Service manual

57
-15 V On A2 Control Board. Voltage regulator A2U4 regulates the voltage across resistor A2R32 to be 1.25Vdc.
Circuit Included. -15 Vdc bias supply circuitry from connector pin A2P1-30 through test point A2J3-21 on A2 control
board.
Setup. The Main Troubleshooting Setup, Page 48. Apply the ac mains voltage to the bias transformer, and set the external
supply to 0Vdc.
Input:
NODE (+) MEASUREMENT SOURCE
A2U4(IN), A2C16 (-)
≈ - 24Vdc
A1U1, A1C1(+)
Outputs:
NODE ( + ) NODE (-) MEASUREMENT
A2U4 (ADJ) A2U24 (OUT) l.25Vdc
A2VR2 (cath.) A2VR2 (anode) 11.7Vdc
A2R33, A2R34 A2VR2 (cath.) 2.05Vdc
To check if load on -15V is shorted, remove jumper A2W3.
Refer to Down Programmer, for the + 8.9Vdc bias supply, and refer to OVP Circuit, for the + 2.5V bias supply.
Troubleshooting Down Programmer
The down programmer loads the output when either MASTER ENABLE is low or CV ERROR is more negative than about
- 6Vdc. Comparator A4U3B triggers down programming when the voltage at A4U3B-5 is less than about 3Vdc. The
collector-emitter current through transistor A4Q6 increases as the output voltage decreases because of feedback from
voltage divider A4R24-A4R27 at A4U3A-2
Circuit Included. Down programmer and 8.9V bias supply on A4 power mesh board.
Setup. The Main Troubleshooting Setup, Page 48, except connect the external supply to the unit's + OUT ( + ) and - OUT
( - ) terminals. Apply the ac mains voltage to the bias transformer. Set the external supply (EXTERNAL) and adjust the
unit's voltage setting (INTERNAL) as instructed below.
Outputs:
Set Voltage (Vdc)
NODE External Internal Setup Measurement
A4U4 (OUT) - - 8.9Vdc
A4U3B-7 0 2 unplug TS1 0Vdc
A4U3B-7 10 0 reconnect TS1 0Vdc
A4U3B-7 0 2 7.8Vdc
A4U3A-2 0 2 unplug TS1 0.43Vdc
A4R26 0 2
≈0.41Vdc (6038A)
0.2Vdc (6033A)
A4Q6 (base) 20 2 1.0Vdc
A4U3A-1 20 2 4.0Vdc
A4R26 20 2
≈0.34Vdc (6038A)
≈0.11Vdc (6033A)