SECTION VI
SERVICE AND MAINTENANCE
a. Replacement of Tubes: Replacement or the rectifier and voltage
The vacuum tubes in the Model 59 ere con- regulator tube will seldom be required. Aged regu-
lator tubes sometimes become erratic and their out-
servatively operated and should provide long ser- put voltage will fluctuate, resulting In erratic
vice life unless tie instrument is abused by rough
handling. Aiter several hundred hours of opera- grid current. Occasionally new regulator tube: may
tion; however, the 955 oscillator tube may show also be erratic. Any tube which exhibits jitter or
unstable operation should be replaced.
signs of reduced output by lower readqgs or grid
current on the highest frequency coil. When this
occurs it may be necessary tomplace the 955 in
b. Circuit Failure:
the probe.
To replace the 955, remove the tuning knob Most circuit failures wlll be evident from
indications of the grid current meter and reference
and the three screw8 around the edge of the probe to the schematic diagram, Figure 17. Failure to
assembly. Then lift the dial cover from the probe;
thus exposing the calibrated tuning dial, etc. oscillate will usually result, if the probe ha6
Next remove this calibrated dial, taking care not been dropped and one of the ceramic variable tunlng
eapacltor supports has been broken. It is advisable
to touch and mar the calibration numbers. (This
dial should be kept face up and covered during the to return the instrument to the factory for repair
time it remains out of the probe to prevent dam- in this event. Replacement of the ceramic is a
major repair and will require re-callbratJon of
age). The cathode Clip must be slipped loose radi-
ally ffrst. The 955 can then be removed by a Ellght the frequency dial. If return is not possible,
twisting, counter-clockwise motion, after disen- after replacement of the ceramic support, it wlll
gaging the cathode clip. Next insert the new 955 be necessary to center the stator approximately
and twist slightly into place. Take care to see and then set the frequency callbration as closely
that all four pins line up properly. Do NOT FORCE as possible by the roiiowlng process. Put the
into place. Forcing is not necessary, if the lowest frequency coil in position (with the serial
radial tube pins are properly lined up with their numbered side UP tmard the tULng dial side of
respective clips; application of excessive force the probe). Be sure it is fully inserted or
wlll probably damage the alignment of the tuning bottomed in the jacks. Then allow about 5 minutes
condenser. After twisting the new tube into place for the instrument to warm up. Rotate the tuning
properly, slide the cathode (center pin) contact dial until it reads approxlmately 5 mc. and locate
clip into place. Next replace the tun- dial and exact zero beat note with W. Then turn dial
three mounting screws, again exercising care not down to 2.5 mc. and note displacement of beat note
to smear the printed numbers. Replace the dtal from the calibration mark. Rotate cover and fidu-
cover and knob and leave the three cover screws cial to registration, if the displacement is about
slightly loose, 80 that it will be possible to the same as at 5 mc. If the amount is more or
rotate the dial cover (permitted by the three slots less than that at 5 mc., split the difference by
rotating the cover and fiducial.. Tighten the
around the fastening screws). Turn the tuning knob
as far as it will go clockwise and adjust the po- three screws so this adjustment will not slip.
sition of the fiducial so that it falls over the A slight readjustment of the dlal cover md ilh-
long end mark on the dial. Tighten the three case cia1 may be required.
SC rews .
During the above procedure, care should be
exercised to prevent placing the probe and oscil-
lator coil too close to any metal object which
mlght seriously affect the frequency or the oscil-
lator. It 11 not necessary to couple closely to
the receiver or frequency meter for zero beat
against the standard frequency source.
22. |
ACEC
