HP 740B Operating And Service Manual page 35

Section IV
4-110. METER DRIVER AND FEEDBACK DIVIDER.
4-111. From A2 pin 10, the de signal is fed through
A2R17andA2R19* to A2Q6. A2R19* (typically 22 kn)
is factory selected to set the gain of the Meter Driver.
(Paragra ph 5-86 describes the procedure for selecting
A2R19*. ) A2Q6 amplifies the de level and drives A2Q8.
A2Q7 provides temperature compensation for A2Q6.
The signal is amplified by A2Q8 and A2Q9 and applied
to the class AB push-pull output stage, A2Q10 and
A2Q11. A2CR3 and A2CR4 keep A2Q10 and A2Q11
slightly forward-biased under no-signal conditions in
order to prevent crossover distortion. Degenerative
ac feedback from the emitters of A2Q10 and A2Q11
to A2Q6 causes the Meter Driver to have good noise
rejection. DC gain of the Meter Driver is controlled
by degenerative dcfeedback through A2R32 and A2R31
to A2Q6. DC gain is approximately 200.
4-112. The output of the Meter Driver drives the Meter,
M1. C13 bypasses any sharp transients around M1.
A10R3 and A10R10 match the meter characteristics
to the output of the Meter Driver. A10R10 calibrates
the meter end-scale deflection. The Meter Driver
output voltage is also applied across R14 to the RE-
CORDER OUTPUT terminals. R14 varies the voltage
from 0 to approximately 1 V for an end-scale meter
deflection, allowing the output voltage to be matched
to an external recorder or monitoring device.
4-113. The Meter Loop Feedback Divider controls
the Meter Loop gain. The divider is comprisedof
S2R1 through S2R5 and the Sensitivity switch, S2.
When the Xl SENSITIVITY button is depressed, all of
the Meter Driver output voltage is fed back to the
Modulator, A16V1/v2, setting the effective gain of
the Meter Loop at 1 (unity). Depressing the X10
through X10 4 pushbuttons decreases the feedback by
a factor of ten each time, resulting in an end-scale
sensitivity of 100 Jl
V on the X10 4 Sensitivity.
The
divider resistors (S2R1 through S2R5) are precision
wirewound resistors. The divider is calibrated by
adjusting A10R9 which changes the effective resistance
of S2R5.
4-114. Zero offsets in the Meter Loop are nulled out
by a controlled opposing offset voltage from A10R12.
The voltage from the wiper of A10R12 is fed through
A10R5 to the Meter Loop Feedback Divider.
4-115. POWER SUPPLIES (Figure 7-3).
4-116. Power is supplied to the Model 740B from an
external 115 Vac or 230 Vac source through the Power
Input Receptacle, J3. The power is applied through
L6 and L7 to the POWERON -OFF Switch, S12, and the
115-230 Switch, S13. S12DSllightswhenS12 is in the
ON position. S13 switches the primary windings of T1
from series -connected to parallel-connected when 115
Vac operation is selected. The voltage across one
primary winding of T1 isfed to the primary of T2. T1
provides power to the Unguarded Power Supplies; T2
provides power to the Guarded Power Supplies.
4-117. GUARDED POWER SUPPLIES.
4-118. Floating 12VPowerSupply, plo A5(Figure7-9).
The Floating 12 V Power Supply consists of a full-wave
4-10
Model 740B
rectifier (A5CR6 and A5CR7) and a filter (A5C7). A
secondary winding of T2 provides approxin1ately 30 V
rms to the power supply. The Floating 12 V Power
Supply keeps Zener Diode A5CR4 in permanentbreak-
down. A5R18 limits the breakdown current. The volt-
age across A5CR4 introduces a permanent 12 V offset
inthe -Outputstage of the Main Loop, allowing linear
operation at low output voltage (Paragraph 4-71).
4-119. ±22 V, ±10V, -30 V and +34 V Supplies. A6
(Figure 7 -10). T2 provides approximately 40 V rms
to bridge rectifier A6CR1 through A6CR4. The +out-
put of the bridge is filtered by A6C 5 and the resultant
+ 22 V is fed out A6 pin 13. The - output of the bridge
is filtered by A6C11 and the resultant - 22 V is fed out
A6 pin 12. The + and -22 V supplies are unregulated.
4-120. The + 10 V Supply consists of a Darlington-
connected series regulator, A6Q1 and A6Q2. A6CR5
provides the reference voltage for the control amp-
lifier, A6Q3. A6Q3 provides the correct bias for the
series regulator. The + 10 V Supply obtains primary
power from the + output of the bridge rectifier, A6CR1
through A6CR4. The regulated + 10 V output is fil-
tered by C1 and fed out A6 pin 21.
4-121. The -10 V Supply is similar to the + 10 V Sup-
ply. The major difference is the voltage reference
for the control amplifier, A6Q4, which is derived from
the output of the + 10 V Supply dropped across A6R11
and A6R10. The - 10 V regulated output is fed oul A6
pin 18.
4-122. The - 30 Vand + 34 V Supplies consist of brid[':e
rectifiers A6CR10 through A6CR13 and A6CR6 through
A6CR9. The -30 V outputisfed outA6 pin 14 and fil-
tered by C12. The +34 V output is filtered by A6C13
andfedoutA6 pin 3. The +34 V Supply is referenced
to
W
but may be measured with respect to
\y
in
Standard mode when the two grounds become eleclri-
cally common. The - 30 V and + 34 V Supplies are
unregulated.
4-123. UNGUARDED POWER SUPPLIES.
4 -124. - 42 V Supply (Figure 7-3). T1 provides pri-
mary power for Zl located on the main chassis. The
rectified output of Zl is filtered by C1, L1 and C2.
An interlock on A7 pins 13 and 14 interrupts the - 42
V
Supply whenever A7 is removed. The - 42
V
Supply
provides power to the Power Switch (Q1 and Q2) and
supplies energizing current for the Output Rotary
Switch, K1. The output of the - 42 V Supply is re-
ferenced to
w·
(wand ware electrically common).
4-125. ±16. 5 V Power Supply, pia A7 (Figure 7-11).
T1 supplies approximately 26 V rms to bddge-recti=-
fier A7CR4 through A7CR7. The
I
output of the
bridge is filtered by A7C2 and connected to A6 pin 17.
The - output of the bridge is filtered by A7C 1 and
connected to A7 pin 18. The Output Switch Interlock
is connected across A7 pins 17 and 18. The interlock
consistsofrelayA7K1 and A7R19. The interlock in-
terrupts the - 42 V supply r)utput tr) the Output Switch,
Kl, when the instrumenl is turned off. K1 wr)uld
otherwise remain ene rgized fr)r several secrmds due
to the long discharge time r)f the - 42 V Power Supply
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