Boiler PCB 407676

by Nick  

At risk of turning into a boiler specialist, I wanted to share some useful info provided to me by John B. He has a Potterton 16/22 Netaheat Electronic LPG boiler and had similar symptoms to my dodgy old boiler. His fan and pilot ignition were OK. However, the ignitor was clicking every second up to two minutes plus before the main burner cut in when starting from cold. When hot, it only took 2-3 seconds for the burner to light. He'd replaced both the electrode and the thermostat to no avail.

Unlike my boiler though, his had a different PCB, a Potterton 407676 (the reverse side of the PCB also shows the number 90911/4). What seemed conclusive to me of an electrolytic capacitor problem was that a gentle tap (when cold) on the PCB triggered the main burner. John kindly took some photos and provided technical details of his PCB for me to post.

407676 PCB case

The original electrolytic capacitors were marked as follows:

C9: 220uF, 16V/85°C
C5: 100uF, 40V/85°C
C4: 22uF, 50V/105°C

407676 PCB with electrolytic capacitors highlighted

John replaced them with capacitors rated to cope with slightly higher voltages (Maplin part nos. VH42U, VH39N, DT57M respectively). His boiler now works perfectly again!

If this is helpful to you please leave a comment, and link if you can, to make it more likely to be picked up by search engines for other people. If it's saved you the cost of a replacement boiler and you'd like to express your thanks with a donation (secured by PayPal) to support my hosting costs, it'd be very welcome. I'll donate 50% of your contributions to National Energy Action, the UK's leading fuel poverty charity, campaigning for warmer homes - it seems like an appropriate cause.

Disclaimer: Don't electrocute or incinerate yourself. This page is for information only. Landlords and employers are legally obliged to use a registered CH engineer for all gas work. I accept no responsibility for any accidents or problems.


Comment from: Peter Beighton
Peter Beighton

Were the capacitors marked “uF” rather than “mF"? It’s just that the symbol “m” is milli rather than “u” for micro. (Should really be a Greek “mu” symbol which is like a “u” but with a long tale at the left of the symbol). BTW, great instructions.

24/12/09 @ 12:41
Comment from:

@peter, no idea! I’m quoting verbatim what I was sent as I’ve never seen the PCB in question. mF does sound large though… Looking up the part number I was given I think it probably should be uF after all. I’ll change the original post.

24/12/09 @ 15:25
Comment from: Brian

The PCB shown here is from a Netaheat and is different to Profile PCBs they are NOT interchangeable, anyone know the Maplin part numbers for the Profile resisters ? Also check the 100 ohm resister as the go open circuit and cause fan hunting.

26/01/10 @ 09:06
Comment from:

@Brian, thanks for this. I hope it reinforces to readers that you can’t put a differently numbered PCB in a different boiler and hope it works!

26/01/10 @ 12:10
Comment from: IanJef

I found another common problem on these boilers is that the reducer (a brass plug in the plastic pipe that goes the the chassis) gets blocked.
After changing the PCB, the boiler would only ignite if I sucked on the pipe the goes from the front of the diaphragm to the reducer.
I removed the reducer and inspected it with a watchmakers eyeglass, I could see a tiny hole in it which was blocked.
So I cleaned the hole with a pin, it’s so small that only the tapered part of the pin would go through it.
When I reconnected it hey-presto it sprang into life.

Don’t listen to an engineer who recommends replacing this boiler, it’s rated very highly, several engineers I have spoken to say it’s the best boiler ever made.

Just a knowledge of the easily repairable faults can keep it going for over 30 years.

19/02/10 @ 12:41
Comment from: NickS

Replacing the electrolytics fixed the problem “fan runs and pilot light lit but no main burner” described by John for me. A few years ago I also had the “fan runs but no pilot light” issue due to the reducer being blocked - probably by the oil used as a lubricant to slip the plastic tube over the reducer.

I’d add that I found the3-way (gas valve) terminal block had dry joints and needed to be re-soldered.

23/02/12 @ 22:51
Comment from: CLIFF THOMPSON

WHOOPEE !! Now works like a dream !!

After hitting (tapping?) the base of the boiler control box on many occasions when it was really cold (boiler in garage) to get it going, I searched the ‘net and I found this blog.

I bought the electrolytics cited above before dismantling only to find there was a different style of control board with different electrolytics !!

Although my control board differs from that shown, I went through the same procedure of changing the electrolytics (different values from those recorded above) resoldering the -3way gas valve terminals (yes, they were dry - too much metal/heat-sink for the manufacturer’s flow solder machine I’ll warrant).

Thank you guys - the repair means I don’t get an elbow in the ribs at 5 o’clock in the morning ‘cos the boiler hasn’t lit !!!!

23/12/12 @ 09:54
Comment from: Mike Travis
Mike Travis

Have refurbed my 407677. Changed RLY`s, Caps, GDT etc Re-soldered all PCB joints. Perfect now.

14/09/17 @ 12:02