Tektronix 491 Maintenance Manual page 62

Fig. 4-17. Preparation to remove the oscillator and tuner assembly.
b. Slide the oscillator tube out of the mounting flanges by
gently pushing the tube with a plastic or fiber dowel. See Fig.
4-19B.
c. Insert the new tube and slide into place. Do not bend
the two contact fingers for the filament buttons.
d. Replace the cover and secure all mounting screws.
Tighten the screws uniformly.
e. Refer to the Calibration section to calibrate the oscil-
lator and check its operation.
2. Band C oscillator tube replacement (V42)
The procedure to replace this tube is described and
illustrated in Fig. 4-20A and 4-20B.
Component Replacement
The physical size and shape of the replaced component
may affect the performance of the circuit, therefore it is best
to duplicate the original component as much as possible.
Parts orientation and lead dress should also duplicate those
of the original part since many of the components are
mounted in such a way as to reduce or control circuit capaci-
tance and inductance. After repair, the circuits of the instru-
ment may need recalibration.
4 - 1 4
Fig. 4-18. Replacing the mixer diodes.