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Same thing...if you cut one open you'll see it for yourself...ok its not a conventional off the shelf diode but it can only be arranged in the same way...embedded or not... As to the movement...I'll take your word on that one....never seen if for myself....but then with my eyesight..well lets...

Its four diodes in an epoxy resin mix....its not going to move about...but can break down like anything else prone to failure

It was a PBL405 bridge rectifier...

It was the fuse on the Fan PCB that kept blowing....not the main fuse...it was very frustrating as I thought it was the fan going overcurrent through wear and tear...but it was definitely the Bridge....I think one of the diodes in it was hovering around a short which taking out the fuse.....but...

That long..!!! You are more optimistic than me then !! :shock:

Spot on....it was the Fan PCB....and for those a little bit braver it was the bridge rectifier going short circuit under load...replaced and its a happy boiler again....well as happy as a 120L can get !!

Does anybody know what the switch and the two potentiometers do on the back of the fan control pcb on a potterton suprima 120L. I have such board where the 250mA fuse blows every 5 or 6 days, have replaced the fan and still it blows. I presume it is now either the transformer or the PCB or both...

Yes please Graham....just need your email address...I changed the electrodes on mine a couple of months ago and its been as good as gold....until now...so guess they have heated up and slightly moved out of critical alignment..!!!

I eagerly await your reply....I have searched for months on this problem too....I have this boiler in my house supplying HW/CH and it does exactly the same as yours....I have had a potterton engineer run the same tests..replaced the PCB but still no joy......I have recently replaced the probes...