HP 11710A Operating And Service Manual page 14

Page 10
Model 11710A
62. A schematic diagram of converter-amplifier
Al is shown in igure 11. Note that there are
three adjustments: A1Y1 (Xtal Adj), A1R12
(Gain Adj), and A1R5 (5 MHz Null Adj). These
adjustments are set as specified in the Performance
Test and Adjustment procedures.
58. Principles of Operation
59. A block diagram of the 11710A Down Con ­
verter is shown in igure 9. Input power is applied
to Power Supply A2 through switch S2. Power
Supply A2 consists of a voltage rectifier and regul ­
ator circuit. This circuit provides the regulated
+12 Vdc power.
60. Converter-amplifier Al down-converts the
input signal. Relays KI and K2, and range switch
SI select the output signal range. To select the
down-converted signal, switch SI is set to the
5 — 500 kHz position. In this position the +12Vdc
is applied to converter-amplifier Al and relay K2.
The closed contacts of KI connect the output of
Al to the R OUT connector ( J2) via cables W2
and W5. Since relay KI is not energized when
switch SI is set to the 5 — 500 kHz position, the
open contacts connect the unit R IN connector
(JI) to the input of Al via cables W3 and Wl. The
input signal is mixed with the 5 MHz local oscilla ­
tor and the down-converted signal is then ampli ­
fied and filtered. Only the difference frequency is
allowed to pass through the filter to the output.
56. SERVICE
57. Service instructions consist of principles of
operation, troubleshooting, and repairs.
61. When range switch SI is set to the 0.5 —
1024 MHz position, +12 Vdc is applied to relay KI
but not to converter-amplifier Al or relay K2.
Converter-amplifier Al is disabled and bypassed
and the input signal is routed directly to the out ­
put connector via cables W3, W4, and W5.
63. A schematic diagram of the power supply
circuit (A2) is shown in igure 13. The power
supply is a series pass type with QI being the series
pass transistor. Comparison amplifier A2Q4 and
Q5 compares the divided down supply voltage
against the reference A2VR2 and drives QI
through A2Q2 to bring the base voltage of A2Q5
equal to the base voltage of A2Q4. A2Q3 is a cur ­
rent limiting transistor that is normally off. If the
supply current is large enough, the voltage drop
across A2R5 will turn A2Q3 on. This in turn shuts
A2Q2 and QI off. A2VR3 and A2Q6 form a crow ­
bar to protect the output from voltages that are
too high. If the output exceeds the breakdown
voltage of A2VR3, it conducts and fires SCR
A2Q6 which shorts the output and initiates current
limiting. Variable resistor A2R9 is the output
VOLT ADJ control and is set as specified in the
Performance Test and Adjustment procedures.
64. Troubleshooting
65. 11710A Down Converter circuits are conven ­
tional and are not complicated. Significant circuit
stage functions and operation levels are identified
in the schematic diagrams. Therefore, trouble ­
shooting can be accomplished by using all the in ­
formation in the Principles of Operation and the
schematics and by conducting the Performance
Test and Adjustment procedures. By using this
approach, the user can quickly isolate a malfunc ­
tion to a chassis-mounted or PC board-mounted
component.
66. Repair
67. In some instances, repair consists of merely
making the required adjustments to bring the
instrument up to specification levels. In other
cases, repair requires the replacement of malfunc ­
tioning component with a known good component.
Assembly and chassis component locations for the
instrument are shown in igure 14. Parts locations
for PC boards Al and A2 are shown in igure 10
and 12, respectively. To gain access to the chassis ­
mounted components and PC boards, remove the
top cover. The side panels and bottom cover are
also removable (see igure 8). The Al Assembly
may be extended by use of the 12-pin extender
board located inside the chassis. A 30-pin extender
board (HP 08640-60036) useful for extending the
A2 Assembly is available from your nearest
Hewlett-Packard office.
55. To order a part that is not listed in the re ­
placeable parts table, include the instrument model
number, instrument serial number, the description
and function of the part, and the number of parts
required. Address the order to the nearest
Hewlett-Packard office.