Vokera 20-80 RS Flowmatic - Gas Pressure - Help!

[Robbie1980]

Just looked at the manual online - i was thinking there was a small PCB behind the mod coil infront of gas valve but there must just be the main PCB and a second one - which is what you recommend changing.

Have done some electrical work before so DIY OK?

I don't suppose anyone knows a part number for the small PCB as stuggling to find one from the manual

I'm confused as to whether you are suggesting changing the main PCB, a smaller PCB or indeed it's possible to change a capacitor only with some nifty soldering.

The noise/vibration is not continuous - in fact i haven't heard it tonight - seems to happen when the heatings has been on a long time. The capacitor is large and easily accessible. If this was the cause it would be an easy change.

Rob

 

[Robbie1980]

Heard the vibrating noise again this morning and temp guage was about 75C

When i turned the thermostat for the heat down a touch on the dial on the front of the boiler, the noise goes away.

Does this help diagnose?

 

[DP]

Robbie already gave you the cause/ source of the noise.

 

[Robbie1980]

DP,

Do you think it's worth changing the capacitor only first, before a new PCB?

I am also confused about the talk of 'small PCB'. There is only one i can see from the manual and looking myself which is at the front left under a cover and is about 6" by 4"

Do you mean this PCB DP? Appreciate your patience!

Thanks
Rob

 

[WDIK]

//www.diynot.com/forums/viewtopic.php?p=863266#863266

 

[DP]

Robbie, without digging too deep a hole for myself, all I am going to say is that raw/ unrectified/ unsmoothed low voltage is the issue. With your electrical knowledge to back you up, you should be able to locate the defect or change the component. While remote diagnosis can pinpoint area of malfunction, final clear can only be found when on site. Look at full wave simple low voltage smoothing circuits.

Your boiler will only have one PCB. Later versions of this boiler were fitted with a timer that sat on plastic pillars on the main board

 

[Robbie1980]

Changed the 470 micro farad caopacitor, so will see if it has any affect - not had the vibrating since, but was intermittant before.

The voltage across the two terminals on the mod coil changed from around 6V to 10V with the new capacitor fitted.

I wonder if the wrong potential difference was exciting the assembly...

DP - i guess if the problem doesn't go away, the next step would be other components like bridge rectifiers.

How on earth do manufacturers get away with charger over 100£ for a PCB, transformer and basic components!

 

[Agile]

The manufacturer wants to make a profit! You dont think a wing mirror for an Audi costs £300 to make do you?

The smoothing capacitor does just that to the DC supply.

If its failing there is a 100 Hz ripple superimposed on the DC and under certain mod coil conditions will make it buzz.

Tony

 

[Robbie1980]

Back again,

The noise is back from the coil area - not as noisey after changing the capacitor.

Which components would you change next? rectifiers?

Guessing it's just a case of trial and error?

Cheers,
Rob

 

[DP]

Not trial and error, elementary Mr Watson

 

[tonybhoy]

I've a second hand one which I fitted new to a boiler I replaced. PCB was no more than six months old when I removed the boiler. I have two you can have the two of them however I can't vouch for the other one which was also removed from an old boiler.


Yours for £35 including postage if your interested Pm me.

 

[Robbie1980]

Not trial and error, elementary Mr Watson

DP - i was referring to which smoothing component to try next - thats the advice i was after.

I would have thought the bridge recetifier as the capacitor has been changed.

I'm guessing it could be something else inherrently wrong with the PCB
Rob

 

[DP]

Mains goes to primary coil of transformer through some kind of limiting device. Secondary output is full wave rectified. Smoothing done by capacitor. Resultant voltage used for controls that you are interested in. Rest of PCB is just a termination board save the relay that provides two contacts for switching.

 

[Robbie1980]

Can anyone tell me the voltage i should expect across the modulating coil?

This way i will know if the rectifying compoents are working correctly.

Could it possibly be the relay making the vibrating noise DP? The reason i say this is that when CH is on, sometimes the boiler will fire then low flame, fire then low flame...or does this just happen when the water is just reaching then dropping below the temperature required to be pumped round to heat the house?
Rob

 

[Agile]

The voltage to the mod coil varies according to the power required from the boiler.

The important aspect is that it should be smoothed DC and not have any superimposed ripple which can cause the coil to buzz.

Did you replace the smoothing capacitor with a brand new one of the same value?

Tony