Service manual

STP 11-25R13-SM-TG
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Performance Steps
e. Common causes of blooming are a weak high voltage rectifier or too little bias on the CRT.
(1) No high voltage means no high brightness; insufficient high voltage causes low brightness.
(2) Eliminate corona and arcing by separating all high voltage connections from the chassis.
Arcing can cause streaks in the picture and can usually be heard as a snapping sound.
Corona produces a sizzling sound.
f. If the picture screen is dark but the sound is normal, check the horizontal (H) output.
g. For no synchronization, check the H and vertical (V) deflection oscillator.
h. A black screen with power ON indicates that both the H and V syncs are missing; check the
sync separator circuit.
i. No drive signal to the H output circuit, no high voltage or boost voltage, and a dark picture
screen are symptoms of the H oscillator not oscillating.
(1) These same symptoms can also be produced by failure of the H output circuit, low voltage
power supply, or picture tube.
(2) If the H drive signal is absent or abnormal, but there is good sync and comparison pulse
input, the problem is in the H oscillator.
j. Identify sweep circuit problems.
(1) For a complete sweep failure, check the oscillator output, drive input, drive output, and
power output.
(2) An absent or abnormal oscillator wave indicates that the problem is in the oscillator stage.
(3) A weak amplifier in the output circuit causes insufficient height. Reduced DC supply
voltage for the amplifiers can also cause this problem.
k. A horizontal line indicates a vertical problem.
l. A weak picture or snow indicates video amplifier stage problems.
m. For picture pulling, weak brightness modulation, bending, or washout, check the automatic gain
control (AGC) circuit, a common cause of trouble in the picture while the raster is normal.
(1) Too much AGC bias reduces gain, which causes a weak picture without snow.
( a) A weak antenna signal produces a weak picture with snow.
( b) The sound also cuts off because of the intercarrier sound circuit.
(2) Too little AGC bias can cause high contrast and bending in the picture just before the point
of a reverse picture out of sync.
(3) Not enough AGC bias causes overloading on normal signal levels, but has a normal
picture with a very weak signal.
4. Isolate the fault to a component.
a. For problems in the low voltage power supply, measure DC voltage on each branch. If one
branch is open, voltage on the other branches may or may not be affected. If one branch is
shorted, low output voltages will probably affect the remaining branches.
b. With power OFF and the power cord unplugged, check resistance of semiconductors in the
power supply for short circuits with a multimeter.
c. For thermal or warm-up problems, use a can of cold spray and a heat gun to identify
components whose characteristics may be drifting with temperature.
d. Troubleshoot the H output circuit.
(1) On an oscilloscope, measure the input waveform from the H drive circuit at the H output
transistor base. If the waveform is normal, the trouble is in the H output circuit.
(2) If both the HV boost voltage and the drive signals are good, the problem is in the H output
transistor or related circuit parts, capacitor, or flyback transformer.
e. Measure the waveforms at all outputs and inputs of the H oscillator and related circuits.
(1) Check all DC voltage measurements at the H oscillator and related circuits.
(2) Check the H oscillator or multivibrator circuits for correct frequency with an oscilloscope.
f. Troubleshoot a marginal failure of the sweep circuit.
(1) Try to adjust the VERTICAL control from maximum to minimum.
(2) If adjusting the VERTICAL control does not correct the problem, suspect leaking or open
capacitors, faulty transistors, faulty diodes, or worn potentiometers.
g. Troubleshoot video circuit problems.