Power Supply Circuit - Tandy 102 Service Manual

Power Supply Circuit
The Tandy 102 logic circuit uses +5V for VDD, -5V for VEE and +4 to +5V for VB. These voltages
are supplied by the DC/DC converter. Also, the power supply circuit has the automatic power off
function and reset circuit.
DC/DC Converter Circuit
OT2 is a converter transformer which oscillates T21 and T22 and generates voltages at the
secondary side of the transformer. At the same time the power is switched ON, a very slight
collector current flows to T21 and T22. As the current flowing through OT2 is increased, the voltage
induced between pin 8 and pin 9 of the converter transformer causes pin 9 to be positive. The
positive voltage is applied to the base of T22 passing through R126 and C81 to activate T21 and
T22.
Fully charged, C81 stops the primary current. The secondary magnetic field begins to collapse,
reversing the polarity of the induced voltage, causing pin 9 to be negative. By being applied to the
base of T22 through C81, this voltage is used to turn OFF T21 and T22, as C81 discharges.
Discharged C81 allows the transistor to turn ON again to repeat this cycle. The switching frequency
is determined by R126 and C81.
The output of this circuit is derived from the secondary winding. VEE is from pin 9, rectified by D15,
filtered by C85, and VDD is from pin 7, rectified by D13, filtered by C84.
Also, VDD is fed back to the base of T13 through zener diode D4 to maintain VDD
of +5V.
Low Power Detection Circuit
The low power detection circuit illuminates an LED warning lamp when the battery voltage
decreases. If it continues to decrease, the system power will be switched OFF just before the
voltage becomes so low that the converter cannot operate.
There are about 20 minutes between the time when the LED lamp illuminates and the system is
switched OFF.
Battery voltage is detected by splitting the resistance of R105, R108, R144 and R116. When the
battery voltage (VR) becomes 4.2V ± 0.1 V, T16 is switched OFF. T17 is switched ON, T19 is driven,
and the LED illuminates.
When VL becomes 3.7V ±, T14 is switched OFF, T15 is switched ON, and the LPS signal changes
from H to L. This signal is inverted by M34 and fed to the TRAP terminal of the CPU. If the CPU
acknowledges this signal, it sends the PCS signal passing through the PB4 of the PIO after the
internal operations.
When the PCS becomes H, the Q output of M28 becomes H, T20 operates and the oscillation of the
converter is stopped.
If there is no operation for 10 minutes or more (awaiting a command for 10 minutes or more), the
PCS is output from the PB4 of the PIO.
When the power switch is switched OFF, T18 is switched OFF, the RESET terminal of M28 becomes
H and oscillation is resumed by switching the power switch ON. If, however, the power is reduced
by the PCS signal, a battery replacement is necessary.
RESET Circuit
This circuit supplies the RESET signal and also the RAM RST signal as the protecting signal for the
contents of the RAM when the power decreases. C78 and R103 delay the introduction of input
power so that T11 is switched ON and T10 is switched OFF after VDD is activated, with the result
that the RESET signal changed from L to H. In the same way, the RAM RST signal is generated by
T9 and changes from H to L. Thermistor TH2 suppresses the RESET signal fluctuations due to
temperature.
T25 receives the signal from the Q terminal of M28 during automatic power OFF, short-circuiting
both ends of C78, and resets the system. The RESET signal is active L and the RAM RST signal is
active H.