Tweco 15XC CE Merlin PAK - Manual (Page 102)

SERVICE 4 - 30 Manual 0-2529
If a diode fails, check the potential causes of diode failure
to make sure the replacement diode will not also fail when
it is installed:
1. Isolate and check each diode separately to determine
which individual diode has failed.
2. High frequency protection for the input rectifier diodes
(D1-6) is provided by capacitors C1 - C6 and MOV 1-3,
which are located between each side of the diode
heatsink on the input filter PC board. Except for the
shorts, these components can not be checked with a
volt/ohm meter. To be safe, the complete input filter
board should be replaced any time an input diode fails.
3. Diodes can overheat if air flow over the heatsink is not
adequate or if the diode is not properly fastened to the
heatsink. Check that all small diodes (D7-14) are
torqued to 20-25 in-lbs (2.3-2.8 Nm) and all large diodes
(D1-6) are torqued to 100-125 in-lbs (11.3-14.1 Nm).
Apply a light film of electrically conductive heatsink
compound between the diode and heatsink. Make sure
air passages in and out of the unit are not obstructed.
4. Diodes that are faulty at the time of manufacture are
difficult to diagnose. These diodes generally fail within
the first few hours of operation. Before deciding that
this was the case, be sure to check out other possibili-
ties.
1. Thermal sensors TS1, TS2, and TS3 are connected in
series to J1-9 on the logic board. Check the voltage from
J1-9 to test point TP1 (or J1-8) on the logic board for less
than 7.5 vdc. If the voltage is greater than 7.5 vdc, the
unit is overheated or a temperature sensor is faulty.
2. If the unit still operates but the TEMP indicator is red,
the problem may be on the LED PC board. If the voltage
is less than 7.5 vdc, check the voltage from J3-4 to test
point TP1 (ground). If the voltage is greater than +4
vdc (and the TEMP indicator is lit red), replace the LED
PC board. If the voltage at J3-4 is less than 4 vdc,
replace the logic board.
Pressure switches PS1 and PS2 are connected in series to
J1-7 on the logic PC board. PS2 is jumpered out by SW3-B
when the unit is set to the O
2
or water secondary mode.
1. Check the voltage at J1-7 for less than 1V with gases
flowing and operating pressure greater than 35 psi (2.4
bar).
D. Diode Check
(continued)
E. Thermal Sensing
Circuit Check
F. Pressure Sensing Circuit