Dead Oven Control Board, Possibly Transformer?

[Bobby78]

Six long years ago, @tom66 helped me save my plasma TV here on BADCAPS, and it's still going strong. Fingers crossed for another win!

I have a Frigidaire oven, rarely used, that I noticed was dead at some point during the summer, possibly after one of the bad storms. The light still comes on when the door is opened, so the circuit has power, but no sign of life from the LCD or the button panel. The control board part number is 318010700 and does not seem to be available any longer from any supplier, but there are companies that will repair them for $150-$200. Obviously I'd like to do it myself.

I think the problem may be the transformer, but I'm just a random tinkerer, so not much confidence in my conclusion. The primary side looks toasty, but I'm not sure if that's just cosmetic? Anyway, I probed it and got 120V on the primary side, but only 0.1-0.2V on the secondary pins. I'm not accustomed to measuring such low AC voltage, the lowest range on my MM is 200 V, but I'm pretty sure the secondary is supposed to be around 10V because there are some remote 9V relays for the fan and oven light. The label on the transformer shows Robert Shaw PT# 10-84-00 and V&F PT# R14P20, but I'm having trouble finding a replacement or even the specs for it.

Any help would be greatly appreciated!

[petehall347]

https://www.vf-transformer.com/products/

[Bobby78]

Originally Posted by petehall347

https://www.vf-transformer.com/products/

Thanks, but I'm afraid a link to their website isn't all that helpful... A search for their own part number, on their own website, comes up with "Sorry, nothing found." If I've missed something, then I sincerely apologize; please provide further details if possible.

[stj]

remove it and check the resistance of the windings

[sam_sam_sam]

Here is what I would if I were doing this repair this is only a recommendation

I would recommend first removing the transformer and check the secondary winding side of transformer as mentioned earlier

If you have a bench power supply ( find the diodes that are on the secondary side of the power supply rail you are going to have use the pins that were for the transformer and you inject power into the pins on the board start at one volt and slowly turn the voltage up until your display power on )

You need to very careful do this procedure and watch for any and all devices around the the transformer secondary side pins for anything that is getting warm or hot do not ignore any issues

[petehall347]

Originally Posted by Bobby78

Thanks, but I'm afraid a link to their website isn't all that helpful... A search for their own part number, on their own website, comes up with "Sorry, nothing found." If I've missed something, then I sincerely apologize; please provide further details if possible.

are the contact details no use to you ? i would be emailing them for details and availability . you never know you might be lucky and get a free one .

[CapLeaker]

The connector on the red wires and F1 look black?

[goontron]

Originally Posted by CapLeaker

The connector on the red wires and F1 look black?

Well, i would expect it to look black when its PVC sleeved :P

[Bobby78]

Originally Posted by petehall347

are the contact details no use to you ? i would be emailing them for details and availability . you never know you might be lucky and get a free one .

Yeah, I suppose I should consider contacting them. I just figured if they were at all interested in providing support for the thing then they would at least have some basic specs on the website.

[R_J]

Check the resistance of the primary winding on the transformer If it is getting 120vac and there is no secondary voltage the primary winding could be open or there is a thermal fuse in the primary that is open.

[Bobby78]

Ok, transformer removed from circuit... I'm getting open circuit from pin 3 (line-in), to pin 6 which looks to be the other end of that winding. So the primary side has two windings (of different gauge) connected in series at pins 5-6? And the secondary looks to have two independent windings of different gauge, the lighter of which is center-tapped to ground at pin 12, do I have that right? I also tried to power the transformer out of circuit (after jumping pins 5 and 6 as they are on the board) and got nothing on the secondary side. Issue found, right?

As far as checking the board... with 5 pins on the secondary side of the transformer I don't have a clue which pin does what, other than 12 is connected to ground and I think 9/10 are twice the voltage of 11/13, right? I tried to start tracing the circuit but, unsurprisingly, it quickly led to me needing a stiff drink. I don't have a bench power supply, but I'm willing to MacGyver it with batteries and potentiometers or other power adapters if that's possible.

