Akai VS-X9EGN

VII. EXPLANATION OF POWER SUPPLY CIRCUIT
8-1. OVER VIEW
A switching regulated power supply has been employed
for the VS-X9, that is an RCC (Ringing Choke Con-
verter) type. The arrangement has enabled the VS-X9 to
operate at a broad range of mains power voltages (110V
AC to 240V AC) without any voltage switching require-
ments.
After the direct rectification and smoothing of mains
power, the voltage serves as the switching regulator
power input. Pulses generated by the switching regulator
induce power in the secondary coils of power trans-
former T1, and after being duly rectified, the voltage of
ACIIOV
—240V
RECTFIER
FILTER
8-2. OSCILLATING PRINCIPLES
1) TR1: ON MODE
Initially, start-up current ‘‘is’” flows through R1 and
R3 to the TR1 base. It causes collector current ‘‘ic”’
to flow through TRI. See Fig. 8-2.
The current will be a temporally linear current that
may be expressed by ‘ic = 7” See Fig. 8-3.
By assuming a negligibly small coil Lp resistance and
an equally infinitesimal internal resistance of TR1 in
F : ta, GY ae
its on mode, the relationship: “ic = ip: may
actually be held valid.
The flowing of current “ic” through coil Lp causes a
voltage with the polarity illustrated to be generated
in coil Lb. See Fig. 8-2.
SWITCHING
CIRCUIT
SERVICE MANUAL
these coils will individually be output as a DC voltage.
In addition, the secondary voltage will also be fed back
through a photo-coupler to the switching regulator. By
detecting the secondary voltage fluctuations due to
fluctuations in the mains power and load, and regulating
the magnitude of switching pulses with the detected
output, the feedback arrangement holds the output
voltage at a steady level. See Fig. 8-1.
Thermistor TH! in the circuit diagram serves to prevent
the breakdown of D1 or blowing of the fuse caused by
an inrush current in a C3 charging mode.
REG 16V
REG 40V
REG 6V
By denoting the mutual inductance between Lp and
Lb with Mpb, voltage “vb” in coil Lb may be
expressed by:
2 lt pe ME pb
vb = Mpb dt Mpb at “Lp t) ip Vv
which is a steady level irrespectivel of time.
Due to the steady “‘vb” level, base current “ib” of
TRI will also be made steady in level, as follows:
ib = (vb—VD3—VBE)/R3 = vb/R3 (is << ib)
_ Mpb
= ig
See Fig. 8-3.
Vv
TB"
(hfe.ib)
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a aS ee a eT eee ae ee ee Ne Mae