Maintenance
Troubleshooting
5
5-15
Transformer/Rectifier/Filter 5-19.
The transformer/rectifier/filter functional group is composed of the transformer
assembly, the overvoltage detect circuit, the rectifier bridge, and the capacitive filter.
The transformer provides a secondary voltage of 25V rms, or about 30V pk.
The best way to troubleshoot this section is to use an oscilloscope at TP501 and at the
secondary input connector J501. The ripple voltage at TP501 should be approximately
400 mV p-p. You can check the overvoltage detect circuit by using a variable ac source
with some current limiting. The overvoltage detect circuit should short out the secondary
when it trips.
Use the following guide to symptoms and possible faults:
Symptom 1: No voltage at TP501
Check: Cable, secondary voltage, diodes CR501-504, C502, C503, or C504.
Symptom 2: Charger puts out approximately 12V dc but will not go higher.
Check: To see that the ac line voltage selector is in the 120V ac position. These
symptoms will occur if the selector is in the 220V ac position and 120V ac is applied.
Battery Charger 5-20.
The battery charger is a dual mode charger. In one mode it acts as a current source of
234 mA. In the other mode, it acts as a voltage source with a finite source impedance. IC
U501, pins 6, 7, and 1 act as the sensing comparator for switching between these modes.
If the battery charger doesn’t work correctly, start by removing ac power and the battery
connections and replacing the battery with a 51Ω, 3W resistor or resistor network. Power
the unit with ac and check to see that the voltage across the resistor is consistent with a
current of 234 mA (11.93V dc nominal). This checks the whole current source section of
the circuit.
If the voltage across R502 or R503 is not 1.25V dc in the current mode, first check to see
that SW502 is closed, then check U500 or the protection diode CR505. The sensing
circuit is supposed to switch from the current mode to the voltage mode when the
voltage across the battery is 14.6V dc. This is equivalent to a battery impedance of
62.4Ω. You can simulate this impedance using a variable resistor from 50 to 70Ω, but
make sure the resistor is able to dissipate the required power. If the circuit doesn’t switch
from the current mode to the voltage mode, check the U501 comparator and Q502
switch.
+11
.
5V DC Preregulator 5-21.
The +11.5V DC Preregulator circuit is a constant-voltage supply set by U504 and
resistors R531 and R532. The output is nominally +14.9V dc. If the voltage is too high,
check resistors R531 and R532, and U504 and CR507.
The battery switch relay circuit should switch the state of relay K500 when the voltage at
the output of U504 falls below about 14V dc. At this point, the zener stops conducting,
and transistor Q503 switches off, thus de-energizing relay K500. Check the trip point of
the relay switching circuit by reducing the voltage. Use either method 1 or 2:
1. Insert a potentiometer in parallel with R532 to reduce the effective resistance of
R532 to reduce the voltage in a controlled manner.
2. Parallel R532 with a 4.12 kΩ resistor. This will drop the voltage to 12V dc.
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