Performa PCB

[Softus]

Hi Guys

Just looking at a Performa 24 which has spontaneously stopped working. Recent rad addition, by another plumber, but no reason to think this is the cause. So far, fault finding reveals:

1a. Pump is running.
1b. APS is ok.
2. Fan not starting.
3. No supply to fan.
4. R90 on PCB shows open circuit.

Anyone seen one of these faults, or similar?

If so, have you repaired or replaced the PCB?

If repaired, was it successful, or is the failed resistor indicative of another failed component, in which case am I onto a hiding to nothing?

 

[gazthepottertonengineer]

try pushing the overheat stat in

 

[Softus]

Thanks gaz - that was the first thing I tried.

However, have just this moment discovered the fault - it was me.

I misread the scale on the DVM and the stat resistance is 1.1 Ohms, not 11KOhms, so it's a dead 'un. Hope this doesn't mean something got blown on the PCB, but we'll see on Tuesday when I can get a new stat.

 

[Agile]

The O/H stat resistance is MEANT to be low!!!

Tony

 

[Softus]

The O/H stat resistance is MEANT to be low!!!

It isn't clear what you mean by the word "low", but p39 of the Potterton Installation Manual states that the resistance, when cold, should be 11K Ohms.

 

[gazthepottertonengineer]

i think your getting your wires crossed as one of you is talking about the o/heat stat and the other about the thermistor

 

[Softus]

Agile - I apologise - it seems that it was me I crossed the wires.

Back to the fault-finding flowchart, according to which I should be checking the sensor next, and this ought to have a resistance of 11K Ohms.

 

[Agile]

There are two sensors, one DHW the other CH, they should have a cold resistance of around 10 K.

Any O/H stat should have a low resistance, i.e. less than ONE ohm!

I am not confident that you have correctly measured R90 on the PCB and I have not had the opportunity to look at one of these PCBs in our stores ( as its a holiday today! ).

It should be reasonable easy to read its colour code or marking to see what the resistance should be and to follow the tracks to see what its circuit function is.

Tony

 

[Softus]

There are two sensors, one DHW the other CH, they should have a cold resistance of around 10 K.

11K according to the Potty manual; also according to the manual there is only one such sensor, which I have to admit surprised me (initally).

Any O/H stat should have a low resistance, i.e. less than ONE ohm!

I concur, now, having got my facts right - the upper limit of resistance for the o/h stat is 1.5 Ohms.

I am not confident that you have correctly measured R90 on the PCB and I have not had the opportunity to look at one of these PCBs in our stores ( as its a holiday today! ).

Very astute of you indeed - my mistake with the DVM extended to the false measurement of R90. I was given to measure it when I noticed a very suspect looking solder joint nearby - the joint is the by far the worst on the entire PCB, but I currently have no reason to suspect either the joint or R90.

It should be reasonable easy to read its colour code or marking to see what the resistance should be and to follow the tracks to see what its circuit function is.

The colour code indicates that its value is 10 Mohms, +/-5%. I can't follow the tracks tonight 'cos I don't have the board with me now.

 

[Paul Barker]

Just desoulder one leg and measure it with a very good meter. Cheap ones are hopelessly inacurate above 1 meg. Why not just put a new 10 meg resistor in anyway and resoulter the poor joint to eliminate that?

then see where you are.

Before fan starts aps has to state that it is working properly. Could be that.

 

[Softus]

Thanks PB, I'd like to try all those things.

Can you expound on the behaviour of the APS? I thought it was simply a switch with no resistance when closed? I notice now though that it has three wires - can you tell me more about it?

 

[gazthepottertonengineer]

There are two sensors, one DHW the other CH, they should have a cold resistance of around 10 K.


Tony

only one on this model your getting confused cos there is 2 on the 28 model the 24 uses the central heating thermistor for the hot water as well

 

[Agile]

Thanks, Gaz, thats confusing putting different bits on the 24 and 28 !!!

I think I noticed there are different gas valves on the Baxi and Potty versions or was it a 24/28 difference?

If the resistor is 10 meg ohms then you will need a good meter with a 20 M range as many cheaper ones only go up to 10 M or even 2 M.

With such a high valve its probably in the flame detection circuit.

Most APS circuits require the APS to be in the resting position before comencing the ignition sequence. This is to prevent RGIs trying to bypass it !

Tony

 

[Paul Barker]

Yes Tony put it right

"Most APS circuits require the APS to be in the resting position before comencing the ignition sequence. This is to prevent RGIs trying to bypass it !"

First chjeck a boiler makes is that aps is one way then it allows fan to start then it checks if aps is other way, that way it kjnows aps hasn't stuck in one position or as Tony says been bipassed.

Certainly this is the sequence on that boiler.

I believe that dc flame rectification replaced flame conduction for the same anti tamper benefit. Anyone can bipass a conductive circuit but you can't make a short circuit rectify.

Sorry I should elaborate a peice of wire will conduct ac and dc, but a rectifier converts ac into dc. In flame rectification flame proving the pcb sends out an ac signal and is looking for a dc signal in exchange. A short circuit would return an ac signal and pcb would not be satisfied that a flame was present.

I think we have all been to the boiler with the oht stat shorted out and a sign on the front saying only to be used on low!!!!!!!!!!!!!!!!!!!!!!!

 

[Softus]

Update:

APS - resting position - conducts between one terminal and common.
APS - activated (by mouth) - conducts between 'other' terminal and common.
o/h stat - still reads 0.5 Ohms
temp sensor - 55K Ohms at about 10 degC.
Conclusion - temp sensor faulty.
Plan - fit new one in the morning.