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Service Instructions
General
The following instructions are intended as an aid for the
electronic technician, who is carrying out readjustments on
the HM 205, if the nominal values do not meet the specifica-
tions. These instructions primarily refer to those faults,
which were found after using the Test Instructions. How-
ever, this work should only be carried out by properly qual-
ified personnel. This concerns the digital storage part in
particular. 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 HAMEG for
service or repair (see also Warranty, page M2).
Instrument Case Removal
The rear cover can be taken off after unplugging the power
cord's triple-contact connector and after two cross reces-
sed 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held, the entire chassis with its front panel can withdrawn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
ail 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 accel-
eration voltage for the CRT — 2000V — and to the
operating voltages for both final amplifier stages — 150
and 130V. Potentials of these voltages are on the CRT
socket, on the lower horizontal PCB, and on the Z-
board. 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 6 sec-
onds after switching off. However, with a defective
instrument an interruption of the load is not impossi-
ble. Therefore, after switching off, it is recommended to
connect one by one all terminals of the check strips on
the upper PCB across 1kQ 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
POWER button depressed) is — after each intervention
— undergo a voltage test with 2000V, 45 to 65Hz,
between accessible parts to both mains/line supply ter-
minals.
This
test
is dangerous
and
requires
an
adequately trained specialist.
Operating Voltages
Besides the two AC voltages for the CRT heating (6.3),
Component Tester and line triggering (12V) there are ten
electronically requiated DC operating voltages generated
(+12V,
+5Vy, +5Vy,
+5Vig, +5Vs, —12V,
+130V (or
+118V in storage mode), +150V, —1900V, and 22V for the
unblanking circuit). These different operating voltages are
fixed voltages, except the +12V, which can be adjusted. All
other voltages are dependent from the accuracy of this volt-
age (and also from some resistors with close tolerances).
Only 22V in the unblanking circuit is stabilized with Z-diode.
The variation of the fixed voltages greater than +5% from
the nominal value indicates a fault. Excepting 22V, +130V,
and —1900V, the other DC voltages have no more than
+2% variation on the average. These voltages are mea-
sured on the checkpoint strip with reference to ground (see
Adjusting Plan). Measurements of the high voltage may
only be accomplished by the use of a sufficient highly resis-
tive voltmeter (>10MQ). You must make absolutely sure
that the electric strength of the voltmeter is sufficiently
high. The 22V for the unblanking circuit can be measured as
the difference between two high voitages with reference to
ground. 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 of 500kQ each, located on the Z-
PCB,
are used
for these adjustment
procedures
(see
Adjusting Plan}. They may only be touched by a properly
insulating screwdriver (Caution! High voltage!). The adjust-
Subject to change without notice
$1 205
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