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The PCB generates a wide range of different voltages both ac and dc. With care these can
be checked from the multi pin sockets as shown on the circuit diagram. In most cases
when checking voltages the return (neutral) will be to earth and occasionally in parallel
(across the two connections), this applies to the thermistors and the modulating gas valve
solenoid.
Components such as variable resistors and thermistors can be checked on the ohms
range on the multi meter. Flame detection signal can be checked by using micro amps ac
or ac volts.
If a fault develops on the boiler it is necessary to determine if the problem is internal or
external. Internal refers to the PCB and external refers to all the controls/components
which are attached to the PCB. Both internal and external systems must work together in
order for the appliance to operate correctly.
Many of the external controls are associated with the safe operation of the boiler, and
should a fault develop the self diagnostic codes will indicate the faulty component. This
system will not indicate faults on external controls which are used as demand detectors
e.g. Microswitch, room thermostat and time clock.
Whenever the PCB is powered up in the relevant mode, voltage is supplied to the external
controls/components responsible for detecting demand and safe operation of the boiler.
Ignition sequence will not begin if;
A Demand from external controls is not detected.
B A component is faulty or out of range (temperature detection).
C Lock out occurs before any ignition sequence commences.
ELECTRICAL COMPONENT TESTING
The PCB contains sensitive electronics and software programs that will become
permanently damaged if mains power 230V AC is allowed to enter the low voltage
circuits. This can only occur if care is not taken when testing the PCB. When working on
the PCB it is possible to check for voltages from the multi pin plugs which are identified by
the prefix 'J'. By using the circuit diagram and accompanying notes from each component
you can select the correct range on the multi meter for testing purposes.
DO NOT ATTEMPT to test individual transistors, chips, etc. which are built into the PCB.
When checking for output from the PCB the multi pin plug must be left in position, and the
tip of the test lead probe must make contact with the exposed part of the cable connector
at the entry point of the multi pin plug.
DO NOT ALLOW wires to short out to earth when changing components, even if they are
low voltage as damage to the PCB will occur.
To test for voltage from the PCB preferably use the main earth connection as negative or
the boiler chassis for both AC and DC tests.
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