Zenith ZVM-121 Service Manual page 16

Page 5-2
POWER SUPPLY (Figure 5-2)
Power transformer TX201 and bridge diodes CRX701,
CRX702, CRX703, and CRX704 develop approxi-
mately 17.5 volts DC on capacitor CX707, with 1.7
volts of ripple present. QX703 acts as a variable series
element, dropping more or less voltage across it in
order to maintain the output voltage at a constant 12.7
volts.
When the output voltage increases or decreases due to
line voltage fluctuations or load variations, this volt-
age change appears at the base of Q701. The voltage
divider network that feeds this voltage to the base is
made up of R704, R706, R707, R708, RX709, and
C709. The potentiometer is configured to minimize
voltage drift caused by tolerances or temperature var-
iations. The AC ripple is coupled to the base by C709.
This voltage is compared against the reference of
CR706 (4.7 volt zener) and C708. Any difference
causes the collector current to increase or decrease
SX201
FX701
2.SA
FX201 CX707
4A 2200
2SV
+ CX706
I
10
CRX703
= -
SOV
-
+
proportionally. This current is amplified by Q702 and
fed to control transistor QX703. If the output voltage
tries to rise, Q701 will try to turn off, causing Q702
and Q703 to turn off. Conversely, when the output
voltage drops, Q701 turns on harder, turning onQ702
and Q703 and raising the output voltage back to nor-
mal.
A unique feature of this circuit is its ability to switch
from a regulator to an active filter at low line voltages.
The voltage at the collector of Q702 is proportional to
the ava,ilable input voltage to the regulator. When the
input is too low to maintain 12.7 volts, the voltage at
the collector of Q702 drops below the reference zener
voltage, causing CR705 to conduct. This additional
current sinking through R701 disables the zener re-
ference, causing the DC output voltage to drop and no
longer be regulated for DC variations. However, the
AC reference remains in control; so AC ripple regula-
tion continues - in effect - producing an active fil-
ter.
R701
R703
CX711
2.2K
7S0
R707 + C709
CR70S
2.2K 10MF
220
- 16V
16V
Q701
+
R702
ERROR
R704
100
RX711
22
AMP I
'hW
R708 9.1
10% 33K
RX709
'hW
10K
B+
ADJ.
BLACK!
4
BROWN
CRT
FIL
-
R706
12K
-
QX703
REGULATOR
RX712
15
7W
-
Figure 5-2