Does anyone know the resistor R15 value for a Potterton Control Unit Part No.407677 which has partly burnt out, so missing the 3rd. colour code band! I have Brown/Black/....../Gold.

I too had the senario of my Potterton Profile 60e attemting to startup, that is, the fan continually cycling on & off, but by replacing Capacitor C7 (22uF.), now ALL OK! Thank you very much!!

Andrew P, April 2015

Thanks Killerbee & HB. My boiler had similar symptoms to the original post with the fan cycling many times before it finally lit. Replaced C4, C6 and C7 with new 105deg parts from RS Components.
One other consequence of the continual fan cycling was that one of the relays had worn out and become 'sticky', so sometimes the boiler never fired up at all. It doesn't take many months to reach the rated 100k switching life of the relay when it was cycling 30-50 times before the boiler fired up, so not really surprised at that. I replaced both relays at the same time as the capacitors and now it fires up first time, like new. :-)
On testing the old capacitors, only C7 had dropped in value significantly, measuring 15.5uF instead of the rated 22uF. The other two were still within specification. However, with the cycling occurring while C7 was at 70% of its rated value, I'm now wondering if it really needs to be 33uF or even larger for improved life...

RKM, March 2023

R15 is 100k. It's brown, black, and yellow.

What normally happens is that either C7 (22uf), C4 or C6 loses capacitance over time. This causes the power rail used to energise both relays to fail. Without relay RL1 energised Normally Open (NO) of the air pressure does not get mains live. So when the air pressure switch is activated, the fan loses mains live and shuts down. When the air pressure switch deactivates, the fan gets live through the air pressure Normally Close (NC) and starts again. So the fan flip flops start stop start stop.

What should happen

When power is supplied to the PCB, the capacitive power supply gets energised. C9 is the capacitive voltage dropper. This starts transistors T2 and T2 and energises both relays. The fan starts and causes the air pressure switch to move to NO. Because both relays are energised, NO of RL1 now has live, so NO of the pressure switch will also have main Live. The air pressure switch moving to NO, now keeps the fan working and causes the pilot gas valve to open. At the same time capacitor C8 starts to charge through diode D4 and resistor R15. When the correct voltage is reached this capacitor discharges through SP1 and the primary of the TF1. This causes a spark, which in turn causes the pilot flame to come into existence. Flame rectification causes a DC voltage to be passed back through R10. This causes T1 to switch on and switch off T2 and T3. This in turn switches off RL2 stopping the spark and switching on the main gas valve (Gas valve White wire). RL1 does not switch off because it still receives power through R4. R4 can maintain the energised relay LR1, but cannot cannot switch it on. i.e safety circuit. Please see the power requirements of activating a relay vs maintaining an activated relay.

Killerbee, December 2022

My post about C7 was not quite correct. Relay 1 should energise immediately on a call for heat. C7 is the smoothing capacitor for the -50V DC zener diode based circuit which energises relay 1. If this circuit malfunctions, the relay fails to energise and the fan is disconnected from live as soon as the air pressure switch is triggered. The fan then stops, the air pressure switch switches back and reconnects the fan to live and it starts again. This causes the characteristic fan cycling. I believe a deteriorated C7 is the most likely culprit if your fan is cycling.

HB, May 2021

It looks as though C7 is a timing capacitor which slowly charges up after a call for heat. If the ignition sequence is not completed during that time delay, I believe relay 1 opens and disconnects the fan. On my failed PCB I had the same fan cycling problem. C7 read only 4.5 uF, not 22uF, so I am guessing the delay became too short to allow all of the other ignition steps to complete. If on the other hand, the ignition is completed, I am guessing that the flame rectification circuit inhibits the charging up of C7, holding relay 1 closed. I can't follow the schematic 100% as it's confusing but it gives a lot of clues.

HB, May 2021

Based on the schematic that I have traced out, R15 appears to be feeding mains live from the air pressure switch (terminal T4) to the spark generator via another 100K resistor (R12) and diode (D4), charging up C8, which discharges through spark gap SP1. Once the pilot flame is established, it appears that a flame rectification detector circuit triggers relay 2 to turn on, stopping the spark generator and opening the main gas valve (GV2) by shorting terminal T1 to neutral T2.

HB, May 2021

Hi this was an invaluable post for me like several people I too had the senario of my Potterton Profile 80e attempting to startup, that is, the fan continually cycling on & off, but by replacing Capacitor C7 (22uF.), now ALL OK! Thank you very much!!

I found the best way to do it was testing the capacitor via a cheap multimeter with capacitor testing capabilities Model N59Lk cost £14.99 from maplins . Although the capacitor was still functioning the reading was v low compared with a new capacitor . Simply replaced it and hey Presto - Thanks for the great post just thought I'd add my experience . Ps This is the second PCB unit that I've had in the boiler which is about 10 years old . The last pcb cost me about £140 so this fix ( hopefully if it lasts has saved me over 100 quid )

Dave, May 2015

R15 is 100k ¼W (Brown Black Yellow) and is located next to Relay RL2. It is prone to overheating and you might replace it with 100k ½W.

While you have it apart replace the electrolytic capacitors C4 & C7 both 22uF 63V and C6 4.7uF at 63V. Be sure to observe the polarity of the capacitors and get them the right way round ±.

These capacitors are prone to degradation due to being in an over warm environment all the time. Electrolytic capacitors can be badly affected by heat. Try to get the same or higher voltage rating (but not lower) and 105 ºC temperature rating for longer reliability/life or at least 85 ºC.

An article here tells all about it - with expanding pictures.

http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler

I believe that R15 is placed across the gas valve as a bleed resistor to help the valve close more quickly - as such it takes current as long as the gas valve is open - a burnt R15 is a feature of this PCB - they all burn in time and is nothing much to worry about. I think the boiler will work even without R15 in circuit - the gas valve will just close more slowly

All these components are available very cheaply from a hobby electronics shop like Maplins in the UK.

Good luck..., April 2015