the voltages at the —1675-volt test point and the
+2325-volt test point at the top rear of the oscilloscope. If
there is no voltage at either of these points, remove the
high-voltage shield containing the two test point openings.
WARNING
Always turn the instrument o ff before removing or
replacing either high-voltage shield.
Check for heater glow in the three high-voltage rectifier
tubes, V822, V832, and V842. If there is no heater glow in
any of these tubes, the high-voltage oscillator circuit is not
oscillating. If there is heater glow in any or all of the three
rectifier tubes, measure the voltage at the grid of the Oscil­
lator tube, V800. If this voltage is about —55 volts, the oscil­
lator is operating properly. If it is significantly less than —55
volts, the oscillator is not operating properly.
If the oscillator is not operating properly, and replacement
of V814 and V800 does not correct the trouble, check the
other components associated with the circuit, including the
primary and secondary of T801.
If there is no voltage at either of the high-voltage test
points, but the high-voltage Oscillator is found to be oper­
ating properly, then the trouble lies in the secondary of T801
or in one of the high voltage rectifiers. If this is the case,
measure the voltage at the plate of V832. (This is the high-
voltage rectifier tube nearest the centerline of the instrument;
the plate is the connection at the forward end of the tube.)
The voltage at this point should be in the vicinity of —1775
volts. If it is not, then trouble exists in some part of the cir­
cuit common to all three high-voltage supplies. If the volt­
age at the plate of V832 is in the vicinity of —1775 volts,
then trouble exists in some part of the circuit peculiar to the
—1675- and +2325-volt supplies.
If there are voltages present at the —1675-volt test point
and the +2325-volt test point, but they are not of the proper
value and cannot be brought to the proper value by ad­
justment of the HV ADJ. (R841), the regulator circuit (V814)
is probably faulty, or the supply is being loaded down by
a short somewhere in the circuit.
If the high-voltage supplies are normal but no spot is
visible on the crt, check the continuity of the cathode circuit.
Also, check the ASTIGMATISM control, R864, for proper re-
5-6