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PROBLEM
No voltage at negative variable output.
No or low voltage at negative variable
output with load.
No +12V at output.
No +12V at output with load.
No –12V at output.
No –12V at output with load.
No +5VDC at output
No +5VDC at output with load.
POSSIBLE CAUSE
1. Measure for an AC voltage of 18VAC at cathode of D8, D10.
A. Transformer and/or secondary connection to PC board
defective.
2. Measure DC voltage of –28VDC at pin 2 of U5 LM337.
3. Set voltage for minimum –1.25VDC and measure pin 3 of U5.
A. Voltage adjusts only from –7.8 to –9.8V R2 open or
defective.
B. Voltage –27V, check VR2 connections.
1. Check to see if capacitor C5 1000µF is inserted in the
correct polarity.
2. Check ripple on pin 2 of U5. 6VP-P max.
A. Capacitor C5 and/or diodes D8, D10 defective.
1. Measure an AC voltage of 15VAC at anode of D1, D3.
A. Transformer and/or secondary connection to PC board
defective.
2. Measure for a DC voltage of 21VDC at pin 1 of U2 LM7812.
A. Diodes D1, D3 in backwards or defective, check
capacitor C1.
3. Measure for a DC voltage of 12VDC on pin 3 of U2.
A. U2 LM7812 defective or open ground.
1. Check capacitor C2 1000µF is inserted in the correct
polarity.
2. Check ripple on pin 1 of U2. 7VP-P Max.
A. Capacitor C2 or diodes D1, D3 defective.
1. Measure an AC voltage of 15VAC at anode of D2, D4.
A. Transformer and/or secondary connection to PC board
defective.
2. Measure for a DC voltage of -21VDC at pin 2 of U4 LM7912.
A. Diodes D2, D4 in backwards or defective, check
capacitor C4.
3. Measure for a –12VDC voltage on pin 3 of U4.
A. U4 LM7912 defective or open ground.
1. Check capacitor C4 is inserted in the correct polarity.
2. Check ripple on pin 2 of U3. 7VP-P Max.
A. Capacitor C4 and/or diodes D1, D3 defective.
1. Measure an AC voltage of 9VAC at anode of D5, D6.
A. Transformer and/or secondary connection to PC board
defective.
2. Measure for a DC voltage of 12VDC at pin 1 of U3 LM7805.
A. Diodes D5, D6 in backwards or defective, check
capacitor C3.
3. Measure for a 5VDC voltage on pin 3 of U3 LM7805.
A. U3 LM7805 defective or open ground.
1. Check that capacitor C3 is inserted in the correct polarity.
2. Check ripple on pin 1 of U3. 4VP-P Max.
A. Capacitor C3 and/or diodes D5, D6 defective.
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