Sencore SC61 Waveform Analyzer Tech Tips

#131
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Means Success In Electronic Servicing
Servicing Startup And Shutdown Problems With The SC61
Most television receivers and mon itors use
at least one safety shutdown circuit and
circuits to start the horizontal stages when
power is first applied. This Tech Tip
explains how to use the Sencore SC61
Waveform Analyzer™ and the PR57 AC
" POWERITE "® to isolate problems in
these circuits .
Why Use Shutdown Circuits?
Safety shutdown circuits prevent the high
voltage stages from operating unsafely .
Their main function is to safely shut down
the set if a problem causes the high voltage
to rise too high . They also prevent power
supply problems from damaging other
components in the receiver.
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To
~ Safety
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Fly back
"flyback Pulse Winding"
The safety circuits have sensors which
monitor the signal at one or more test
points related to the horizontal circuits.
Four typical signal sources are: 1) A
flyback transformer winding , 2) The DC
from the regulator, CRT screens or focus ,
or video outputs, 3) The current returning
from the picture tube , and 4) The current
in the emitter of the horizontal output
transistor . Some receivers use only one of
these sensing methods , while others may
use several.
If a detector finds a signal which is too
large, it kills the horizontal output. The
three most common shutdown methods
are: 1) Shorting the drive signal to the
output stage, 2) Stopping the B
+
feeding
"High Voltage Return"
"DC From Regulator"
the output transistor , and 3) Blocking the
DC to the horizontal oscillator or driver.
Since the horizontal oscillator operates
with scan-derived power from the flyback
transformer , any method which blocks the
horizontal signal results in a comp lete
shutdown of the horizontal circuits.
The Startup Circuits
The set needs startup circuits because the
horizontal oscillator receives its power
supply from the flyback transformer . This
forms a closed loop , which cannot get
started without help from an external
power source. There are two kinds of
startup circuits: 1) Kick starters , and 2)
Trickle starters.
To
Safety
Circuits
"Emitter Current Sensing"
Fig. 1: Safety circuits use one of these four methods of determining if one of the horizontal circuits is producing too much voltage or
current.
#131