Tandy 1000 SL Technical Reference Manual page 205

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 R2 supplies transistor Ql's base, then Q1
turns ON.
When transistor Q1 is On, the Q1 current excites the
transformer T1 and voltage rises in the bias coil of Tl(5-6)
which leads transistor Q1 positive bias, then transistor Q1 turns
ON.
When transistor Q1 turns ON, collector current charges the energy
to primary inductance of transformer T1 (1-3). Increasing the
collector current of transistor Q1 to the point of:
I
>
I .hfe
C = B
Then, transistor Q1 immediately turns
OFF.
In a moment,
transformer T1 will have negative voltage which will be supplied
to the secondary circuit through a rectifier.
A Short Circuit
Protector is provided to protect transistor Q1 from excess
amounts of current when the secondary circuit becomes shorted.
When transistor Q2 detects the voltage drop at R12, the collector
of 42 shorts the base and emitter of Q1. Then Q1 stops working
so that the circuit protects Q1 from over current.
The over current protector in the -12V line is provided by the
three terminal positive voltage regulator IC1 (built-in current
fold back protection 1 , which protects Q1 against excessive
current from the -12V line.