Tektronix TSG 95, Service Manual

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Theory of Operation
3–6
TSG 95 Service Manual
supply when a DC input voltage is applied to J5. F1 is a fuse used for safety pur-
poses. C64 helps with EMI. Q7 is on during normal operation. If a reverse volt-
age is applied to the DC input, Q7 shuts off to protect the power supply circuit.
VR1 limits the gate-source voltage of Q7. C57 and C56 control ripple on the in-
put voltage.
U22 allows the microprocessor (U2) to turn off the power supply. The power
supply for U22 is connected to a regulator (U25), which is connected to the input
voltage. This allows U22 to stay powered up even when the instrument is
switched off. When the ON key is pressed U22–10 gets pulled high; normally it
is pulled low by R30. U22B is connected to act as an inverter pulling U22A–4
low. This sets U22A–6 low, enabling the power supply controller (U27) through
U27–5. When the ON key is pressed again, U2 detects the key press, stores in-
formation for power up, and then drives the gate of Q5 high and then low. This
causes the drain of Q5 to be pulled low and then high, clocking the flip-flop
U22A. Since the Q output of U22A is tied to its input, the flip-flop toggles its Q
output high. This disables the power supply, shutting the instrument off. R70 and
C78 keep the instrument from coming on automatically when an adapter is
plugged in or new batteries are put in the instrument.
The –5 V supply voltage is generated off the +5 V supply through transformer
L9. CR1 and CR2 keep the current from flowing on the inactive parts of the
switching cycle for each supply. L9 also helps to filter the +5 V and –5 V sup-
plies along with C53, C54, and C77. L2 and L3 are used to filter the supplies
which are used in analog portions of the circuitry.