Stoves 900EF trips mains when cooling fan stops

Hi

we have an approximately 20 year old Stoves 900EF dual oven. The other day, a little while after successfully cooking something, the mains tripped. The cooker has it’s own breaker but it was the main RCD that tripped (earth leakage as opposed to over current?). It wouldn’t reset without isolating the oven.

Today I’ve come to look at it and I could provide power to the oven without it tripping the RCD. HOWEVER, after heating up the oven for a minute or so and turning it off again, 15 minutes later the mains tripped. Again, I had to isolate the oven before I could get the mains back on. But some time later (not sure how long) I could power up the oven again.

So, I disconnected the cooling fan (the one sucking air in through the back wall) and tried the test again – heating the oven for a short time and then turning it off. This time it did NOT cause the mains to trip. So I tried the same again and again no trip. I re-connected the cooling fan and that made it trip again.

And just to confirm, the cooling fan itself seems to work fine until the mains goes. That is – when power is applied the cooling fan spins up for a second or so, and it is working at what seems like the right speed all while the oven is heating and for the 15 minutes afterwards.

I have looked at the circuit board that looks like it’s controlling the cooling fan – the one at the top of the oven, marked APCO Model No. D9751. There are no visible signs of damage

I’d be grateful if someone could suggest an explanation and possibly a way to fix it

thanks