296
8.
MICROSCOPY
reference, instabilities can be expected to occur as the batteries age, and
often before there is much change in the rated voltage. The voltage instability is usually of sufficient magnitude so that tiny "kicks" can be seen
on a standard voltmeter if the battery itself is tested.
As with any complicated electronic device, tube failure is an occasional cause of circuit failure. Tube failure, of course, can be a relative
thing so that the tube functions to some extent, but below critical levels.
It is not difficult to check tubes by substitution. Any laboratory that is at
all distantly removed from a service source should stock a complete extra
set of tubes. The investigator can substitute them in blocks. Thus, all
those tubes connected with the objective lens circuit might be substituted
at one time if this circuit is suspected. If the difficulty is corrected, the
particular faulty tube can be determined by successive elimination.
8.14. Trouble Shooting
When an electron microscope behaves in an erratic manner a "differential diagnosis" is called for. We have already considered most of
the common ailments in detail, but scattered through this and previous
chapters. There follows a summary which, in essence, can be regarded as
a check list.
(1) Image instability! This is most apt to be from dirt or contaminating films on the objective lens aperture or pole-piece. If the movements vary with beam intensity, these areas are particularly suspect.
If it is the aperture that is involved, moving it slightly may give stability
temporarily. Dirt or lint near the specimen, either on the supporting
grid, in the bore of the specimen holder, or on the lip of the cap may be
responsible. Then the instability is apt to vary as the stage is moved
about. Contamination lower in the column, particularly in the projector
pole-piece, is not uncommon. Electronic instability may be the cause.
Dirty focusing potentiometers frequently are at fault. Poor contacts in
switches and plugs may cause the difficulty. Tubes may be functioning
inadequately. Dry cell reference batteries may be "noisy." Electronic
difficulties producing image movements might be found in the high
voltage, objective lens and projector lens circuits.
(2) Flickering illumination with a stable or nearly stable image! This
indicates difficulty above the specimen level. The anode cap probably is
dirty. This is apt to be associated with unduly high beam currents which
8.
MICROSCOPY
reference, instabilities can be expected to occur as the batteries age, and
often before there is much change in the rated voltage. The voltage instability is usually of sufficient magnitude so that tiny "kicks" can be seen
on a standard voltmeter if the battery itself is tested.
As with any complicated electronic device, tube failure is an occasional cause of circuit failure. Tube failure, of course, can be a relative
thing so that the tube functions to some extent, but below critical levels.
It is not difficult to check tubes by substitution. Any laboratory that is at
all distantly removed from a service source should stock a complete extra
set of tubes. The investigator can substitute them in blocks. Thus, all
those tubes connected with the objective lens circuit might be substituted
at one time if this circuit is suspected. If the difficulty is corrected, the
particular faulty tube can be determined by successive elimination.
8.14. Trouble Shooting
When an electron microscope behaves in an erratic manner a "differential diagnosis" is called for. We have already considered most of
the common ailments in detail, but scattered through this and previous
chapters. There follows a summary which, in essence, can be regarded as
a check list.
(1) Image instability! This is most apt to be from dirt or contaminating films on the objective lens aperture or pole-piece. If the movements vary with beam intensity, these areas are particularly suspect.
If it is the aperture that is involved, moving it slightly may give stability
temporarily. Dirt or lint near the specimen, either on the supporting
grid, in the bore of the specimen holder, or on the lip of the cap may be
responsible. Then the instability is apt to vary as the stage is moved
about. Contamination lower in the column, particularly in the projector
pole-piece, is not uncommon. Electronic instability may be the cause.
Dirty focusing potentiometers frequently are at fault. Poor contacts in
switches and plugs may cause the difficulty. Tubes may be functioning
inadequately. Dry cell reference batteries may be "noisy." Electronic
difficulties producing image movements might be found in the high
voltage, objective lens and projector lens circuits.
(2) Flickering illumination with a stable or nearly stable image! This
indicates difficulty above the specimen level. The anode cap probably is
dirty. This is apt to be associated with unduly high beam currents which