Radio Shack TRS-80 Service Manual page 50

____ c_o_1_o_r __ c_o_m_p_u_t_e_r ___ 2 _________
"TFl!3-E3Cl
® ___________
s_e_r_v_1_·c __ e __ M_a_n_u_a_1 __
The final function of the DAC chip is to provide the output
signal for recording of cassette data. This is, quite
simply, a buffered output of the DAC which is attenuated to
produce approximately 1 volt p-p into a 2K ohm load.
Therefore, it is up to the microprocessor to produce the
necessary FSK signals through the DAC and the proper
software.
4.10 SALT Circuitry (Ul)
The SALT chip Ul (Supply and Level Translator) is
responsible for supply regulation, RS-232 interface level
translation, cassette read operations, and driving the
cassette relay, as is shown in the block diagram in Figure
4-12.
Figure 4-13 shows the complete power supply circuit. AC
voltage is brought into the primary of transformer Tl
through fuse Fl. The secondary of the power transformer
provides 17.2 VAC, center-tapped, at 1.8 amps RMS to the
Color Computer 2 circuit board. If switch Sl is closed,
this AC voltage is applied to the cathodes of CRl and CR2,
and to the anodes of CR3 and CR4. CRl and CR2 form a full
wave center-tapped rectifier with a negative output. This
is filtered by electrolytic capacitor Cl. This negative
voltage is then applied to pin 15 of the SALT chip, where it
is internally regulated to -5 VDC and used for the RS-232
output drivers. The negative voltage is not used anywhere
else in the computer.
CR3 and CR4 form a full wave center-tapped rectifier with a
positive output which is filtered by electrolytic capacitor
C2. This positive voltage is applied to the collector of
pass transistor Ql and is used to power the SALT chip at pin
16. The SALT chip internally regulates the positive voltage
to +5 VDC and provides the base drive current for Ql. The
current for the computer is drawn from the emitter of Ql
through resistor R2. The voltage at this point is monitored
by pin 3 of the SALT chip and the base drive adjusted to
keep the voltage at a steady +5
voe,
+/- 5%. Electrolytic
capacitors CS and Cll are used to prevent oscillation of the
power supply.
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