Warning, I mirrored the back of the PCB in the pic below, sorry if that causes confusion.

[Bobby78]

Hmm, I just saw that little square bulge in the primary winding... fuse?

[Bobby78]

In case anyone needs a good laugh, here was my pathetic start at trying to follow the circuit. Big Clive, I am not.

[budm]

Originally Posted by Bobby78

Hmm, I just saw that little square bulge in the primary winding... fuse?

That will be thermal fuse.

[Bobby78]

Progress is being made! Bypassing the thermal fuse gets me secondary-side readings of 9V across pins 11/12 and 12/13; 18V across pins 11/13; and 4V across 9/10. And that explains what I thought was two primary windings in series lol.

So at ~68 Ohms is the primary winding still good? I would think so since I got the 9V I was expecting?

If that's the case I can just replace the thermal fuse, yes?

[budm]

U26 2.5A 250V 169c.
https://www.aliexpress.com/i/32261363545.html

I am sure you can look around EBAY/MOUSER/DIGIKEY for it.
You can use KAPTON tape to wrap it back up.

[sam_sam_sam]

This is a great find to what your problem is but now the question is what got the transformer to get this hot to open the thermal fuse that is the real problem

So now that you know what the voltages of the winding I would check and see what amperage is on each winding and see if you have an issue on one of winding pulling to much current

One other possibles is that the the transformer itself is shorting out one way to check this is to power the transformer ( without a load ) and see if the transformer starts to heating up run it for an hour or so and note that temperature it should be room temperature or a little warmer than that but it should not be hot if it is the transformer has issues and should not be used

One note this is a thermal fuse / current limiting fuse

You need also check the primary side of the transformer to make sure that the device is not pulling more than two amps of current

I ran into this problem a few times in the past and once it was that lighting fried the transformer because it opened the thermal fuse so I bypass it to check the transformer after running it for 10 minutes ( with out a load ) the temperature got so hot that you could not touch it anymore I threw out the transformer and bought a new one

Funny thing was that the output voltage of the transformer was correct and it would power up a light bulb


One other note is this “ You can use KAPTON tape to wrap it back up.” this is very important thing to do if you find that you have NO issues with the transformer and are going to reuse it because if you do not do this the protection device will not work correctly and you will not be protected

One final note if you find your self temped to run this transformer with out the thermal fuse then at minimum put a 2.5 fast acting fuse instead of the thermal fuse but I would caution against doing this because of the fact that power is applied to it continually to the transformer and not very easy to get to the board where it is located at

I hope this helps you

[Bobby78]

Originally Posted by budm

U26 2.5A 250V 169c.
https://www.aliexpress.com/i/32261363545.html

I am sure you can look around EBAY/MOUSER/DIGIKEY for it.
You can use KAPTON tape to wrap it back up.

Thanks, but unfortunately that listing says no longer available. I've contacted the seller to see if they will be getting more in. I have been unable to find the part anywhere else either.

[Bobby78]

Struggling to find a replacement transformer or thermal fuse with the correct specs. I've searched Digikey, Mouser, Ebay, AliExpress, and whatever else I could find with Google. The AliExpress fuse listing was no longer available so I sent a message to the seller. I've also sent emails to V&F, Robert Shaw, and applianceboardrepair.com to see if any of them can help.

After 1.5 hours powered with no load, the transformer is just warm to the touch, about 100F. Is that ok?

I guess if the transformer is good and I can't find the exact thermal fuse then I could select one with the closest temp rating, but higher amp rating and then put a 2.5A fuse in series?

[R_J]

If you search for U26 2.5a you will find them, you can also use the cylinder type thermal fuse. I see these fuses open all the time and there is no problem with the transformer or motor they are installed in, after prolonged heat over time they just open.