EG&G ORTEC 402A Instrukcja obsługi i serwisowania - Strona 13

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to increase the current through Q2 and to then decrease

the output voltage. Q2 drives emitter follower Q1 to

supply drive current for series pass element Q101.

In the current limiting mode, the voltage across sense

resistor R101 is applied between the bases of Q6 and Q9.

When the current level is less than the rated current, Q6

and Q7 conduct. If the output current exceeds its rated

value, the voltage at the base of Q6 goes higher than the

voltage at the base of Q9 and Q9 conducts. This turns Q8

on to increase the current through Q2 and to decrease

the current drive to the base of Q1. The amount of current

that the series pass element, Q101, can supply is

reduced. When the over-current condition is corrected,

normal regulation returns automatically.

The ±6 V regulators operate much the same as the +24 V

regulator. The circuit for the ±6 V regulators is shown on

schematic 402D-0301 -SI. The sense amplifier of the +6 V

regulator includes Q11, Q10, Q9, Q26, Q4, Q5, Q2. Q1,

and Q3. The operation is the same as for the +24 V regu

lator. The addition of Q9 and Q26 provides a constant

current for reference diode D11.

In the current limiting mode, operation is the same as for

the +24 V regulator. Resistors R105 through R108 are

used to balance the current in the four series pass tran

sistors, Q105 through Q108. R108 is also used as a sense

resistor. The four transistors are used to keep the power

dissipated in each device below the maximum rated value.

The "crowbar" circuit is used for over-voltage protection.

It includes Q12, D12, and SCR D107. If the output voltage

exceeds the zener voltage of D12 (~6.8 V), Q12 turns on

and triggers the gate of the SCR. When the SCR turns on,

it appears as a short across the supply output and causes

the current to fold back. After the cause of the over-

voltage has been removed, the supply must be switched

off for two seconds and can then be turned on again to

restore its normal operation.

MAINTENANCE

6.1.

ACCESS

The top cover of the 402D Power Supply is attached with

sixteen 4-40 Phillips-head machine screws. When all 16

screws are removed the cover can be lifted for access to

the plug-in regulator boards. Each regulator board has

keying pins to prevent reversal of position in the connec

tor and to prevent insertion into the incorrect location. Be

very careful to prevent breaking or removing the keying

pins from the connector during either removal or return

of the board.

There are copper clips attached to some of the transis

tors on the printed circuit boards. Their purpose is to

ensure equal operating temperatures for these transis

tors. Do not remove the copper clips from these transis

tors.

Most of the components mounted on the heat sink can be

replaced easily. If access is required to the chassis side of

the heat sink, remove the 14 mounting screws that secure

it to the chassis and carefully fold the heat sink away

from the chassis to prevent breaking any of the wire leads

or the soldered connections.

6.2. TROUBLESHOOTING

Any of the three transistor boards may be operated

separately without requiring the other boards to be

installed in the power supply.

If an output voltage is high and cannot be adjusted down

to its proper level, its series pass transistor, mounted on

the heat sink, may be shorted. This condition will usually

cause damage to the regulator that is mounted on the

printed circuit board. Check the emitter-base junction of

the series pass transistor with an ohmmeter if a short is

suspected. Do not install another regulator board if the

transistor is shorted, or the new board will be damaged.

Since the positive 12/24 V regulator board is identical to

the negative 12/24 V regulator board, the boards can be

interchanged for troubleshooting. Replacement regula

tor boards are available from EG&G ORTEC.

6.3. TYPICAL VOLTAGES

Tables 6.1 and 6.2 list typical voltages that were meas

ured with respect to ground potential and they are given

here as an aid in troubleshooting. These voltage levels

are typical of a circuit that is operating properly, but the

precise values will vary between individual circuits.

The 12/24 V table indicates the voltages on the board

when it is installed at the positive location, furthest from

the heat sink. Line voltage is 117 V ac. The 24 V supply is

loaded for 1 A and the 12 V supply is loaded for 2 A. All

measurements are referenced to chassis ground and are

made with a voltmeter that has a 10 MO input impedance.

The regulator board for the ±6 V supplies operates in the

location closest to the heat sink. The following voltages

assume a line voltage of 117 V. Each supply, +6 V and —6

V, is loaded for 8 A. All measurements are referenced to

chassis ground and are made with a voltmeter that has a

10 Mn input impedance.