Tektronix 2430 Service Manual

3-81
4. Control of the display functions (TV CLAMP and
FAST CLAMP).
3. Selection of correct polarity of the offset signal via
the CH2 INV signal.
2. Choice of triggering on positive- or negative-sync
input signals(NEG-SYNC).
1. Selectionof trigger field (Field1or Field2).
Video Option Control Register
The Video Option Control Register (U750 on
diagram20) is written to by the System Processor
(address-decodedlocation 6012h) to control operational
setup of the Video Option. The Video Option Control
Register is initializedon power-up and provides for control
of the following functions:
Probe Power
The Probe Power outputs on the rear panel provide
access to three of theinstrument power-supply voltages
and may be used to power approved voltage- and
current-probe accessories. Contact your Tektronix sales
representativefor a list of approvedprobe accessories.
As long as the BELL line remains HI, the speaker pro-
duces an approximate 2 kHz tone. In practice, the System
ILPsets the BELL line HI for a short time (=4 ms), turning
0588 on, starting the tone and rapidly charging C590.
When BELL returns LO, C590 gradually discharges
through R594. As the capacitor discharges,bias on 0592,
and thus current through the speaker, is reduced,causing
the sound to gradually fade out in a pleasing"bell-like"
tone.
Since LS498 is inductive, the current decay portion of
its cycle (0594 off) tends to force pin 1 of the speaker
above the +5 Vo supply level. Diode CR594 becomesfor-
ward biased in this case and shunts the decay current
back to the+ 5 V 0 supply, protecting transistor 0594 from
overvoltageconditions.
When BELL goes HI, transistor 0596 turns on, which in
turn, turns on 0588. With 0588 on, the base of Darlington
transistor 0592 is pulled HI. Now, whenever the oscillator
transistor 0594 is on, proper biasing conditions for 0592
are established and current flows from the +5 Vo supply
to ground through Darlington 0592, the speaker LS498,
and transistor 0594. When 0594 turns off, current flow is
interrupted until the oscillator turns 0594 back on.
Theory of Operation-2430 Service
The BELL line from the MiscellaneousRegister (U760,
diagram1) is usedto gate this oscillator signalthrough the
speaker to produce the audio output. As long as BELL is
LO, transistors 0596, 0558, and 0592 are off, and current
is cut off to speaker LS498.
The oscillator consrsts of timer U274, configured as an
astable multivibrator (oscillator), and output transistor
0594, used to buffer the oscillator output. Current flowing
in R274 and R276 charges C372 up until it crosses the
trigger level at pin 2 of U274.This sets the output applied
to the base of 0594 LO, turning the transistor off, and
sets the discharge output at pin 7 to ground potential.
Capacitor C372 now discharges through R276 until the
threshold level at pin 6 is reached,at which time the out-
put at pin 3 goes HI and the dischargepin goes to a high-
impedance state. Capacitor C372 begins to charge
through R274 and R276 again, completing the cycle. The
cycle continues as long as instrument power is applied,
alternately turning 0594 off and on with an approximate
50% duty cycle.
Bell
The Bell circuit allows the 2430 to produce an audio
tone to draw the operator's attention to certain warning
and error conditions.The circuit consists of a free-running
oscillator whose signal is gated through the output
speaker.
The remaining inputs and outputs of Control Register
U754 are used to control the GPIB Status LEDs and to
reset GPIB Controller U630.
The WDTTL output is applied to the trigger circuits of
the 2430 where, if selected as the trigger source, it pro-
duces a scope trigger event. The trigger signal is buffered
to the rear panel by U844D, 0720, and the associated
components. Output levels are TTL compatible, with the
maximum HI level being set by R716 and VR717. Output
impedances are 47 ohms LO and 227 ohms HI. Diode
CR722, zener VR717, and resistors R717 and R718 pro-
vide protection of the output circuit should an out-of-range
voltage be appliedto the output connector.
Register U754 are used to set the recognition mode of the
Word Trigger Probe. Forty bits of serial data are applied to
the W DATA (word data) line and clocked into the serial
shift register in the word probe by toggling the W CLOCK
(word clock) line. Once loaded, the Word Trigger Probe
outputs a trigger pulse each time (and as long as) the set
conditions are met.

Tektronix 2430 Specifications

General

Brand	Tektronix
Model	2430
Category	Test Equipment
Language	English