55
fig. 97
GW 421/0
fig. 98
GW 528/0
7.22.4 Replacement of CPU
board
•
Remove rectifier and mother boards
as in 7.22.2 and 7.22.3.
•
Pull off central heating control knob
(2, fig. 97) from front of control
box.
•
Remove lower control box cover
(5, fig. 94) by undoing two screws
(6, fig. 94).
•
Lift all cable grommets (6, fig. 96)
out of grommet channels.
•
Lift lower control box cover
(5, fig. 94) clear.
•
Disconnect three cable plugs
(1, fig. 98) and flame sensor lead
(2, fig. 98).
•
Unplug ribbon cable (3, fig. 98)
from relay board.
•
Remove two screws (4, fig. 98).
•
Squeeze together the board retai-
ning clips (5, fig. 98) and lift board
out of control box.
•
Reassemble in reverse order.
NOTE: When refitting the CPU board,
ensure that the user control spindles
(a, fig. 99) are aligned with the poten-
tiometer hole (b, fig. 99) and that the
locating rib of the potentiom-
eter correctly engages in the groove
of the control knob spindle.
•
Recommission boiler (see Section 5)
7.22.5 Replacement of relay
board
•
Remove on/off control knob.
•
Remove rectifier and mother boards
as in Sections 7.22.1 and 7.22.2.
•
Remove lower control box cover
(5, fig. 94) by undoing two screws
(6, fig. 94).
•
Lift all cable grommets (6, fig. 96)
out of grommet channels.
•
Lift lower control box cover
(5, fig. 94) clear.
fig. 99
GW 528/0
