Service Instructions

General

The following instructions are intended as an aid for the electronic technician, who is carrying out readjustments on the HM 604, 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 HAM EG. 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 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 While the instrument case is firmly held, the entire chassis with its front panel can withdrawn forward.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 —12.5kV— and to the operating voltages for both final amplifier stages — together 140 V. Potentials of these voltages are on the CRT socket, on the upper and the lower horizontal PCBs, and on the lateral PCB directly beside the CRT neck. High voltages of max. 2000 V are at the INTENS. and FOCUS potentiometers. 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. 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 seconds after switching off. However, with a defective instrument an interruption of the load is possible.

Therefore, after switching off, it is recommended to connect one by one all terminals of the check strip across 1 kQ 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).

Operating Voltages

Besides the two AC voltages for the CRT heating (6.3V) and graticule illumination, Component Tester and line triggering there are eight electronically regulated DC operating voltages generated (+12V, +5V,-5V,-12V, +68V, + MOV,-1800V, and + 10.4 kV). These different operating voltages are fixed voltages, except the + 12 V, which can be adjusted. The variation of the fixed voltages greater than ±2 % from the nominal value indicates a fault. This voltage is measured on the checkpoint strip (located on XY Board) with reference to ground. Measurements of the high voltage may only be accomplished by the use of a sufficient highly resistive voltmeter (>10M£2). You must make absolutely sure that the electric strength of the voltmeter is sufficiently high.

Minimum Brightness

The variable resistor VR601, located on the Z-PCB, is used for this adjustment procedure. It may only be touched by a properly insulating screwdriver (Caution! High voltage!). Correct adjustment is achieved, when the trace can be blanked while X-Y 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 VR603, located on the Z-PCB (see Adjusting Plan). As a precaution however, the voltage for the vertical deflecting plates (approx. +41,5V) should firstly be checked, because this voltage will affect the astigmastism correction. Use the 1 MHz square-wave signal from the built-in calibrator via 10: 1 probe for this correction. Firstly adjust the sharpness of the horizontal square-wave lines using the FOCUS control. The sharpness of the vertical lines should then be corrected with the aid of the Astigm. pot VR626. Repeat the correction several times in this sequence. Adjustment has been correctly carried out when, on using FOCUS control only, the sharpness is not improved in eitherdirection.

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 voltage 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 the power transformer, to front chassis parts, to CRT socket, to trace rotation coil (inside of CRT's shielding), to the 3 BNC connectors at the rear chassis, and to the control potentiometers and switches on top of and beneath both 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 troubleshooting, also check the external power source.

If the instrument fails completely, the first and most important step — after checking the mains/line voltage and power fuse — 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»42V and X —67 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.

Replacement of Components and Parts

For the replacement of parts and components use only parts of the same or equivalent type. Resistors without specific data in the diagrams have a power dissipation of 0.25 Watt and a tolerance of 2 %. 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 the gate-diodes 1 N4154, and all amplifier transistors, which are contained in push-pull circuits (including the FETs). If a selected semiconductor is defective, all gate-diodes or 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 HAMEG 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: Instrument type and serial number, description of the part (including function and location in the instrument), quantity desired.

Replacement of the Power Transformer

Should it be necessary to replace the mains/line transformer, the correct terminal sequence (color identification) for primary and secondary windings must be followed (see diagram "Power Supply" and the figure below). In addition, the relevant Safety Regulations must be observed. Here, we refer only to those requirements relative to the parts conductively connected to the supply mains:

The construction of the instrument shall be such as to prevent any short-circuiting or bridging of the insulation, clearances or creepage distances between those parts connected to the supply mains and any accessible conductive parts due to accidental loosening or freeing of the wiring, screws, etc.

The rigidity of the mains wiring connections, which may be subject to mechanical stresses, shall not be dependent upon the soldering alone. To meet this requirement, the bare ends of the wires must be pushed through the holes in the respective soldering tab, bent over with a pair of pliers, and subsequently fixed by soldering.

— The minimum cross section of the protective earth connection between the instrument's power inlet and the connecting soldering tab on the rear chassis must be 0.81 mm2 in North America and 0.75 mm2 in Western Europe. The connecting soldering tab on the rear chassis has to be secured mechanically against loosening (e.g. with lock washer).

After replacing the power transformer, all remaining bits of wire, solder and other foreign matter must be removed from the PCBs, the vicinity of the power transformer and from within the insulating connecting box by shaking, brushing and blowing. Finally, the top plate of the insulating connecting box has to be replaced. Before connecting the instrument to the power supply, replace the possibly defective fuse, press the POWER button and make sure that there is an adequate insulation state between chassis (= safety earth conductor) on the one hand, and the live/line pin as well as the neutral pin, on the other. Only after proper insulation has been established may further function tests with open chassis follow, but with appropriate precautionary measures.

Adjustments

As advised in the Operating, Test and Service Instructions, small corrections and adjustments are easily carried out with the aid of the Circuit Diagrams and Adjusting P/an. However, a complete recalibration of the scope should not be attempted by an inexperienced operator, but only someone with sufficient expertise. Several precision measuring instruments with cables and adapters are required, and only then should the pots and trimmers be readjusted, provided that the result of each adjustment can be exactly determined. Thus for each operating mode and switch position, a signal with the appropriate sine or square waveform, frequency, amplitude, risetime and duty cycle is required.

Connection of the safety earth pin via long sold. tab to rear chassis

Appliance Inlet

Safety Class I

Plug with earthing contact

Connection of the safety earth pin via long sold. tab to rear chassis

Appliance Inlet

250v

Rear View of Power Switch and Appliance Inlet with Voltage Selector and Fuse

250v

Rear View of Power Switch and Appliance Inlet with Voltage Selector and Fuse

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