Scr Removal/Installation - Carrier 19XR Start-Up, Operation And Maintenance Instructions Manual

5. Using an ohmmeter, perform the following resistance
measurements and record the results:
SCR PAIRS
MEASURE
BETWEEN
CHECKED
T1 and T6
T2 and T4
T3 and T5
If all measured values are greater than 5K ohms, pro-
ceed to Step 10. If any values are less than 5K ohms,
one or more of the SCRs in that pair is shorted.
6. Remove both SCRs in the pair (See SCR Removal/
Installation).
7. Using an ohmmeter, measure the resistance (anode to
cathode) of each SCR to determine which device has
failed.
NOTE: Both SCRs may be defective, but typically, only
one is shorted. If both SCRs provide acceptable resis-
tance measurements, proceed to Step 10.
8. Replace the defective SCR(s).
9. Retest the ''pair'' for resistance values indicated above.
10. On the right side of the firing card, measure the resis-
tance between the red and white gate/cathode leads for
each SCR (1 through 6). A measurement between 5 and
50 ohms is normal. Abnormally high values may indi-
cate a failed gate for that SCR.
If any red or white SCR gate leads are removed
from the firing card or an SCR, care must be taken
to ensure the leads are replaced EXACTLY as they
were (white wires to gates, and red wires to cath-
odes on both the firing card and SCR), or damage
to the starter and/or motor may result.
11. Replace the SCRs and retest the pair.
SCR REMOVAL/INSTALLATION — Refer to Fig. 39.
1. Remove the SCR by loosening the clamping bolts on each
side of the SCR,
2. After the SCR has been removed and the bus work is loose,
apply a thin coat of either silicon based thermal joint com-
pound or a joint compound for aluminum or copper wire
connections to the contact surfaces of the replacement SCR.
This allows for improved heat dissipation and electrical
conductivity.
3. Place the SCR between the roll pins on the heatsink as-
semblies so the roll pins fit into the small holes in each
side of the SCR.
NOTE: Ensure the SCR is installed so the cathode side is
the side from which the red wire extends. The heatsink is
labeled to show the correct orientation.
4. Hand tighten the bolts until the SCR contacts the
heatsink.
5. Using quarter-turn increments, alternating between clamp-
ing bolts, apply the appropriate number of whole turns
referencing the table in Fig. 39.
RECORDED
BEING
VALUE
3 and 6
2 and 5
1 and 4
Care must be taken to prevent nut rotation while tight-
ening the bolts. If the nut rotates while tighting the
bolt, SCR replacement must be started over.
6. Reconnect the red (cathode) wire from the SCR and the
white (anode-gate) wire to the appropriate location on
the firing card (i.e., SCR1 wires to firing card terminal
G1-white wire, and K1-red wire).
7. Reconnect all other wiring and bus work.
8. Return starter to normal operation.
NUT
CLAMPING
BOLT
ALUMINUM
HEATSINK
ROLL PIN
SCR
SCR PART
CLAMP
NUMBER
SIZE
BISCR
6601218 1030
6601818 1030
8801230 1035
8801830 1035
15001850 2040
15001850 2050
220012100 Consult Benshaw Representative
330018500 Consult Benshaw Representative
Fig. 39 — SCR Installation
83
LOOSEN
AND
TIGHTEN
BOLTS
A
FROM
THIS END
A BOLT
NO OF
DIMENSION LENGTH
TURNS
(in.) (in.)
2.75 3.0
1 ⁄
1
2
( 70 mm) ( 76 mm)
2.75 3.0
1 1 ⁄
2
( 70 mm) ( 76 mm)
2.75 3.5
1 3 ⁄
4
( 70 mm) ( 89 mm)
2.75 3.0
1 ⁄
3
4
( 70 mm) ( 89 mm)
4.00 4.0
2 3 ⁄
4
(102 mm) (102 mm)
4.00 5.0
2 3 ⁄
4
(102 mm) (127 mm)