Scr Diodes (Scr1 / Scr2 / Scr3); Control Module (A1) - HindlePower SCRF SERIES Operating And Service Instructions

2. Note that semiconductors usually short in pairs in bridge circuits, seldom as single units, and it is rare that
all six semiconductors in a bridge are found defective. When diodes fail it usually is because of surge voltages.
Therefore, surge suppressors (SS1, SS2, SS3 & SS4) should also be checked to determine that they are operative
(refer to Section III, 3, d).
f. SCR Diodes (SCR1/SCR2/SCR3) - also see Section III, 3, e
1. These devices are either part of the SCR-diode modules or are individual components as described in
Section III, 3, e above. Power regulating devices (SCR1, SCR2 & SCR3) are silicon-controlled rectifiers, which
cannot be checked using the same method as used with rectifier diodes by forward and reverse resistance checks,
since the SCR will always show a high resistance until triggered.
2. The operation of the SCR can be checked with an oscilloscope. The control module assembly (A1)
produces the gate-firing voltage signal. The oscilloscope should be GROUND ISOLATED for these tests. This is
normally done, using a line isolation transformer, in which the secondary that powers the oscilloscope is
UNGROUNDED. Alternatively, a battery-powered portable oscilloscope can be used. The gate signal may be
checked on the control module (see Section III, 3, g) or where the twisted-pair trigger leads terminate on the SCR-
diode module. The same signal should appear at both points. Absence of the signal indicates that the control module is
defective, not the SCR (see Section III, 3, g).
3. If the charger AC input breaker (CB1) trips immediately, and a shorted SCR is suspected, a simple check
with an ohmmeter can be made. Turn off CB1 and CB2. Connect to the anode and to the cathode, and adjust to the
direct reading scale of the ohmmeter. If a low resistance is observed, reverse the leads and again check the resistance.
If this reading is also low, the SCR is shorted and should be replaced.
4. An SCR can also be checked for operation with a Simpson Model 260 volt-ohmmeter. With the black lead
in common and red lead in +, put polarity switch to + DC. Connect the red lead to the anode and the black lead to the
cathode. The meter should now indicate high resistance above 50,000 ohms (when on the R x 10,000 scale). With the
leads connected as above, set the function switch to R x 1 and touch the gate to the anode. This should fire the SCR
and give a reading of approximately 5-20 ohms. This shows the SCR has been turned on. On very small SCRs, this
reading will hold after removing the gate lead. This is latched-on and can be unlatched by opening the cathode lead.
Larger SCRs will not stay on with the current available with Simpson meter. The Simpson may not have enough
current to gate or turn on extremely large SCRs (400 amp and up).
5. If the charger output is too high, unplug the control module and turn on the charger. With no gate signals
the charger should have zero output. If there is still current output, one or both SCRs are defective.
6. The above checks can be used to confirm that a suspected SCR is indeed bad. However, occasionally an
SCR might check OK in all these tests and still break down or fail in the charger circuit during normal operation. Any
SCRs suspected should be replaced.
g. Control Module PC Board Assembly (A1)
One preliminary note that should be kept in mind: The action of the overall feedback circuit controls the
battery charger output voltage so that the feedback voltage from the slider of "FLOAT ADJUST" potentiometer (R3)
to the control module board matches the 6.4 volt reference voltage on the module. Measure this voltage and if it is not
approximately 6.4 volts the feedback circuit or related circuits are not functioning. Proceed as follows using the
GROUND ISOLATED oscilloscope described in Section III, 3, f, 2 above. Make the following tests in the order
shown.
1. If the gate signals were not checked as described in Section III, 3, f, 2, do so at this time. If one or more of
the gate signals are absent when attempting to operate the charger in a normal manner, make the following checks.
Refer to FIGURE 1 on page 29 for component layout and test point locations on the trigger board. See schematic on
page 30 for related circuit diagram.
2. Check DC voltages at test point P10 [+12 VDC, +/- 0.5V]. If zero, check input and output to miniature
transformer TR1 [120V:12V AC] and check DC voltage at IC7, pin 1 [18 VDC]. Replace defective components. If
OK, proceed as follows.
3. Check three phase input voltages at resistors (R1, R2 & R3). The voltage should be 120 VAC between any
two resistors in three phase relationship. If not present, check J1 plug connections and voltages appearing between
terminal pins 1, 3 and 5 (also 120V - three phase).
Note: The following wave form tests must be made using the oscilloscope set with vertical sensitivity set
at 5V/CM and the horizontal time base set at 5mS/CM. Compare the waveforms observed with those
shown in FIGURE 2 on page 31 for the various test points indicated.
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