Tandy 1000 HX Technical Reference Manual page 341

Theory of Operation
AC Input Circuit
This circuit is composed of an AC Power Switch, a fuse, a line
filter, and an inrush current limiting circuit and rectifying
smoothing circuit. The inrush current limiting circuit controls
the charging current to electrolytic capacitors when power is ON.
The line filter reduces noise that leaks from the power source to
the AC line or that returns from the unit to the power souce; it
satisfies the specifications of noise regulations.
Control Circuit & Power Converter Circuit
This circuit is a self oscillation switching system, generally
called an R.C.C. (Ringing Choke Converter). The R.C.C. circuit
does not fix the oscillating frequency. Whenever input voltage
is high or the load becomes light, the oscillating frequency will
be high.
The current through R3 supplies transistor Ql's base, then Ql
turns ON. When transistor Ql is On, the Ql current excites the
transformer Tl and voltage rises in the bias coil of T K 5 - 6 )
which leads transistor Ql positive bias, then transistor Ql turns
ON.
When transistor Ql turns ON, collector current charges the energy
to primary inductance of transformer Tl (1-3). Increasing the
collector current of transistor Ql to the point of:
I > I .hfe
C = B
Then, transistor Ql immediately turns OFF. In a moment,
transformer Tl will have negative voltage which will be supplied
to the secondary circuit through a rectifier. A Short Circuit
Protector is provided to protect transistor Ql from excess
amounts of current when the secondary circuit becomes shorted.
When transistor Q2 detects the voltage drop at R 8 , the collector
of Q2 shorts the base and emitter of Ql. Then Ql stops working
so that the circuit protects Ql from over current.
The over current protector in the -12V line is provided by the
three terminal positive voltage regulator ICl (built-in current
fold back protection ) , which protects Ql against excessive
current from the -12v line.