tune the RF CENTER FREQ to any harmonic signal. Depress
the LOCK CHECK button and adjust the FINE RF CENTER
FREQ control to establish a lock mode on the harmonic
signal. See Operating instructions.
4) Decrease the DISPERSION to 50 kHz/div, keeping the
signal centered on screen with the IF CENTER FREQ control,
5) Slowly adjust the FINE RF CENTER FREQ control until
the LO loses its lock. The signal may shift screen when the
LO loses phase lock. Re-establish phase lock by adjusting the
FINE RF CENTER FREQ control to return the signal on screen,
6) Slowly adjust the INT REF FREQ VARIABLE control.
Note the signal shift across the dispersion window as the
reference oscillator frequency is changed. Range of the
control is approximately 0.1% of the dial frequency. See
step 27.
f. Remove the signal and harmonic source from band B
RF INPUT connector and apply the signals to band C Co-
axial Mixer. Switch the band selector to C and set the DIS-
PERSION to 2 MHz/div.
g. Check dial accuracy over scale 4, band C. Check oscil-
lator and mixer operation and local oscillator phase lock
as the dial accuracy is checked. Check these parameters by
repeating the procedure described in step d for band B.
NOTE
There is no need to check dial calibration for the
upper scales of band C because they are multiples
of scale 4.
h. Apply 10 MHz (.1 µs) marker signals and harmonics to
the band A RF INPUT. Switch the band selector to A. Set the
DISPERSION to 1 MHz/div.
i. Check the dial accuracy, oscillator and mixer perform-
ance, and LO phase lock operation through band A. Check
by using the same procedure that was used to check bands
B and C.
j. Check phase lock operation with an external reference
frequency. Use the procedure described in step 38A (k).
RF AND LOCAL OSCILLATOR CALIBRATION
NOTE
The following procedures are NOT part of the
routine calibration. They only provide a means
of calibrating the RF section after minor repair,
such as oscillator tube replacement. If possible we
recommend the complete RF assembly or the com-
plete unit be returned to Tektronix for repair,
See your local Tektranix Field Office or representa-
tive.
Dial Tracking
1. Apply power to the oscillator and allow 20 minutes
warmup time for the oscillator to stabilize.
2. Push the LOCK CHECK button and adjust the RF FINE
FREQ control for a centered (vertical) trace. This should
provide a varactor bias reading of +7.0 volts. See Fig. 6-48.
3. Use an accurate frequency meter or
the A band oscillator to exactly 375.5 MHz
a dial reading of 175.5).
counter to tune
(200 MHz above
Fig. 6-47. Harmonic frequency markers used to check dial accuracy.
6-45
Summary of Contents for 491
Page 4: ...i i Fig 1 1 The Type 491 Spectrum Analyzer ...
Page 24: ...2 16 Fig 2 17 Control set up chart ...
Page 34: ...Fig 3 1 Function block diagram of the Type 491 3 2 ...
Page 42: ...Fig 3 10 Block diagram of the video detector and vertical amplifier 3 1 0 ...
Page 48: ......
Page 53: ...Fig 4 4 Power Supply Circuit board assembly with wiring color code 4 5 ...
Page 54: ...Fig 4 5 Horizontal Display circuit board assembly showing color code to pin connectors 4 6 ...
Page 55: ...Fig 4 6 IF Control board assembly Wiring color code to pin connector 4 7 ...
Page 64: ...Fig 4 20A Tube subassembly removal procedure 4 16 ...
Page 65: ...Fig 4 20B Tube subassembly installation procedure 4 17 ...
Page 68: ...Fig 4 22 Power supply board assembly with component call out 4 2 0 ...
Page 69: ...Fig 4 23 Vertical Amplifier and Blanking board assembly with component call out 4 21 ...
Page 70: ...Fig 4 24 IF control board assembly with component call out 4 22 ...
Page 71: ...Fig 4 24 IF control board assembly with component call out 4 23 ...
Page 72: ...Fig 4 25 Horizontal display board with component call out 4 24 ...
Page 73: ...Fig 4 25 Horizontal display board with component call out 4 25 ...
Page 74: ...Fig 4 26 Phase lock board with component call out 4 26 ...
Page 88: ......
Page 90: ...Fig 6 1 Test equipment recommended for calibration of the Type 491 6 2 ...
Page 138: ......
Page 192: ......
Page 195: ...SECTION II B 3 ...
Page 196: ......
Page 200: ......
Page 201: ...SECTION 9 DIAGRAMS MECHANICAL PARTS LIST ILLUSTRATIONS ACCESSORIES ...
Page 202: ......
Page 203: ...9 1 ...
Page 204: ...9 3 ...
Page 205: ...9 5 ...
Page 206: ...9 7 ...
Page 207: ...9 9 ...
Page 208: ...9 11 ...
Page 209: ...9 13 ...
Page 210: ...9 15 ...
Page 211: ...9 17 ...
Page 212: ...9 19 ...
Page 213: ...9 21 ...
Page 214: ...9 23 ...
Page 215: ...9 25 ...
Page 216: ...9 27 ...
Page 217: ...FIG 1 9 29 ...
Page 218: ...FIG 2 REAR 9 31 ...
Page 219: ...9 33 FIG 3 IF CHASSIS PHASE LOCK ASSEMBLIES ...
Page 220: ...FIG 4 POWER CHASSIS 9 35 ...
Page 221: ...FIG 5 TIME DIV SWITCH OSCILLATOR ASSEMBLIES 9 37 ...
Page 222: ...FIG 6 CRT SHIELD ASSEMBLY 9 39 ...
Page 223: ...FIG 7 CABINET ASSEMBLY HANDLE 9 4 1 ...
Page 224: ...FIG 8 491 STANDARD ACCESSORIES FIG 8 491 STANDARD ACCESSORIES 9 43 ...
Page 225: ......
Page 226: ...PIN 028017 000 ...