Jumpers - HP 82718A Service Manual

HP 82718A Functional Description 3-3
When Vp drops sufficiently low, the sense voltage falls below the reference voltage, and the output of UlA
• ^ goes to 0 volts; Q4 is turned off and Q3 is turned on. The energy-storing sequence then begins.
Energy is stored in Tl by the battery current flowing through Q3. This is a ramp current of up to approxi
mately 1.2A maximum; it flows from the battery through Q3 and the primary winding of Tl. The current
induced in the secondary of Tl flows through R3 and C2 and provides a positive feedback voltage to the
base of Q4, quickly driving Q4 into saturation. Since the current in the primary of Tl is no longer increas
ing, the current in the secondary of Tl starts to fall, thereby applying a negative-going voltage to the base
of Q3, quickly turning Q3 off. The energy stored in the primary of Tl then flows through diode CR2 into
C5, causing Vp and the sense voltage to rise to or above the reference level. The energy-storing sequence
is then completed until the sense voltage again falls below the reference level.
Capacitors 03 and 05 and transistors Q3 and Q4 are returned to B- rather than ground because the series
resistance of R5 would limit the amount of energy that could be stored in Tl and the speed with which
this energy is transferred to capacitor 05. The feedback network consisting of resistor R3 and capacitor
02 is designed to permit a collector current of at least 1.2A to flow in Q3. R18 increases the voltage on the
base of Q3 when the modem is turned on to provide for the additional energy required by the modem.
Pod battery number one is located electrically in the HP-75 battery circuit, and pod battery number two is
effectively connected in parallel with it. Therefore, it is possible for the HP-75 battery-detect circuit to
detect the level of the pod batteries. The BATT annunciator appears first; then the "WARNING: low
batteries" message appears, and the system shuts down. The batteries can be charged by plugging an ac
adapter into the HP-75.
Resistor R1 is connected between the negative terminals of the two batteries to limit the equalizing cur-
^0^ rent that will flow if the batteries are not charged to the same voltage. R5 is a 1/8W resistor that burns
out quickly if Vp is shorted to ground thereby acting as a fuse to protect the circuitry on the PCA. (R5 on
the Pwr/Mem PCA in the HP-75 could also be burned out.) It also acts as a charging-current limiter when
the batteries are charged from the HP-75.
3-6. Jumpers
Jumper Wl is connected to ground in units used in the US. It will be connected to Vm in a European unit
that includes the modem, although there are no plans to develop such a product at this time.
Jumper W2 connects resistor R12 to Vp in units containing U2s with on-board ROM. In units with ex
ternal ROM, W2 grounds R12.
Jumper W3 permits adjustment of the 6301 modem controller (U2) to operate with modems of differing
noise characteristics. It connects RXO (U2-11) to either RXl (J2-11) or Vp (U2-9). Pods using the 82718-
60907 modem should have W3 connecting U2-11 to J2-11.