Model 686C Section IV
Paragraphs 4-18 to 4-19
B. SWEEP-OUT SAWTOOTH pip at the low end of the hand and another at the high
end. The end of the sweep should occur at the top of The sawtoothvoltage at the SWEEP OUTPUT jack
the wavemeter pip. Switch to TRIG. and operate the
(5203) is linear (see figures 4-6 and 4- 17) andgenerally
MAN. TRIG. button. Adjust R203 to obtain the same will not be out of specifications. Should trouble be
sweep length on MAN. TRIG as on RECUR. This is experienced, however, the points discussed below are
easy to determine if the sweep ends on the wavemeter
among the possible sources.
pips.
4-18. REPLACING NEON DC COUPLING
ELEMENTS.
Throughout the sweep oscillator there are a number
of dc coupled circuits which use small neon lamps as
coupling devices. These lamps maintain a constant
voltage drop across themselves and thus couple to-
gether points of different potential. Since all power
supplies are regulated and absolute voltage levels are
critical, these lamps are important in maintaining
exact voltage relationships in the system. The lamps
are aged, polarized and selectedat the factory. Lamps
are aged on dc to insure stability of voltage level and
T .2 CM MAX.
to establish a polarity characteristic. The lamps are
RO selected for proper voltage drop and color coded. The
color tells the voltage and also tells the correct pol-
arity. The paint mark is located on the positive lead,
Figure 4-17. Sweep Out Voltage Linearity This is important, as improper polarity will result in
a different and unstable voltage drop.
Three troubles which may occur would cause a very
distorted wave shape and thus can be easily detected. When replacing a lamp, the same color code lamp
should be installed and care should be taken to observe
If W203 (6AL5) or V204 (6AN8) is defective, the saw- the correct polarity. Improper connection may de-
tooth will be distorted. The defective tube generally
stroy the lamps' stability even if later reconnected
has high heater-cathode leakage which modulates the in the proper manner.
sawtooth SWEEP OUTPUT voltage. Observing the saw-
tooth SWEEP OUTPUT voltage on an oscilloscope, The factory specifications on the lamps used in this
distortion can easily be detected. Diode CR202, as-
sociated with the FREQUENCY potentiometer circuit, sweep oscillator are as follows:
will cause limiting on the sawtooth if the diode has a @ Stock No. Color Voltage RanFe Circuit Reference
low front-to-back ratio. If diode CR201 is defective,
the cathode of V202B will not follow the grid. The G84B Blue 57 to 60V V6 through V11,
grid will then draw current which will disable the V209, V210
swept circuit.
G84E Red 61 to 65V V110, V211
NOTE
When in good condition, GllA diodes will 4-19. POSITIONING FREQUENCY DIAL
typically have a forward resistance of less ON POTENTIOMETER SHAFT.
than 100 ohms and a back resistance greater
than 500,000 ohms. The actual resistance If R236 has to be replaced, it is necessary to reset the
proper relative position between the dial hub mechani-
values read for diode CR201 or CR202 will
vary with the internal voltage of the ohmmeter cal stop and the mechanical stop on the potentiometer
rotor. Proper positioning of the stops will cause the
used. The above typical values are those read dial stop to hit just before the internal rotor stop.
on an hp Model 410B vtvm. The forward re-
sistance is measured onthe Rxl range and the This prevents possible damage to the precisionpoten-
tiometer if the shaft is forced.
back resistance on the RxlKor higher range.
Proceed as follows:
If the sweep output voltage at 5203 is too low or high,
under approximately 20 volts or over approximately 1) Turn the FREQUENCY dial counterclockwise to the
25 volts, the Schmitt Trigger is probably not trigger- mechanical stop.
ing reliably. This will also cause a non-linear sweep.
In this case, V201 and/or V202A may be defective. 2) Record this position with a pencil mark on the dial.
C. SETTING THE SCHMITT TRIGGER BIAS
3) Remove the long set screw on the dial hub (do not
The trigger bias should be adjustedwithR203 (see touch the short set screw! )and turn the FREQUENCY
figure 4-15). The equipment should be connected as dial counterclockwise until it'is stopped by the mech-
in figure 4-2. Set the A FREQUENCY = 4.4K and anical stop of the potentiometer rotor. Be careful not
SWEEP RATE to 3.2 kmc/sec. Adjust one wavemeter to use force.
00034-1 4-21 |
ACEC
