28
Subject to change without notice
+85V) should firstly be checked, because this voltage will
affect the astigmastism correction. While the adjustment
is being carried out (with medium brightness and a 1MHz
square-wave signal), the upper horizontal square-wave
tops are firstly focussed with the
FOCUS
control. Then
the sharpness of the vertical lines are corrected with the
47k
Ω
Astigm. pot. The correction should be repeated
several times in this sequence. The adjustment is finished,
when the
FOCUS
knob
exclusively
brings no improve-
ment of the sharpness in
both
directions.
Trigger Threshold
The internal trigger threshold should be in the range 0.3 to
0.5div. display height. It is strongly dependent on the
comparator IC. If there are compelling reasons to replace
this comparator, it may be that triggering becomes too
sensitive or too insensitive caused by the IC gain tolerances
(see Test Instructions: “Triggering Checks”, page T3). In
extreme cases, the 3.32k
Ω
hysteresis resistor of the
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
premature 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
transformer (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 voltages for the final stages
are highly dangerous. Therefore it is recommended to use
totally insulated extended probe tips
, when trouble-
shooting the instrument. Accidental 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
overheating). Check to see that all circuit board connections
are making good contact and are not shorting to an
adjacent circuit. Especially inspect the connections
between the PCBs, to front chassis parts, to CRT PCB, to
trace rotation coil (inside of CRTs shielding), and to the
control potentiometers and switches on top of and beneath
the PCBs. This visual inspection can lead to success much
more quickly than a systematic fault location using
measuring instruments. Prior to any extensive trouble-
shooting, also check the external power source.
If the instrument fails completely, the first and important
step
−
after checking the power fuses
−
will be to
measure the deflecting plate voltages 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 approx.
80V and X approx 71V). 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.
Replacement of Components and Parts
For the replacement of parts and components use only
parts of the same or equivalent type. Resistors unspecified
in the diagrams have a power dissipation of 1/5 Watt
(Melf) or 1/8 Watt (Chip) respectively and a tolerance of
1%. Resistors in the high voltage circuit must have
sufficient electric strength. Capacitors without a voltage
value must be rated for an operating voltage of 63V. The
capacitance tolerance should not exceed 20%. Many
semiconductors are selected, especially all amplifier
transistors, which are contained in push-pull circuits. If a
selected semiconductor is defective, both push-pull
transistors of a stage should be replaced by selected
components, because otherwise there are possibly
deviations of the specified data or functions. The Service
Department can give you advice for troubleshooting and
replaceable parts. Replacement parts can be ordered by
letter or telephone from the nearest HAMEG Service
Office. Please supply the following information: Instru-
ment type and serial number, description of the part (type
and part number on the circuit drawing).
