6.5 OSCILLATOR TROUBLESHOOTING HINTS
As a
rule
the
only
test
equipment
carried by
service
representatives
is a Digital Volt Meter (DVM).
The
oscillator
-
circuitry is
generally
difficult, if
not
impossible to
troubleshoot in the field using a DVM.
Various power supply pins
may be checked at specific
IC's
on the oscillator
circuit
and
output pins may be checked for "relative" output level. In
this
instance relative refers to an estimated voltage output dependent
on the
frequency
selected and the speed at
which
the
DVM's
circuitry measures its input signal.
Generally when measuring a
0 Vdc to
5
Vdc square wave with a
50/50
duty cycle the dc voltage
should measure
2.5
Vdc.
A measurement of
>4
Vdc
or<1
Vdc
would
indicate steady state output.
Likewise an ac RMS measurement of
2.5
VRS would be normal and an ac reading of 0 V would
indicate
steady
state.
This type of information allows us to
check
for
the
presence or absence of signal flow.
Using this information
can sometimes point to open connections,
defective amplifiers and
switches,
or other hardware failures.
The most effective way to troubleshoot the oscillator is with an
oscilloscope.
The ocsilloscope enables visualization of the
sinewave
or square wave.
Distortion or frequency errors become
obvious and signal level may be observed at the same time.
The
oscillator circuit is located on the Analog Board
Schematic
#1.
The oscillator output is routed to the attenuator
circuitry
via
the
Filter
Multiplexer (U 200).
The
Filter
Multiplexer
routes
the
appropriate
signal to the input of
the
attenuator
circuit.
This signal is either the oscillator output Band
Pass
filtered (BP),
Low Pass filtered (LP), or a test
signal
input.
The
antialliasing filters round the edges of
the
square
wave
clock input prior to filtering.
If the oscillator circuit is non-functional verify that the square
wave clock signal is present at
U511
-
pin 10.
This is a
O-5
Vdc
(2.5 VRMS)
square
wave
which should be the
same
frequency as
indicated
on the front panel.
The output of
the
antialliasing
filter
U203
pin
-
7 should be a square wave with
rounded edges
and its voltage swing should be approximately -3.5 V to
+3.5
Vdc
(2.5 VRMS).
The output of the Switched Capacitor filter
should
be a clean undistorted
sinewave
with a voltage swing of -5 Vdc to
+5
Vdc (2.5 VRMS).
6.6 TROUBLESHOOTING THE PROCESSOR
(U503)
There are some processor related problems that can be repaired in
the field.
The processor on the audiometer board is a
multi-
combination device.
It has its own internal EEPROM, D/A
converter,
RAM, watch dog circuitry,
Priority Interrupt function,
and a free running timer.
Processor errors are generally
associated as lock-ups.
Lock-ups occur for various reasons. A
shorted or loaded address or data line will stop the processors
execution process, the result is a 'lock-up'.
When this occurs
the processor will generally continue to strobe the address and
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