Suprima 120 ignites and then goes out

[russell.townsend]

Suprima 120:

Started cutting out: Ignited, but spark generation continued, then shut down (after 2-3 secs).
Assumed that PCB was faulty, so I have put in a new one.
(I put in a complete ignition kit, inc. electrodes and adjusted fan speeds)
Problem continues - exactly the same.

At first I thought that new PCB was faulty, but when I think about it - perhaps something else is at fault. I'd like to confirm before I send the PCB back as faulty.

Found some detail on an old post about measuring the flame sensor, but not enough to help.
How can I measure to see if the flame sensor is working ?
Could it be anything else ?
Is there a circuit diagram of the PCB ?

 

[Rob]

Check continuity through electrode and lead and that electrode is clean and positioned correctly.

Check polarity aswell

 

[Rob]

...

 

[baxpoti]

Suprima 120
and adjusted fan speeds

What have you set the fan speeds to?

 

[russell.townsend]

Checked electrode continuity:
both are 0.2 ohms from electrode tip to back of PCB.
checked resistor values on PCB (they are the same as on old PCB)
Performed a visual check of PCB and no obvious cracks / dry joints etc..
checked resistance between electrodes >10 Mohm


Fan speeds:
With red switch down:
Slow speed set to -4.0 mbar
High speed set to -17.5 mbar
then put red switch back up

 

[russell.townsend]

Forgot to say:

Thanks for the replies. Really appreciated that you can take time to reply to me !

 

[Agile]

Most usually a leak causing water on the flame electrode.

If its removed it can be visually checked.

Continuity has to be to the TIP of the electrode not just to the connector tag.

Tony

 

[ChrisR]

How far does the gas pressure on the inlet of the gas valve drop when it gets to full gas rate? I know the initial rate is much lower, but I "have one", doign the same as yours, which has everything changed and checked and adjusted, and the only thing anyone can suggest is inadequate gas supply. It drops to about 18mbar on first lighting and then dives to about 14 on full. Somehow the gas governor got adjusted up a couple of mbar which helped...

Are you thinking Ideal Response 120, Tony?

 

[russell.townsend]

Thanks for the replies.

Input pressure:
26mbar off -> 22mbar on
doesn't stay on for long enough to check full on pressure.

I looked carefully at the PCB and realise that the "chip" U5 is a LM393N and this is connected, via resistors to the flame sense electrode. By putting a meter between pins 5 (ground) and 7 (output) I can see if the flame is being sensed. (0v == no flame ; 3v == flame).
When the boiler ignites this senses the flame for about 1 second, then reverts to no flame.
To double test this I put an insulated (safety for me !) switch and 10Mohm resistor in series between the electrodes. If I switch this on before ignition the boiler shuts down in error. If I switch it on as the boiler ignites then the boiler stays lit.
So: I think that the flame sensor is not working correctly...
When I use my switch to keep the boiler on - I observe the flame and it seems steady and regular (looks like I expect it should).

I've given the electrodes many inspections/ cleans and they seem OK, so the next thing is to take out the jet/burner assembly to see if a foreign object has got in and is restricting the flames in one particular area.

 

[baxpoti]

Did you change both PCBs at the same time?

 

[russell.townsend]

Yes, both PCBs and the new wiring / electrodes all came in a kit.

I didn't find anything suspicious in the burner.

 

[baxpoti]

What is the gas valve offset reading?

Confirm you have no flow switch fitted, opt extra after 2003
(for installations containing plastic pipe)

 

[scatmanjohn]

Most usually a leak causing water on the flame electrode.

funnily enough i was going to say that.

About a year ago i went to one of these, it had 3 new boards in the last year.

Me thinks, hang on a minute, they are sheeite but not that sheeite.

Took out the burner and removed the flue hood, blinking hek.

Heat ex had been leaking and was virtually solid with rust.

Have you had a look at the heat exchanger?

 

[russell.townsend]

Hello thanks for the replies.

Not sure how to measure the gas valve offset on this valve.
But there is no trouble igniting / no improper noise, so I don't suspect this.

Heat exchanger is clean and dry.

I still suspect that the new PCB is faulty.
I'm going to borrow an oscilloscope to check over the weekend.

 

[ChrisR]

A 10M resistor shouldn't work, if it's looking for rectification .
Wouldn't it be embarrassing if it turned out to be a reverse polarity problem

Spose you use your blowlamp on the detection electrodes?

What the frig is a gas valve offset?

By the way some of this is getting inappropriate for a diy forum. The Workbench section of the ARGI website would be better as it's rgi's only, but many of the good guys on this forum don't seem to visit there.