F3 Series Charger Manual
15
Refer to Figure 5.2. Disconnect all the leads to the SCR and its heat sink. Connect a voltmeter across
the 1K
Ω resistor to measure the voltage drop. With the battery connected as shown, Vdrop should
read approximately 2.3V (Vsource-0.7V). Remove the voltage source to the gate, but keep it
connected to the 1K
Ω resistor and cathode. Vdrop should equal zero. Reconnect the gate and reverse
the batteries polarity. Vdrop should read zero volts. Readings other than these indicate a defective
SCR.
Test #5: Due to the modest cost of the control circuit, we recommend that the entire unit be replaced
rather than attempting to repair it. If the troubleshooting guide has not revealed any defective
components (tests #1-4), the control circuit should be replaced as a unit.
Test #6: Remove all wires from the current shunt. Place a milliohmmeter across the two terminals.
The following formula should be used to determine the correct resistance:
Resistance in ohms should equal 0.5/Output current rating of the charger, except in 35 amp units when
resistance should equal 0.1 ohms.
If the resistance is more than 20% too low, the current shunt should be replaced.
FIGURE 5.2
SCR test setup
