HP 6038A, Service Manual

Page: 59 / 111

59
If the failure symptoms include output current oscillation, check if the differentiator circuit is at fault by removing resistor
A2R16. If oscillations stop, the differentiator is probably at fault.
Troubleshooting OVP Circuit
Comparator A2U14D sets, and gate A2U17A resets flipflop A2U14B-A2U14C. TTL low at A2U14-1,8,13 inhibits the
PWM.
Circuit included. OVP Circuit and 2.5V bias supply on A2 control 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. Adjust the unit’s OVP limit to 15Vdc. Set the external
supply (EXTERNAL) as instructed below.
Outputs:
SET VOLTAGE
NODE EXTERNAL (Vdc) SETUP MEASUREMENT
A2U9 (OUT) - 2.5Vdc
A2U14-10 10 1.0Vdc
A2U14-11 - 1.5Vdc
A2J3-13 10 Hi
A2J3-13 20 Lo
A2J3-13 10 Lo
A2J3-13 10 cycle power Hi
Note:
Connecting a test probe to either input of either comparator in the OV Flipflop (pins A2U14-1, 6, 7, 8, 9,
14 or A2U11-13) may cause the flipflop to change states and cause the probed input to be low.
Troubleshooting PWM & Clock
The inputs to Inhibit Gate A2U19A and PWM gate A2U19B are the keys to PWM troubleshooting. The 20KHz Clock
starts each PWM output pulse, and the pulse stops when any of the inputs to A2U19A or A2U19B goes low. The PWM is
inhibited and prevented from initiating output pulses as long as any of the eight inputs are low.
Circuit Included. Pulse Width Modulator (PWM), Inhibit Gate A2U19A, Off-Pulse Oneshot, On-Pulse Oneshot, A2U17B,
20KHz Clock.
Setup. The Main Troubleshooting Setup, Page 48. Apply the ac mains voltage to the bias transformer and switch on the
LINE switch. Adjust the unit’s current setting above 1.0 Adc. Set the extemal supply (EXTERNAL) and adjust the unit’s
voltage setting (INTERNAL) as instructed below.