Tektronix 2246 1Y, Service Manual

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Theory of Operation—2246 1Y and 2246 Mod A Service
for changes to the control settings. Those control
voltages are held constant between refreshes by
sam ple-and-hold circuits formed by a capacitor (to
hold the voltage) and a voltage follower (to buffer
the voltage held by capacitor). The voltage follower
circuits are provided by the operational amplifiers of
U2304 and U2305. Each of the hold voltages are
protected from humidity degradation of the voltage
follower’s high-im pedance input by an active guard
shield on the circuit board. Extra noise filtering for
two of the control voltages (REF DELAY and DELTA
DELAY) is provided by using an RC pi-type filter
input circuit to the voltage follower.
POWER SUPPLY (Diagram 12)
The Power Supply (Diagram 12) provides the various
low-voltages needed to operate the 2246 1Y and
2246 Mod A and the high-voltage required by the
cathode-ray tube (c rt). The supply circuitry is
arranged in the following functional blocks: AC Input,
Primary
Power Rectifier, Start-Up circuit,
Preregulator Control circuit, Preregulator Power
Switching circuit, Inverter Control circuit, Inverter
Power Switching circuit, Low-Voltage Secondary
Supplies, and High-Voltage Supply (see Figure
3 -9 ).
Ac power via the power cord is rectified and filtered
by the Primary Power Rectifier to supply the dc
voltage to Preregulator circuitry. The output voltage
level from the Primary Power Rectifier depends on
the ac supply voltage level and may vary between
about 125 V and 350 V. This unregulated, filtered,
dc voltage is supplied to the Preregulator Start-Up
circuit and the Preregulator Switching circuit. The
Preregulator
Power Switching circuit supplies
+44 Vdc output power to drive the Inverter Power
Switching circuit.
The +44 V Preregulator output voltage is switched by
the Inverter Power Switching circuit to produce an
alternating current through the primary of the
Inverter power transform er. The current source to
the Inverter switching transistors is monitored and
regulated by the Inverter Control circuit to maintain a
constant output voltage level across the transformer
secondaries.
The Low-Voltage Secondary Supplies rectify and fil­
ter the low-voltage secondary ac voltages to provide
the dc power requirements for the instrument. Two
other secondary windings on the Inverter Power
Transformer are used in the High-Voltage Supply, a
high-voltage winding and a crt filament winding.
Voltage from the high-voltage winding is further
multiplied and converted to dc voltage for the crt
anode, cathode, and intensity-grid voltages.
Both overvoltage and overcurrent protection are
provided to protect the oscilloscope circuitry from
further damage if a circuit component fails.
Ac Input
Applied source voltage is input to the Primary Power
Rectifier via surge protection circuitry and noise fil­
tering circuitry. A sealed line filter (FL2201), L2207,
L2208, C2214, C2213, C2216, C2215, R2260,
R2227, and R2228 form a low-pass filter designed to
prevent transmission of high-frequency noise sig­
nals either into or out of the instrument. Bleeder
resistor R2215 across the input line filter drains off
any charge retained by the capacitors in the input
circuitry when the power is disconnected. Ther­
mistor RT2201 prevents a sudden rush of input
current into the rectifier and filter capacitor, C2202,
when the power switch is turned on. The thermistor
presents a relatively high resistance when cold, then
quickly reduces to a low value when warmed up.
Varistor VR2204 acts as a surge limiter to reduce the
effects of any power line surges that may damage
the input circuit components. The varistor is a
voltage-sensitive device that quickly reduces its
resistance value when its voltage limits are
exceeded. Line fuse F2201 protects the instrument
from additional damage in case of of a severe short
in the power supply.
Primary Power Rectifier
Rectification of the input ac source voltage is done
by rectifiers CR2231-CR2234. Simple capacitive fil­
tering of the rectifier output is done by C2202. The
filtered output voltage may range between about
125 Vdc and 350 Vdc, depending on the amplitude
of the ac input voltage. A line trigger signal is picked
off by T2206 for use when the Trigger SOURCE is set
to LINE. Bleeder resistor R2256 drains off the charge
on C2202 when the instrument is turned off.
Start-Up Circuit
The Start-Up circuit provides the operating supply
voltage to the Preregulator. At power on, C2204 in
the Start-Up circuit begins charging through R2203
and R2204 from the output of the Primary Power
Rectifier. When the voltage across C2204 reached
20 V, the voltage at the base of Q2204 is about
6.8 V. This base voltage level causes Q2204 to con­
duct (there is a 6.2 V zener diode in the emitter
path), and Q2211 also is then biased on. Positive
feedback to the base of Q2204 (from the collector of
Q2211 through R2220) then keeps both transistors
on. The dc voltage to U2201 (Vcc) for start up (and
continued running after start up) is provided by the
charge on C2204 via Q2211.
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