Chapter Five: Maintenance
Fault Isolation
35
Set Point Circuits
With the input section zeroed and 10 Volts input applied as described above in “Calibration
of Engineering Units” (place PDR-C-2C rear panel SET POINT switches in “CH 1”
position), check voltages at U20 pin 5 and U16 pin 3 to be 10.000 Volts. If not, check
continuity from R5 and R6 through jumpers E7-E8 and E9-E10 (PDR-C-1C) or through
rear panel switches S7 and S8 (PDR-C-2C).
If 10 Volts is read at U20 and U16 then check the voltage at U20 pin 3 and verify it with
the front panel DVM, with engineering units set to “mmHg” position. Do this by moving
the front panel SET POINT switch to the left for CH 1. Similarly check U16 pin 5 to be
the same as the front panel display with the SET POINT switch to the right (CH2).
Varying the COARSE and FINE set point trimmers should vary the display voltages
accordingly.
By changing the incoming “pressure” signal from the voltage source to be close to the set point
voltage, the outputs of both set point comparators (U18A pin 1 and U18B pin 7) should switch
between the positive and negative supply rails. This in turn should cause U17 pins 10 and 4 to vary
inversely between 0 Volts and the positive supply (be sure the LATCH lines are in a high state-U17
pin 12 and pin 1). Next check the outputs from U19 pins 1 and 7 to ensure switching between rails.
If the above all works, any problem must be with the relay drivers Q5 and Q6 or relays K2 and K3
and associated components.
Channel Selection Circuits
PDR-C-2C only
If no channel selection is indicated, check J113 pin 2 to be connected to digital ground and
J113 pin 1 to be +5 Volts. If digital ground is at fault, check continuity back to the power
supply. If +5 Volts is at fault, check wiring from the front panel DVM (+5 Volt supply
comes from the DVM!). If both are fine and no channel switching occurs, check the
collector voltages of Q1 and Q2 to verify that one is at 0 Volts and the other is at +15
Volts. If this is correct check that relay K1 is switching when changing channel selection
from CH 1 to CH 2 and back. If remote channel selection malfunctions, check U6 pin 4 to
be switching inversely to an a5 Volt logic signal on rear panel Interface connector
(J118) pin C.
If problems exist with the AUTO channel selection circuit, first select AUTO operation and attach
the voltage source to channel 1 Pressure Input as previously described. While monitoring the
voltage at U4 pin 1, vary the incoming voltage from approximately 8 Volts up to 11 Volts noting
that U4 pin 1 should switch high at approximately 10 Volts input. If not, check circuit or adjust
R16. Next do the same test while monitoring U4 pin 7, noting that switching should occur with
approximately 9 Volts input. If this all checks out, next measure U5 pin 4 to be switching high at
approximately 10 Volts as the input increases from 8 Volts. Next decrease from 11 Volts and note
U5 pin 4 to be switching low as the input voltage equals approximately 9 Volts. If this is
happening and automatic channel selection is not occurring, the relay (K1) driver circuits are at
fault.
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