Heathkit H14 Manual page 20

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POWER SUPPLIES
35-Volt Head Supply
Full-wave bridge rectifier 0116, 0119. 0121, and
0122 suppliesapproximately45 volts. TransistorsQl
and Q125 form a Darlington pair pass element. Tran-
sistors
Q126
and QfZ7 form a differential amplifier
which compares a portion arthe 35-volt output to the
S-valt supply and drives the pass element to maintain
the 35-volt output. Transistor Q124 senses an over-
current condition and turns on when the voltage
across R169 exceeds approximately 0.7 volts. This
causes the differential amplifier to make the pass
element conduct less and limits the current to
8
safe
value.
+12 Volt
Diodes 0123 and 0125 full-wave rectify the current,
and c:apacitorC2 filters it. The supply is unregulated.
-12 Volt
Diode
0124
half-wave rectifies the current, and
capacitor
Cl13
filters it. The supply is unregulated.
+5 Volt
Vt is a three-terminal device that changes the +12
volts to +5 volts and limits its output to one ampere.
POWER UP/DOWN CIRCUIT
This circuit keeps power from being applied to the
head and motor drive circuits until the 5-volt supply
reaches 4.2 volts. Capacitor el25 delays the voltage
on the base of transistor Q128 as the 5-volt supply
comes up. The emitter of Q128 is held at 3.6 volts by
ZDl09. When the base voltage of Q128 exceeds 4.2
volts, Ql28 turns on. This turns on Q129 and supplies
12 volts to the pullup resistors of the drive circuits.
Then when the 5-volt supply drops below 4.2 volts at
turnoff, Q128 quickly turns off and removes power
from the pullup resistors.
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