HP 8340A Service Manual page 256

When the trigger pulse (TYOKP, Trigger YO Kick Pulse) is received
from the processor, U9 is reset and Ql is turned on through U7B.
When U9 is reset, pin 8 goes HIGH turning Q4 off through U7D. The
constant current of 64 microamps into the integrator now gives
rise to a negative going ramp at the output of Ul4B with a
constant slope of about -291 volts/second. When this ramp reaches
a value equal in magnitude, but opposite in polarity, to the
voltage at the output of the DAC (Ul5), then comparator
and clocks U9. This effectively sets U9, turning off the kick
pulse and zeroing the integrator.
The only adjustment in this circuit is R36, that varies the
current to the integrator to take out tolerances in the
integrating capacitor, C29, and current setting resistor,
The number written to the pulse width DAC address is related to
the desired pulse width by the formula:
Pulse width (msec)
Since the DAC input itself is bits 1-8 rather than 0-7, the
number that appears at the input of Ul5 is the number calculated
above, divided by two.
Power Supplies
Ll-L5 and Cl-CS form standard low-pass power supply filters. The
inductor type was chosen to have a relatively high series
resistance, while the capacitor was chosen to have a low series
resistance. This results in a Q of about 2 for the filter formed
by the LC circuit.
Ul and U3 are 3-terminal 5 volt and 15 volt regulators.
CR2 are protection diodes. If the input to either regulator is
somehow shorted to ground the discharge currents of C9, C35 and
any onboard capacitance is shunted through these diodes rather
than through the regulators.
regulators results in catastophic failures.
Q2 is a simple voltage follower.
filtering to the base of the transistor.
R2 provides a back-up connection between reference and chassis
grounds so that if the motherboard connection between the grounds
is broken for some reason, this board will continue to operate,
although not necessarily within specifications.
Model 8340A - Service
[(Number ) / 512]
=
H
Reverse current through these
Scans by HB9HCA and HB9FSX
34 msec
*
C7, R3 and R4 provide noise
ua fires
R37.
CRl and
8-341