Tektronix 511A-AD, Instruction Manual, Page: 22 / 75

may be used if available. This method is explained in
detail under servicing instructions for each chassis.
Since a large percentage of troubles experienced will
likely be caused by defective tubes, it is advisable to
check the tubes in the suspected circuits before carrying
out extensive tests of other components. Tube testing is
best accomplished by the substitution of another one
known to be good. A tube tester is useful, but it may not
indicate some defects which cause malfunctioning in the
oscilloscope. If the tube defect has been caused by an
abnormal circuit condition, this should be corrected be ­
fore replacement is made. Although specially matched
or selected tubes are not required in the Type 51 1-A,
some individual tubes which perform normally in less
critical circuits may not function well in certain circuits
of this instrument. Specific conditions are dealt with
under the individual chassis headings. An accurate
voltmeter having 20,000
ohms
per volt or greater sensi ­
tivity, an accurate ohmeter, spare tubes and simple
tools should be available.
CAUTION — WHEN ANY PART WHICH
AFFECTS THE ADJUSTMENTS OR CALI ­
BRATION OF THE TYPE 51 1-A IS RE ­
PLACED, THE AFFECTED CIRCUITS
SHOULD BE READJUSTED AS EXPLAINED
IN THE ADJUSTMENT SECTION.
Absence of Power
When
this
condition
is due to a blown fuse, check line
voltage and if normal replace the fuse (3 amp. slow-
blow), throw the power switch on and check for any ab­
normal condition which may have caused the fuse to
blow. If a short circuit or overload exists, the length of
time required to blow the fuse should provide an indica ­
tion of the trouble. For example, a delay until the high
voltage rectifiers begin to conduct would indicate trou ­
ble in the D.C. supplies or the circuits fed by them. This
condition may be isolated by removing the high volt ­
age rectifier tubes, V21, V22, V23 and V24; and replac­
ing first V24, then V22 and V23, and finally V21 , allowing
several minutes operation between each replacement.
After the faulty D.C. supply is located, disconnect it at
the power supply terminal strip, then at the video ampli ­
fier terminal strip, to further isolate the defect. See the
schematic diagrams for average currents.
A five second delay before the fuse blows might be
caused by a short in the tube heater circuits, and should
also, be isolated by disconnecting at the terminal strips.
A delay of one second or less would suggest a short cir ­
cuit on a high voltage secondary of T1 .
Absence of Spot on Cathode Ray Tube
To determine whether this is due to absence of the
electron
beam
or
to the beam
being
deflected off the
fluorescent screen, connect together and ground all four
CRT
deflection
plates at
the
access
panel. If
a
centered
spot now appears, abnormal deflection plate potentials
are indicated. Remove the short circuiting connections.
With the VERT. POSITION control centered, the HOR.
POSITION control clockwise, the SWEEP STABILITY
control
counter-clockwise, and
no
vertical input signal,
measure the voltage between each CRT deflection plate
and ground, which should be within ±60 volts. The
average potential of both horizontal (X) plates, or both
vertical (Y) plates, should be within ±40 volts of ground
when measured under the above conditions. Abnormal
potential on the horizonal plates indicates malfunction­
ing of the sweep amplifier or associated circuits; and on
the vertical plates, a defective positioning circuit or
shorted coupling capacitor.
If the deflection plate voltages are normal, check the
CRT connections, and observe the CRT heater by view­
ing it through the small hole in the upper rear part of the
CRT
shield.
Next, measure the power supply output
voltages, the CRT bias (pin 2 to pin 3), the first anode
(pin 5) and the second anode (pin 9) voltages. If these
readings are normal, replacement of the CRT is indi ­
cated.
Power Supply
1. — 140, +225 and +450 supplies. If the output volt­
age of one of the regulated supplies is abnormally high
or low, the load current for that section may be meas­
ured to determine whether the defect lies within the
power supply or the external circuits. See the schematic
diagram for values. If the power supply is at fault, the
rectifier output voltage should be checked. Normal rec ­
tifier output would then indicate malfunctioning of the
regulator circuit. See Section III for a description of the
voltage regulator circuits. A regulator performance test
is outlined in
the
Inspection
instructions of
this
section.
A check of the grid-cathode bias of the series regulator
tube will serve to further isolate the trouble within the
regulator circuit itself. Average bias potentials are:
Line Voltage V27, 6AS7G bias
V25, 6AQ5 bias
105
117
125 -14 v.
— 36 v.
— 56 v.
- 8v.
—
16 v.
-22 v.
Figures 5 and 6 are oscillograms illustrating the ampli ­
tude and waveform of the normal rectifier output ripple.
Any unbalance due to defective rectifier tubes or power
transformer is easily observed with an oscilloscope at
the cathodes of V21 and V22-V23.
2. HIGH VOLTAGE
SUPPLY.
If normal — 1500 and
+ 1500 volt accelerating potentials are not restored by
replacement of V31, V32, V33, and the HIGH VOLTAGE
ADJ. control, the CRT heater pin plugs and accelerating
anode clip should be removed to ascertain whether the
power supply itself is at fault.
Next check the +225 volt supply, since the high
voltage output will vary proportionately. Low output
with normal oscillator (V 1 ) grid and cathode voltages,
indicates an open circuit in
the
secondary
windings
of
T2, or other portions of the rectifier circuits. Low volt ­
ages at the oscillator grid and cathode would suggest
shorted turns or excessive load on T2, or defective com ­
ponents in the oscillator primary circuit. Figures 7 and
8
are
oscillograms of the oscillator
waveform.
3.
Heater.
Since relatively large
currents are
present
in the heater circuits, it is important that connections be
clean, tight, and joints well soldered.
TYPE 511-A, 511-AD SECTION 4, PAGE 2