Wersi DX 400, Assembly Manual

Page: 12 / 68

AM 334-01-502
12
2. Plug-in Board PS 12 - Power Supply
The plug-in board PS 12 develops all the voltages
required to run the DX series organs:
+ 5 V - switched and regulated, pins 25 & 26
-10 V - electronically regulated, pin 30
-15 V - electronically regulated, pin 31
All voltages are referenced to GND (=0 V), present at
pins 1,2,7,8, 14, 15,28,29 and 32 of PS 12.
Note: The boards in the DX series strongly
differentiate between A GND (= analog ground) and D
GND (= digital ground), even though both initially
occur together on
PS 12.
Principles of Operation
Fig. 5 is a schematic diagram of PS 12. The AC voltage
from the power transformer (2 x 18 V AC), which enters
the board via pins 17 thru 22, is rectified by bridge
rectifier BR 1 - Unregulated DC voltages PWR + and
PWR - are present across filter capacitors C14/15 and Cl
2, respectively. (These are used as supply voltages for
- the integrated power amplifier on PC board AF
11 or AF 110 when the PS 12 is used with the Alpha
series).
IC3 and lC4 are voltage regulators, developing highly
regulated, short circuit-protected -15 V DC and +15 V DC
supply voltages, respectively. These voltages are
indicated by the green and yellow light-emitting diodes
(LEDs), respectively. Zener diode ZD4 and resistor R23,
wired across the -15 V line, develop an additional supply
voltage of -10 V DC.
Most of the circuitry on PS 12 (over half of the Fig. 5
schematic) is devoted to developing the +5 V DC supply
voltage. The source voltage is the PWR + line (25-30 V
DC). The circuit is a switched power supply, keeping
power losses to a minimum.
04 functions as a switch transistor. It is driven by 02 and Q3,
which are in turn driven by ICi. When 04 is switched
through, memory choke Li builds up a field and electrolytic
capacitor C16 charges up until the output voltage (pins
25/26) r5 V DC. The red LED indicates this.
During Q4's shutdown period, the charge on C16 maintains
the supply current, while diode D4 serves to maintain the
current flow resulting from the collapsing field around Li.
The regulation of the output voltage is achieved by
controlling the duty cycle of the switching pulses, which ICi
sends to switching transistors Q2 and Q3. ICi receives the
reference voltage for this from 1C2 (pin 2). If the supply
voltage at pin 1 of IC 1 varies from the reference voltage at
pin 2, the internal operational amplifier (ICi, right) produces
a corresponding correction voltage to restore the correct duty
cycle.
Further regulation is achieved by current sensing. The left
operational amplifier in ICi senses the voltage drop acrQss
R22 and compares it to a reference voltage at pin 16. This
results in corresponding correction voltages.
The remaining circuit components mainly serve to protect
the power supply and the circuits it is supplying. For
example, as and zener diode ZD3 work together to prevent
an overvoltage condition on the +5 V DC line. If the voltage
rises above 5 volts, as conducts, turning on thyristor Thi This
places a short circuit-across the +5 V DC line, which will
remain until the power supply is turned off momentarily.
Zener diode ZD1 stabilizes the supply voltage for 1C2,
which, besides producing the reference voltage mentioned
above, also produces a reset pulse during organ turn-on. This
pulse momentarily mutes the audio amplifiers on PC board
AF 12 until the digital organ system has stabilized.