- Because
of the temperature changes and vibration encountered in mobile
operation, and because of the crystal problems, many stations can
get off frequency.
- To
remain accurate, the receiver must be checked against a frequency
standard on regular basis.
- An
old but frequently ignored adage in servicing is to check the
obvious things first.
- Are
the tubes lighted, cracked or loose?
- Is
the fuse blown?
- Is
the speaker lead disconnected or broken?
- Has
the power plug fallen out of the socket?
- An
inspection should reveal any overheated resistors, electrolyte
capacitors…
- …that
are swollen or have been leaking fluid.
- If
a discolored resistor is found…
- …the
cause of the excess current should be found and cured.
- The
speaker can be tested with an ohmmeter.
- A
noise should be heard when the probes are connected to the speaker
leads.
- The
audio output transformer can be checked in the same manner.
- Audio
amplifier stages can be tested by applying a signal to the input of
each one.
- A
test signal can be obtained from an audio oscillator…
- …another
receiver…
- …or
from the secondary of a filament transformer.
- The
importance of maintaining a zero, or center frequency, reading at
the discriminator output cannot be overstressed.
- This
saturation is indicated by rectification of the input signal.
- The
operation of the oscillator can be checked by the use of a grid-dip
meter or by a detector probe connected to a VOM.
- Connections
for signal path, dc returns, and chassis ground are not always the
same.
- The
consequences of an adjustment can be noted two or three stages away
from the actual point of making that adjustment.
- Noise
from these sources is most likely to become manifest during periods
of moderate to high wind.
- The
weakest point in any receiver is the front end.
- The
times-out timer starts counting when a carrier is received.
- One
of the first preventive measure to be taken is that of cleaning up
the transmitter.
|
