Tektronix 1730 Series, Instruction Manual

Page: 89 / 258

1730–Series Theory of Operation
4–5
The rear-panel Channel A and B inputs are high-impedance bridging loop-
through inputs compensated for use in 75
W
systems. Each amplifier has its own
DC Restorer that is controlled by the front-panel restorer switch. Restorers are
either both on or off; the ONDCR pulse enables (disables) both U395C (pins 10
& 11) and U395D (pins 14 & 15). When U395C and D close they couple the
back porch sample dc level, from the input amplifier output, to the amplifier
inputs. See Timing (Diagram 3) for more information about the generation of
the BACKPORCH signal.
Because the Channel A and Channel B Input Amplifiers are identical, circuit
numbers for the Channel A Amplifier are used to simplify the remainder of this
discussion.
The Input Amplifiers are inverting feedback operational amplifiers with a gain of
–1. The input resistor (R
i
) is R196 and the feedback resistor (R
f
) is R198. A
plug and jumper is provided to select input coupling. J197 is factory set to the
1-2 position for ac coupling; it can however, be moved to the 2-3 position to
provide dc coupling by bypassing C197, the ac-coupling capacitor.
The DC Restorer is a feedback sample-and-hold circuit. Sampling occurs when
U395A (pins 2 & 3) close at back porch time. When the switch closes, the hold
cap (C398) charges up to the dc level of the amplifier output. If ONDCR, from
the Microprocessor (Diagram 5) is present U395D (pins 14 & 15) closes and the
loop-compensated Buffer Amplifier (U495A) drives the Input Amplifier input
summing junction through R197. The time constant of the restorer does not
attenuate 50/60 Hz hum by more that 10%. The choice of fast or slow dc
restorer time constant is accomplished loop compensating the 2
nd
DC Restorer
shown on Diagram 2.
The Input Amplifier output signals drive the channel switch, U492, through pins
2 & 3. The signal selection is determined by the level of the CH-B signal at
U492 (pin 10). When CH-B is low, CH-B is selected; when it’s high, CH-A is
selected. The Channel Switch output (pin 6) drives a current mirror with three
current sources.
One current source, through Q793, drives the rear-panel Picture Monitor Output
Amplifier (SIG 2). C694 is the response adjustment that compensates the
amplifier to match the 75
W
system input impedance. Q792 is the current source
for the internal sync signal and Q791 is the current source for the remainder of
the vertical. With a 1 V input signal there will be 1.11 mA of signal current
flowing through R392 (or R393) into the channel switch. This signal current is
available to drive the vertical through Q791 and the PIX MON OUT through
Q793.
Only 0.5 mA of signal current is available to drive the sync stripper through
Q792. Its emitter resistor, R694, is twice the resistance of R792 and R693, the
emitter resistance for Q790 and Q793.
Input Amplifiers
Channel Switch