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Set the CURRENT LIMIT ADJ potentiometer to the desired

maximum output current, or set it fully clockwise.

4-3.

TROUBLE SYMPTOMS AND SUGGESTED REMEDIES

a. Circuit faults can be isolated most rapidly by measuring

the voltage and resistance. Use the data given on the schematic

diagram in the Appendix as a first stop in servicing the supply.

CAUTION

WHEN UNSOLDERING SEMICONDUCTORS FOR TEST, USE A

HEAT SINK TO PREVENT THERMAL DAMAGE. A LONG NOSE

PLIERS BETWEEN THE SEMICONDUCTOR AND THE SOLDER

JUNCTION IS ADEQUATE. NEVER OPERATE THE POWER

SUPPLY WITH ANY LEADS DISCONNECTED OR SEMICONDUCTORS

REMOVED. OPERATING POTENTIALS IN THE DC AMPLIFIER

MAY CHANGE RADICALLY WHEN A COMPONENT IS REMOVED OR

DISCONNECTED.

b. Power Supply Does Not Go On : If the OVEN and AC lamps

do not light, check the AC fuse. If the fuse blows repeatedly,

check the oven circuit and diodes CR1, CR2 and CR4 through CR7.

Use an ohmmeter to take a resistance reading across each diode.

Then, reverse the meter leads and take another reading. If one

reading is not at least five times greater than the other, the

diode is defective. If one diode in any pair is defective, replace

both. A short circuit in one will produce high surge currents in

the other, which can result in junction damage.

c. No DC Output Voltage: If both the AC and OVEN lamps light,

but no output voltage is available, insure that the CURRENT LIMIT ADJ

control is not turned fully counterclockwise. Set the meter switch

to MA and increase the output voltage. If no current is indicated,

check the DC fuse and input capacitor C1. If current is present

when the output—voltage controls are adjusted, check safety diode

CR12 and for incorrect programming or sensing connections. Diode

CR12 is connected in the opposite polarity to the DC output voltage.

If the reverse current flow is greater than 1 ampere, this diode

may weld, placing a permanent short circuit across the supply out-

put. Normal operation can be restored by replacing the diode

(located on the amplifier board).

d. Regulator Failure : Check for correct potentials on ampli-

fier transistors, voltage reference, etc. If any voltage appears

incorrect, disconnect AC power and make a rapid check for defective

transistors. This can be done without removing the transistors from

the circuit. Use an ohmmeter sot to its low resistance scale (R x1)

and measure the forward and reverse resistances at the collector-base

and base—emitter junctions. A resistance ratio of less than 5 to 1

indicates that the transistor is defective. Carefully remove it and

check it on a transistor checker.

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