Oscilloscope Functions; Preamplifier - Tektronix 213 DMM Instruction Manual

ever either of these thresholds is exceeded, current flow in-
creases in the associated transistor which causes 0204 to
turn on. 0157 acts as a clamp to prevent the Buffer Ampli-
fier output from going negative enough to saturate 0156.
RMS Converter overrange occurs whenever the crest factor
ratio is exceeded. When this happens, 0195 turns on and
causes 0204 to turn on.
In the A/D Converter, R205 and R206 set a positive thres-
hold for 0206. Whenever this threshold is exceeded by the
positive ramp from U220 there is increased current flow in
0206 which causes 0204 to turn on.
When 0204 turns on, pin 2 of U250A goes LO, pin 3 goes
HI, and 0256 turns on which causes a negative unblanking
signal to appear at the input of 0390. As long as the over-
range condition remains, the count 0000 reset pulse from
pin 8 of U260, which is applied to pin 5 of U250B, cannot
OVERRANGE
STABLE
OCCURS
+
ON
I
0204
OFF
U250A PIN 2 HI
LO
U2508 PIN 5
HIV
LO
HI
U250A PIN 3
U2508 PIN 4
LO
U250A PIN 1
HI
U250B PIN 6
LO
ON
0256
OFF
Theory of Operation-213 Service
reset the overrange latch. As soon as the overrange condi-
tion disappears, pin 2 of U250A goes HI. On the next reset
pulse, pin 1 goes HI, pin 3 goes LO, 0256 turns off, and the
overrange indication disappears.
OSCILLOSCOPE FUNCTIONS
0 <9
Preamplifier
The Oscilloscope Preamplifier is a simple feedback amplifier
consisting of O 170, 0172, and O 174. The input signal is de-
veloped across the 1-2-5 gain switching resistor network
R 128, R 129, and R 178. Amplifier gain is set by R 130,
R132, R133, and feedback resistor R174. Vertical signal
positioning is controlled by current from R 135 through
R 134. This current changes the voltage across R 174 thus
providing position control without affecting amplifier gai.[i.
The Preamplifier output drives the vertical ampl ifi.er section
and provides an internal vertical signal to the trigger circuit
when selected by S315.
0
OVERRAN GE
0
0
STOPS
0
0
+
0
0 0
I
I
V"
r
n
1481-14
Fig. 3-7. Overrange timing diagram.
@
3-9