How to test which pcb ?

[mario500]

Hi fella's.

Been repairing combi's for 12 months or so and seem to diagnose most probs with success. Went to an ocean Alpha 240 today with a very anoying intermittent fault. When it's misbehaving it wont fire on hot water or c/h. It only did this for thirty seconds whilst I was there so didn't have the chance to test for anything dead. I did all the usual checks like fan, aps, but if it fires up at all, then it must be somthing else. My guess is it's the pcb, although there are no dry joints or burned out components. Checked all the multi connectors and all look ok - got continuity at each one in turn. Thinking maybe the relay on the pcb is being a little naughty? But of course this boiler has an ignition module as well as the main pcb. Have any of the more experienced guys got and tips on how to test which one is at fault?

cheers Mario

 

[breezer]

change them both

to identify which component on a pcb takes time = money hence change them both, and what if your diagnosis is wrong?

you have already mentioned the obvious things

when i workd for hp, if these was a suspect pcb, change it, its quicker and customer is alwys happy

 

[Agile]

I have found the APS on this model can be troublesome.

Are you sure the system flow switch is making reliabily all the time?

Tony

 

[mario500]

Thanks for the replies fella's.

Tony, I checked the aps and flow switch. But of course I only had a thirty second window when the boiler actually failed to opperate and found myself trying to make it fail again! oh joy... I guess the only way to solve the problem is to invest a little more time on a second visit.
I was just hoping that you guy's might have had some trick to differentiate faults on main pcb or ignition module.
Of course as pointed out I will just have to have the U gauge and multimeter at the ready and hope to have a eureka moment.

thanks again for the replies.

Regards Mario.

 

[Agile]

A fault finding technique which is apparently rarely used is to measure the pressure applied to the APS ( using a "T" piece ).

The actual pressure varies with the flue length and is not quoted but obviously has to be comfortably above the operating pressure of the APS.

While measuring you can let some presure leak and see at what pressure the switch breaks.

Tony

 

[geoff@cetltd]

CORGIs ... know your limits

 

[corgiman]

CORGIs ... know your limits

belive me raden I do

wish i knew more i do

 

[urbanspaceman]

fan venturi probably full of fluff

 

[Paul Barker]

Depends which part of cycle it fails, but for as many triggers (relays) that there are on the pcb which might be the fault, there are the same number out there in the boiler environment. So the contacts of the flow switch aps have to be suspected but weakness in the mechanical motive force is the first place to look, so diaphragms whether pins are getting stuck and fan venturi have to first be checked out. Cleaning the fan venturi is part of an anual service it is so important. Also start with the simple stuff first, and the very last thought is pcb.

 

[Agile]

Its easy to measure the pressure applied to the APS using the tee piece.

Its also part of the fault finding elimination process to measure the contact resistances of an APS. They should be well below one ohm!

In more complicated situations the operating presssure of the APS needs to be checked. I had an embarassing situation last week when I fitted a new one as a precaution and found it did not operate. Checking it later showed that it needed 1.3 mB instead of the rated 0.89 mB.

Tony

 

[mario500]

Thanks for the replies.

It turned out to be the pcb in the end. Not sure if it was a contact in one of the relays or another electronic component.
Everything else tested ok. don't like being unsure though, it's very un-nerving. Anyway new pcb and away she went.

Tony, I am very interested in your procedure for testing air pressure switches with a tee piece. Are you saying that you connect the U gauge to the tee piece? Could you elaborate please?

Many thanks
Mario

 

[ChrisR]

You have to measure pressures both sides of the aps though one is usually not far off atmospheric. You need a digi manometer of course with a resolution of 0.01 mbar .
You also need to know what pressures the aps is supposed to switch on and off at, they're different pressures.