Service manual

54
Overcurrent Circuit Troubleshooting (Figure 3-9)
This circuit limits the maximum current the load can sink for different input voltages and/or power conditions. The primary
components in this circuit are amplifier U5 (U12 in 60504B/07B) and transistors Q11 and Q12.
At power on the secondary power clear (
SPCLR
) signal provides a High level via D35 to drive U5 (U12 in 60504B/07B)
low, turning Q11 on. With Q11 on, PROG goes high (less negative) and turns off the input power FETs.
When the input voltage is about 6.3V (9V on 60507B; 40V on 60503B) or lower, D33 is forward biased, causing voltage
divider R214, R213, R59, and R58 to hold U5 (U12 in 60504B/07B) at approximately - 7V. This clamps the maximum
input current capability between
30A and 33A for 60501B.
45A and 66A for 60502B.
10A and 11A for 60503B.
102A and 133A for 60504B.
46A and 53A for 60507B.
As the input voltage increases from 63V to 65V (9V to 155V on 60507B; 40V to 260V on 60503B), D53 is reverse biased
and the input voltage will appear across the voltage divider. This causes the voltage at U5 (U12 in 60504B and 60507B) to
decrease from - 7 volts to - 0.8V. At an input of 65V (155V on 60507B; 260V on 60503B), D54 turns on and holds U5
(U12 on 60504B and 60507B) at - 0 8V volts and limits the maximum input current capability to less than
5A for 60501B.
9A for 60502B.
2A for 60503B.
15A for 60504B.
8.5A for 60507B.
When the input voltage reaches 75V (170V on 60507B; 287V on 60503B), the OV circuit goes to -13V and pulls PROG
low (more negative) via D17. The input power stages will now attempt to sink more current and decrease the input voltage.
If the combination of input voltage and current (power) is greater than the power stages can sink when OV condition occurs,
the overpower circuit will override the OV circuit and limit the maximum current capability of the load.
The -15Vx bias voltage is a delayed bias derived from the normal -15V supply. When the load is first turned on, -15Vx is
not present and U5-6 (U12 on 60504B and 60507B) is at common potential. This causes Q11 to conduct pulling IPROG
high. Q12 is also on, connecting Q11 to the + 15V bias. When -15Vx comes on, Q12 turns off causing U5-6 (U12 on
60504B and 60507B) to go more negative than U5-5 (U12 on 60504B and 60507B). This turns off Q11, allowing PROG
to go negative. VR27 supplies Q11 collector current once -15Vx is available.
To troubleshoot the current limit circuit, check test points
, , and using the measurement conditions and
readings specified in Table 3-3.