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merits may possibly have to be repeated, because the func¬
tions of both variable resistors are dependent on each
other. Correct adjustment is achieved, when the trace can
be blanked while
X-Y
pushbutton is depressed and, in addi¬
tion, when the requirement described in the Test Instruc¬
tions are met.
Astigmatism control
The ratio of vertical and horizontal sharpness can be
adjusted by the variable resistor of 47 kQ, located on the
lower PCB (see Adjusting Plan). As a precaution however,
the voltage for the vertical deflecting plates (approx. -I-80V)
should firstly be checked, because this voltage will affect
the astigmastism correction. While the adjustment is being
carried out (with medium brightness and a 1 MHz square-
wave signal), the upper horizontal square-wave tops are
firstly focussed with the
FOCUS
control. Then the sharp¬
ness of the vertical lines are corrected with the 47 kQ
Astigm. pot. The correction should be repeated several
times in this sequence. The adjustment is finished, when
the
FOCUS
knob
exclusively
brings no improvement of the
sharpness in
both
directions.
Trigger Threshold
The internal trigger threshold should be in the range 0.3 to
O.Bdiv. display height. It is strongly dependent on the
710CN comparator 1C. If there are compelling reasons to
replace this comparator, it may be that triggering becomes
too sensitive or too insensitive caused by the 1C gain toler¬
ances (see Test Instructions: "Triggering Checks", page
T3). In extreme cases, the 39.2 kQ hysteresis resistor of the
710 comparator should be changed. Generally, max. halving
or doubling of this resistance value should be sufficient. A
too small trigger threshold cause double-triggering or pre¬
mature trigger action due to interference pulses or random
noise. A too high trigger threshold prevents the display of
very small display heights.
Trouble-Shooting the Instrument
For this job, at least an isolating variable mains/line trans¬
former (protection class II), a signal generator, an adequate
precise multimeter, and, if possible, an oscilloscope are
needed. This last item is required for complex faults, which
can be traced by the display of signal or ripple voltages. As
noted before, the regulated high voltage and the supply
voltage for the final stages are highly dangerous. Therefore
it is recommended to use
totally insulated extended
probe tips,
when trouble-shooting the instrument. Acci¬
dental contact with dangerous voltage potentials is then
unlikely. Of course, these instructions cannot thoroughly
cover all kinds of faults. Some common-sense will certainly
be required, when a complex fault has to be investigated.
If trouble is suspected, visually inspect the instrument
thoroughly after removal of the case. Look for loose or badly
contacted or discolored components (caused by overheat¬
ing). Check to see that all circuit board connections are mak¬
ing good contact and are not shorting to an adjacent circuit.
Especially inspect the connections between the PCBs, to
the power transformer, to front chassis parts, to CRT sock¬
et, to trace rotation coil (inside of CRT's shielding), and to
the control potentiometers and switches on top of and
beneath the PCBs. Furthermore, the soldering connections
of the transistors and Fixed Three-Terminal Regulators
resp. on the rear chassis. This visual inspection can lead to
success much more quickly than a systematic fault location
using measuring instruments. Prior to any extensive trou¬
ble-shooting, also check the external power source.
If the instrument fails completely, the first and most impor¬
tant step -
after checking the mains/line voltage and
power fuse
- will be to measure the deflecting plate volt¬
ages of the CRT. In almost any case, the faulty section can
be located. The sections represent:
1. Vertical deflection.
2. Horizontal deflection.
3. CRT circuit.
4. Power supply.
While the measurement takes place, the position controls
of both deflection devices must be in mid-position. When
the deflection devices are operating properly, the separate
voltages of each plate pair are almost equal then (Y ~80V
and X—71 V). If the separate voltages of a plate pair are very
different, the associated circuit must be faulty. An absent
trace in spite of correct plate voltages means a fault in the
CRT circuit. Missing deflection plate voltages is probably
caused by a defect in the power supply.
Focus
-1725V
(typ.)
(i.V.)
Geo
+95V
+71V
(typ.)
Asti.
Voltages at the CRT socket
S2 203-7
Subject to change without notice
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