Tektronix 2246 1Y, Service Manual

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Theory of Operation—2246 1Y and 2246 Mod A Service
Operational amplifier U930B is a free-running
oscillator circuit with a period of about 1 ms. The
oscillator frequency is determined mainly by the
charging time constant of C935 and R935. The
voltage divider formed by R938, R934, and R939
divides the +15 V supply to provide a positive
voltage on pin 5 of the oscillator to get the circuit
into oscillation. (When the circuit is oscillating, the
feedback signal switches the pin 5 voltage between
about +8 V to 0 V.) The gain of the amplifier is high
enough to drive pin 7 to the positive supply voltage
level at about 14 V, and the signal voltage level on
pin 5 rises to a little over 8 V from the feedback
current supplied by R933. The CLK IK signal taken
from the junction of R934 and R939 is supplied to
U600 and is used to skew the chop-clock frequency.
The skew prevents the oscilloscope from triggering
on the chop frequency when displaying multiple
traces in CHOP Mode.
At that level CR935 is reverse biased, and CR936 is
forward biased (by the output of U930A) to pass the
Probe Adjust high level output signal current.
Charging current through feedback resistor R934
charges C935 up from 0 V toward the output voltage
level. As soon as the charge on C935 (and the
voltage on pin 6 of U930B) reaches the voltage level
on pin 5, the output level at pin 7 drops to about
-5 V, and C935 must then begin discharging to the
new voltage level. At that point CR935 is forward
biased and that reverse biases CR936 so that the
Probe Adjust output voltage drops to 0 V. Resistor
R940, in series with CR935, limits current flow to
protect U930 and CR935 in the event of a static dis­
charge to the PROBE ADJUST output connector.
Crt
The Trace Rotation adjustment, R911, varies the
current through the Trace Rotation coil. The Trace
Rotation coil is located between the crt face and the
vertical and horizontal deflection plates, and it
affects both the vertical and horizontal alignment of
the trace.
The Geometry adjustment, R2784, varies the voltage
level on the horizontal deflection-plate shields to
control
the overall geometry of the display
(minimizes bowing of the display).
The Astigmatism adjustment, R2788, varies the
voltage level on the astigmatism grid to obtain the
best-focused display over the whole face of the crt.
MEASUREMENT PROCESSOR (Diagram 8)
The Measurement Processor circuitry includes the
Processor (U2501), the System RAM (U2521), the
System ROM (U2519), communication bus latches
and transceivers, the Address Decoding circuitry,
and the Power-On Reset 1C (U2502).
Power-On Reset
The +5 V supply is monitored by U2502 to generate
the reset signals throughout the instrument. These
reset signals initialize the states of the logic devices
and ensure that memory writes to any of the RAM
spaces cannot occur until the +5 V supply is up to its
correct operating level. The RESET signal output at
pin 6 is initially high during power up (as soon as the
voltage has reached the operating level of the
RESET 1C, U2502). That high signal is inverted by
U2506C to produce the SYS RESET signal. The
SYS RESET signal holds Processor U2501 in its reset
state.
The SYS RESET signal also resets and initializes the
Readout Processor (U2400, Diagram 9). At pin 5 of
U2502, a RESET signal is generated. That signal
biases Q2507 off to prevent System RAM U2521
from being selected by any random states that
might occur on the address lines during reset as the
voltage is rising.
About 5 ms after the +5 V supply reaches the
operating level required for the Processor, the
RESET condition is removed, allowing the Processor
to operate. At power off (and for a momentary drop
in the +5 V supply), when the +5 V supply falls below
the safe operating level of the logic devices, the
RESET condition occurs to prevent random
operation.
Measurement Processor
FUNCTION. Measurement Processor U2501 is a
multitasking device. Its major functions are the
following:
1. Continually refreshes the front panel indicator
LEDs. One column of the six-column LED
matrix is refreshed every 2.048 ms.
2. Continually scans the front panel switch set­
tings, sensing rotation of rotary switches and
closures or openings of m om entary-contact
switches. One column of the six-column
switch matrix (the same column number of
LEDs being refreshed) is read every
2.048 ms.
3. Communicates with the Readout Processor
and Readout RAM to set attributes for each
readout field, put text into each field, and
turn the readout fields on or off.
4. Scans the front panel pots and sets control
voltage levels. The Measurement Processor
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