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General
The following instructions are intended as an aid for the
electronic technician, who is carrying out readjustments
on the HM303, if the nominal values do not meet the
specifications. These instructions primarily refer to those
faults, which were found after using the Test Instructions.
However, this work should only be carried out by properly
qualified personnel. For any further technical information
call or write to HAMEG. Addresses are provided at the
back of the manual. It is recommended to use only the
original packing material, should the instrument be shipped
to for service or repair (see also Warranty, page M2).
Instrument Case Removal
The rear cover can be taken off after unplugging the power
cords triple-contact connector and after two cross recessed
pan head screws (M4x30mm) with two washers on it
have been removed. While the instrument case is firmly
held, the entire chassis with its front panel can withdrawn
forward. When the chassis is inserted into the case later
on, it should be noticed that the case has to fit under the
flange of the front panel. The same applies for the rear of
the case, on which the rear cover is put.
Caution
During opening or closing of the case, the instrument
must be disconnected from all power sources for
maintenance work or a change of parts or
components. If a measurement, trouble-shooting, or
an adjustment is unavoidable, this work must be
done by a specialist, who is familiar with the risk
involved.
When the instrument is set into operation after the
case has been removed, attention must be paid to the
acceleration voltage for the CRT –2025V and to the
operating voltages for both final amplifier stages
185V and 141V. Potentials of these voltages are on
the PS-Board, the CRT-PCB, on the upper and lower
PCBs. Such potentials are moreover on the checkpoint
strips on the upper and lower horizontal PCBs. They
are highly dangerous and therefore precautions must
be taken. It should be noted furthermore that shorts
occuring on different points of the CRT high voltage
and unblanking circuitry will definitely damage some
semiconductors and the opto-coupler. For the same
reason it is very risky to connect capacitors to these
points while the instrument is on.
Capacitors in the instrument may still be charged,
even when the instrument is disconnected from all
voltage sources. Normally, the capacitors are
discharged approx. 6 seconds after switching off.
However, with a defective instrument an interruption
Subject to change without notice
of the load is not impossible. Therefore, after switching
off, it is recommended to connect one by one all
terminals of the check strips on the upper PCB across
1k Ω Ω Ω Ω Ω to ground (chassis) for a period of 1 second.
Handling of the CRT needs utmost caution. The glass
bulb must not be allowed under any circumstances
to come into contact with hardened tools, nor should
it undergo local superheating (e.g. by soldering iron)
or local undercooling (e.g. by cryogenic-spray). We
recommend the wearing of safety goggles (implosion
danger).
The complete instrument (with case closed and PO-
WER button depressed) is after each intervention
undergo a voltage test with 2200V, DC, between
accessible parts to both mains/line supply terminals.
This test is dangerous and requires an adequately
trained specialist.
Operating Voltages
All operating voltages (+6.3V, +12V, –12V, +141V, +185V,
–2025V) are stabilized by the switch mode power supply.
The +12V supply is further stabilized and used as a
reference voltage for –12V stabilisation. These different
operating voltages are fixed voltages, except the +12V,
which can be adjusted. The variation of the fixed voltages
greater than 5% from the nominal value indicates a fault.
Measurements of the high voltage may only be
accomplished by the use of a sufficient highly resistive
voltmeter (>10MΩ). You must make absolutely sure that
the electric strength of the voltmeter is sufficiently high.
It is recommended to check the ripple and also the
interaction from other possible sources. Excessive values
might be very often the reason for incomprehensible
faults.
Maximum and Minimum Brightness
Two variable resistors (220kΩ and 470kΩ), located on the
switch mode power supply PCB, are used for these
adjustment procedures (see Adjusting Plan). They may only
be touched by a properly insulating screwdriver (Caution!
High voltage!). The adjustments may possibly have to be
repeated, because the functions of both variable resistors
are dependent on each other. Correct adjustment is achieved,
when the trace can be blanked while XY pushbutton is
depressed and, in addition, when the requirement described
in the Test Instructions are met.
Astigmatism control
The ratio of vertical and horizontal sharpness can be
adjusted by the variable resistor of 47kΩ, located on the
lower PCB (see Adjusting Plan). As a precaution however,
the voltage for the vertical deflecting plates (approx.
Service Instructions
27
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