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Protection 2 – D690 & D691
The oscillator transistors (IC601) are protected from a base to emitter
failure. A transistor's base to emitter junction can be punctured (open) or
shorted. By applying an 8-10 volt reverse (b-e) bias voltage from a col-
lapsing magnet field to this junction, the transistor will short. C615, C616
and C613 prevent sharp (high voltage) spikes from leaving the trans-
former. Diodes D690 and D691 prevent the transistor's base to emitter
junction from being reversed.
Protection 3 – C612 & C699
A transistor's collector to emitter junction can be shorted if the maximum
voltage across these terminals is exceeded (Vce). Although the typical
maximum voltage for these transistors is 600volts, it can still be exceeded
when lightning brings in a much higher voltage. The voltage spikes from
a non-direct lightning hit may be high in voltage, but low in current (small
pulse width). They are bypassed to ground with C612 and C699. If a
very high current spike shorted IC601-2 from collector to base, current
would also have flowed through C610 and R612 and they should be
replaced.
IC601 Protection
Protection from internal
D690/D691 – prevents
spikes causing immediate
IC601's E-B junction
failure.
from reverse bias damage
Protection from external
C612/C699 –high voltage low current spikes are
spikes (lightning).
circumvented by this capacitor. This reduces the
voltage to IC601 so the transistor's C-E breakdown
voltage specification is not exceeded.
Protection - Thermal
R606 (fusible resistor).
23
Oscillator Frequency
The oscillator frequency is predominately a function of T603 inductance
and C614 capacitance, forming a sine wave at the junction. Since the
inductance of T603 is changed with a load, the frequency of the oscilla-
tor will be different when the set is turned ON.
Oscillator Testing
After replacing parts in this stage, check the following with an ohmmeter
before gradually applying power:
•
•
Testing Procedure Steps:
1. Plug the set into an isolated variable AC transformer (must contain
C615/C616/C613 –
2. Unplug the degaussing coil so the AC ammeter will only show the TV
rounds off sharp spikes
from rising and collapsing
3. Gradually increase the AC voltage to the TV while observing the fol-
magnetic fields.
•
•
•
4. Gradually increase the AC voltage. The DC voltage at the collector
O s c i l l a t o r C h a r a c t e r i s t i c s
R e s o n a t e p a r t s :
L = T 6 0 3 ( u H )
F r e q u e n c y :
1 0 4 k H z .
T V O F F ( n o l o a d )
Shorts in T603 secondary winding loads (secondaries). Check zener
diode D610 first (see Converter Voltage Output diagram below)
Shorts in a flyback secondary winding loads
an AC ammeter and voltmeter) and set to zero volts AC.
current consumption.
lowing:
The AC current on the variable AC transformer
The oscillator supply voltage (DC) at fusible resistor R606
The DC voltage at the collector of the bottom transistor IC601-1
of IC601-1 will always be half that of the oscillator supply voltage at
R606 if the oscillator is OK. This is true at any time, even when the
AC voltage is being increased. The oscillator will start when there is
about 5-6Vdc at R606.
C = C 6 1 0 , C 6 1 1 , C 6 1 4 ,
C 6 1 5 , C 6 1 6
7 1 . 5 k H z .
T V O N
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