Newbie, so thanks in advance...

Problem: One of these: Controller

From a 30 amp night storage Aga, replacement costs £950 or £250 for repair.

Purpose: to control elements that heat the brick core of a night storage Aga to approx 700 degrees on cheap economy 7 power, air is then circulated around the oven to maintain oven temperature for cooking. Main board takes input from core thermocouple and oven thermocouple to power the 3 ganged relays for the heater elements.

Symptom: board powers up, energises relays and then turns them off again, does this rapidly until fuse blows, relay board fails or essex thermostat trips out or all of the above!

Have replaced controller with a second hand one off eBay (£320) and Aga now works, but I would like to fix the original to keep as a spare (spent too much on the aga already).

Looking at images of other controllers, and the replacement unit I can see that the Mosfet MAR839 has been replaced (centre top of board with heatsink).

So far so good, but has anyone any experience of fixing these boards and is there likely to be other components that cascade fail with this one?

Many thanks to all in advance...

 

... Check to see that Diode D1 on the relay board is making good contact at both ends ... The banded end (cathode) is connected to + voltage ( red wire ) for correct operation.
Possible cold solder connection ... reheat/remelt solder joints to insure good conductivity.... A little more solder helps sometimes.
... If that doesn't accomplish anything, the next likely possibility is something related to the mosfet gate voltage. Usually, there is a minimum gate to source voltage required for the mosfet to switch on.
... Can you measure the gate to source voltage on the mosfet?... seems like it ought to be about 5 VDC for the ON state, as a guess.

 

Thanks for the reply DRC, the controller is sitting on my bench and D1 is OK and well soldered in with no abnormal signs on the board, all solder joints looking sound and no damaged tracks.

Difficult to check gate voltage as the board is on my bench. On another forum, there is a suggestion that the main controller gets confused with one signal saying "call for heat" and another saying "too hot" causing the rapid switching of the relays. Not the thermocouples themselves, but the associated circuitry, of which the Mosfet is part.

I will try and source the Mosfet and see if that alone fixes the problem that lots of people seem to have.

I doubt the company that does the repairs would be willing to divulge the problem components!

Thanks again, Dave

 

... Sounds like it might be related to something called hysteresis ... sort of like turning on at a specific temp, but turning off at a different temp, if properly designed. This factor is adjusted by different resistors ... usually requiring a schematic to fix.
A comparator goes high if a reference voltage is exceeded, and turns off as the temp decreases ... that's where the adjusting resistors come into play.
... Maybe the new mosfet will do it ...

 

Thanks for the responses, on closer inspection it appears that what I thought was a MosFet is actually a voltage regulator here and that makes more sense..
L7812ACV which I had to take a photo of and enlarge, I guess MAR839 is a manufactures code...

Can't believe that charge £250 to replace it!

Dave

 

... If it's possible, take a photo of the main board, so that you can read the numbers on top of the integrated circuit chips.

 

Thanks for the responses, on closer inspection it appears that what I thought was a MosFet is actually a voltage regulator here and that makes more sense..
L7812ACV which I had to take a photo of and enlarge, I guess MAR839 is a manufactures code...

Can't believe that charge £250 to replace it!

Dave

Hi Dave, did you get it working in the end I just ordered a new L7812ACV - I’ve no real soldering experience but it’s definitely worth a go new board is crazy money and repair if it’s just that is crazy too. DIY - and a few YouTube videos on soldering - no worse off than I am already.