Additional Components - Sony CTV-25R1 Manual

When FET Q5001 is turned OFF, its drain voltage rises above the 164Vdc-
supply voltage (to about +300Vdc). This is because magnetic energy
stored in the SBT T5001 winding collapses. The reversed current flow
applies a positive voltage to charge C5003 via D5003:
Current Path from T5001's Collapsing Magnetic Field
Part
T5001
D5003 Anode
C5003
T5001 Pin 1
T5001's collapsing magnetic field induces a LOW voltage that is output
pin 4 to Q5001/gate. This LOW insures that Q5001 turns OFF. In this
path, C5003 is charged to about 300Vdc, representing the crest of the
oscillator waveform (at Q5001/drain).
Voltage Regulator
To maintain voltage regulation, the following parts are used:
P a r t s U s e d f o r V o lt a g e R e g u l a t io n
P a r t
D 5 0 0 7
D 5 0 0 6 – 5 . 6 V z e n e r
Q 5 0 0 2 – N P N
When the oscillator is running, the voltage at T5001/pin 6 is sampled
and rectified. This sampled voltage passes through zener D5006 and is
applied to error regulator Q5002. When the standby voltage is high,
more current is applied to Q5002/base. Its increased conduction re-
duces the Q5001 gate voltage and the FET stays in conduction less.
This change in duty cycle decreases the power available at the SBT
T5001 secondary and the voltage is reduced.
Current Limiter
The current through Q5001 is limited by reducing its gate voltage. Since
Q5001 and R5011 are in series, the current flowing through R5011 rep-
resents the current in the FET. If the current through R5011 drops 0.6Vdc,
Q5002 begins to conduct. Its conduction reduces the FET gate voltage
limiting its current so it does not overheat.
Input Output
Pin 3
Cathode
P u r p o s e
R e c t ifie s T 5 0 0 1 s a m p le v o lt a g e
K e e p s Q 5 0 0 2 O F F s o Q 5 0 0 1 c a n
b e g in o s c illa t in g .
C o n t r o ls t h e v o lt a g e a t t h e g a t e o f
Q 5 0 0 1 fo r r e g u la t i o n
41

Additional Components

There are several additional components used in this circuit that have
not been discussed:
Parts
D5005, R5009, C5008, Back up for voltage error regulator. Prevents excessive
C5006 standby voltage. These parts limit the 7.5Vdc to 33Vdc if
the main regulator path (D5007, R5012, or D5006)
opened.
D5004, R5008 Keeps Q5002 OFF at the start up of the next oscillator
cycle.
Testing
After replacing the Q5001 FET, and checking for shorts, increasing the
AC voltage gradually is a good way to determine if there is an additional
problem. The standby power supply is still connected to the load during
this test so you must monitor the 7.5Vdc output voltage and prevent it
from being excessive. The oscillator should start at about 35Vac. The
normal operating voltages are listed in the chart below:
Normal Standby Oscillator Characteristics – KV27S45
AC Voltage R5001 Vdc
20Vac 8.5Vdc
40Vac 47Vdc
60Vac 79Vdc
80Vac 108Vdc
100Vac 136Vdc
120Vac 164Vdc
Additional Components
Purpose
Q5001/D Osc. Freq.
0 0
180Vp-p 29.3kHz
200Vp-p 50 kHz
240Vp-p 61.6 kHz
270Vp-p 69.6 kHz
300Vp-p 74.2 kHz
Output Vdc
0
7.4Vdc
7.56Vdc
7.6Vdc
7.6Vdc
7.6Vdc