Sime friendly pilot probs

[techno]

Evening gents, got a painfull problem with the above combi boiler, it all started with the pilot light out, gas was present so changed thermocouple.No change ! After testing thermocouple (it was ok!!)Gas vavle I thought ,but before I went on that route just had a general look round and took the cover off to look at the pcb, bingo there was a scorch mark on the cover and the pcb itself, with bits of resisitor hanging around.
The question is would this make any difference to the operation of the pilot light? The only connection i can see is the modureg may not be doing its job due to the burnt out pcb, any ideas?
Before some of you ask yes I am Corgi reg!, always kept away from these boilers due to a friends recommendation that there not so friendly!!
Any help appreciated, cheers

 

[Gasguru]

Perhaps the burnt resistor is allowing the boiler to overheat and the resultant pilot outage is due to the overheat stat tripping?

After opening many pilot overheat stats do not re-close with a low enough resistance to keep gas valve solenoid open.

I have yet to find a problem on a gas valve pilot solenoid. The problems have always been connections/overheat stat etc.

 

[techno]

Thanks Gas for your reply, I didnt make it clear that i cant light the pilot at all, Ive checked the overheat stat which is part of the thermocouple circuit thats fine , maybe the gas valve is faulty afterall.

 

[Gasguru]

Is there gas at the pilot outlet on the gas valve?

Has you friend turned down the pilot adjustment on the gas valve?

Is the pilot injector blocked?

Does the pilot tube nedd purging?

I assume you are attempting to light the pilot with a flame?

 

[techno]

Yep gas , i can light the pilot as normal, when i let go of the pilot button the pilot light goes out!, as i see it it must be the gas valve or burnt pcb thats faulty but which?

 

[techno]

Just realised what I said earlier, I meant i coudnt get the pilot to stay in , not light it at all, sorry for any confusion

 

[Gasguru]

Have you checked the connection on the gas valve - sometimes a tiny strand of solder can bridge the contacts.

I assume the overheat stat is in line with the thermocouple (its years since I've come accoss that particular model). If so change the overheat stat - they are not easy to test with normal mutimeters.

 

[Agile]

You need to test the voltages! Very easy to test!

The thermocouple output is about 25 mV open circuit. When in circuit that drops to 12/13 mV. If there is an o/h stat in series with the gas valve then that drops 1-2 mV leaving about 8-9 mV at the input connection to the magnetovalve.

I have often found a pilot solenoid faulty but this boiler does not have one! Only the standard magnetovalve.

Your voltmeter will tell you whats going on.

Since its a low inpedance circuit, every connection has to be perfect and very low resistance.

Tony Glazier