Tektronix 323, Инструкция по эксплуатации

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Circuit Description— Type 323
Explanations regarding the remaining CRT elements ap­
pear in conjunction with the High Voltage Power Supply
description.
Low Battery Sensing Circuit
The Low Battery Sensing Circuit employs a relaxation
oscillator (R506, C507 and B509) operating at a frequency
of approximately 1 to 2 Hz. When the input power exceeds
+6.25 V, Q505 is saturated and the voltage at its collector
is not sufficient to fire the neon LOW BATT indicator, B509.
When the input falls below 6.25 V, Q505 turns off and
C507 charges toward +100 V until B509 fires and discharges
C507. The cycle then repeats.
Although B509 will blink in any power mode when the
supply is less than 6.25 V, it is of primary concern during
internal battery operation. If the Oscilloscope is left ener­
gized in the internal battery mode for a considerable period
of time after the battery output falls below 6.25 V, the bat­
teries may be damaged to the point where they will no
longer be chargeable. If the light blinks during operation
with external power, it is advisable to check the input volt­
age for at least 6 V. The Oscilloscope itself can be used to
perform this check. Satisfactory external powered operation
can be expected with the power as low as 6 V, even though
the LOW BATT indicator is blinking.
POWER PACK < § >
The Power Pack contains the battery which supplies the
internal power, connectors for applying external AC or
DC power, a transformer and rectifiers for AC operation,
and a battery charging circuit for recharging the internal
batteries from an external AC source. The switching cir­
cuitry which selects the power source is also contained in
the Power Pack.
Block Diagram Description
Refer to the block diagram contained on the Power Pack
schematic diagram page. SW612 is a multiple contact
switch which has three positions— EXT DC, TRICKLE CHG
and FULL CHG.
In EXT DC position, power is routed
from the DC input jacks to the Oscilloscope POWER switch,
SW501. The internal batteries and the battery charging cir­
cuit are disconnected from the rest of the Oscilloscope in
this mode of operation.
With SW612 in FULL CHG or TRICKLE CHG position,
either AC or internal battery operation is possible. With
no AC applied, the batteries supply the power. When AC
is applied, the transformer supplies operating power, and
provides battery charging power during part of each input
half cycle. When the output of the transformer secondary
falls below a certain level, diodes disconnect the transform­
er from the charging circuit and the battery supplies the
operating power until the power supply diodes again con­
duct. In effect, the battery acts as a large filter capacitor
for Oscilloscope operation when AC is applied. In the AC
mode of operation, a reference voltage is developed across
D649. The Comparator Amplifier compares a portion of
this reference to the voltage generated by the battery charg­
ing current which flows through R615. The Comparator Am­
plifier output controls the Driver Amplifier, which controls
the conduction of the Series Regulator Q617, thereby de­
termining the battery charging current. The battery charg­
ing circuit is independent of the POWER switch, operating
whenever AC power is applied.
Battery Charger
Refer to the Power Pack schematic. When AC power is
applied, the output of the upper secondary winding of
T601 is rectified by D605 and filtered by C605. The bottom
of C605 is connected to the positive side of the battery.
The charge on C605 is in series with the battery, and their
combined voltage is applied to the R605-D649 combination.
D649 provides a 6.2 V reference for battery charger opera­
tion.
This 6.2 V reference voltage is applied to R643-R644,
setting the Q636 base voltage. This voltage is compared
to the Q634 base voltage (average voltage across R615)
to determine the division of R635 current between Q634
and Q636.
The lower secondary winding of T601 delivers charging
and operating current through full-wave rectifier D610.
The positive side of the rectifier is connected to the positive
side of the battery, and the negative side is connected
through the series regulator circuit to the battery negative
side. There is a time internal between pulse peaks during
which time D610 does not conduct. See Fig. 3-10 (B). The
current from Q634 and Q636 is then shunted to ground
through D637 and D638, keeping Q634 and Q636 from
saturating.
When the half-cycle output voltage of T601 secondary
becomes large enough to overcome the battery voltage,
D610 goes into conduction, delivering a negative-going
voltage pulse to the battery charging network. R635 cur­
rent starts to flow through R639, R637 and R638. See Fig.
3-10 (A) and (C). The combination of a negative-going volt­
age at the emitter of Q621 and the voltage developed
across R639 causes Q621 to conduct, supplying current
drive to Q620, which supplies current drive to Q617. See
Fig. 3-10 (D), (E) and (F).
Q617 goes into conduction once each half-cycle and the
resulting current develops a positive pulse across R615.
See Fig. 3-10 (G). Voltage divider action causes a portion
of each pulse to be developed at the base of Q634. C636
charges up to the average voltage, thus developing the
Q634 base voltage. The comparison of this average volt­
age to that set on the base of Q636 (by CHARGE RATE
potentiometer R644) determines how much drive current
is provided to Q621. If an increase of line voltage attempts
to increase the charging rate, C636 charges to a higher
average value, decreasing the drive current to Q634 and
Q621. This decreases the drive current to Q617, keeping
the R615 charging pulses (and therefore the battery charg­
ing rate) within design limits.
It should be noted that even though C636 is charged to
the average of the input pulses, almost identical pulses are
present at the bases of Q634 and Q636, so that the cur­
rent division between the two transistors is not upset dur-
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