Tektronix 491, Maintenance Manual

Page: 60 / 226

Fig. 4-15. Removing the Phase Lock assembly.
Removing the Phase Lock Assembly
1. Repeat steps 1 through 3 of the above procedure, then
remove the knob and hardware for the FINE RF CENTER
F R E Q , t h e I N T R E F F R E Q c o n t r o l s a n d t h e L O C K C H E C K
button switch.
2. Remove the mounting screws through the side frame to
the assembly box.
3. Disconnect the three right angle sub-miniature connectors
at the back of the phase lock assembly (J1140, J1150 and
J1160).
4. Slide the phase lock assembly back and lift up and
out of the unit. Set the assembly on a block or the table
top. Power can now be applied to the unit for voltage or
waveform measurements,
If the assembly is to be removed and replaced, disconnect
the pin connectors.
4-12
Removing the Circuit Board Assembly for the
Phase Lock (See Fig. 4-16)
1. Remove the retainer nuts on the connectors J1140,
J1150 and J1160.
2. Remove the two screws that secure the pin connector
to the chassis,
3. Unsolder the wires to the INT FREQ control and remove
the potentiometer.
4 . U n s o l d e r t h e w i r e t o t h e 1 M H z M A R K E R S O U T
connector.
5. Remove the hardware to the RF CENTER FREQ potenti-
ometer and the LOCK CHECK switch button. Push the con-
trols into the box,
6. Remove the five mounting screws.
7. Pull the cable into the box to provide cable slack, then
lift the board up and slide clear of the chassis lip.
When reassembling the board into the box, be sure to
install the wires on the pin connector before replacing
the box assembly.
8. Remount the assembly using the reverse of the removal
procedure. Do not force the assembly into place, Check the
wiring and connectors to avoid wire pinching or strain on
the connectors.
Removing the Line Voltage Selector
Use a pin extractor, Model 107 R-1001; manufactured by
Winchester Electronics Div. of Litton Industries, or equivalent
to remove the wire and pins from the connector. The pins
are re-inserted into the selector with a pair of needle nose
pliers.
Removing the Oscillator Assembly (Fig. 4-17)
1. Remove the tuning knob and the band switch knob,
then remove the flat head screw under the tuning shaft.
2. Disconnect the sub-miniature right angle coaxial con-
nectors and the pin connectors to the oscillator assembly.
3. Remove the two mounting bracket screws under the
coaxial band switch. See Fig. 4-17A.
4. Remove the two nuts securing the drive gear assembly
to the front panel assembly. See Fig. 4-178.
5. Pull the oscillator assembly back and up to remove.
To replace the assembly, reverse the removal procedure.
The two drive gear assembly mounting nuts and the flat
head screw should be installed before the mounting bracket
screws are replaced. To align the assembly, loosen the two
mounting nuts for the bracket assembly and slip the mount-
ing bracket in the slots provided; then tighten the nuts,
Replacing Mixer Diodes
Mixer diodes are sensitive to RF electrical fields and
static charges. Exposure to these fields may damage the
replacement diode.