Dead Oven Control Board, Possibly Transformer?

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

I drew a partial schematic and it all makes much more sense.

This Frigidaire Oven Controller 318010700 oven control board transformer is a custom part, because the vacuum fluorescent display needs that additional winding - the filament winding.

I would say estimate it is around 14VAC center-tapped as the board uses less common 9VDC relays. OP measures 18VAC but at no (relay) load, and 4VAC with the VFD connected?
In a pinch I would use an off the shelf 14VCT transformer and a second (extra) transformer 6.3V and add two resistors to get the filament voltage down to what it should be.

It's great the transformer survived. I read another forum that this is a common failure mode - any power line surge causes these oven boards to collapse and then cook the transformer. I'm still puzzled about what causes the transformer to overheat. There is an additional primary fuse F1 so I would expect that have protected the transformer but the design fails here F1 should only be around 1/2A max. it must be way over that.

I would say if you can't get a replacement thermal fuse, it is acceptable to add a (proper) normal fuse on the transformer's primary for F1. I use around 1/4A to 1/2A slow blow.

Nice, thanks for the analysis. If I continue to have issues with the thermal fuse, I will consider replacing F1 with a 0.25-0.5A slow-blow fuse and see if that spares the thermal fuse. Hopefully I won't have to rig up a two transformer system.

[petehall347]

Re: Dead Oven Control Board, Possibly Transformer?

dont forget those fuses can easily be damaged when soldering .

[sam_sam_sam]

Re: Dead Oven Control Board, Possibly Transformer?

It doesn't say either

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

I did not realize that you were buying thermo fuses that will work it just will not let the transformer to get as hot before it cuts out the power to the transformer which is probably a good thing

[redwire]

Re: Dead Oven Control Board, Possibly Transformer?

I drew a partial schematic and it all makes much more sense.

This Frigidaire Oven Controller 318010700 oven control board transformer is a custom part, because the vacuum fluorescent display needs that additional winding - the filament winding.

I would say estimate it is around 14VAC center-tapped as the board uses less common 9VDC relays. OP measures 18VAC but at no (relay) load, and 4VAC with the VFD connected?
In a pinch I would use an off the shelf 14VCT transformer and a second (extra) transformer 6.3V and add two resistors to get the filament voltage down to what it should be.

It's great the transformer survived. I read another forum that this is a common failure mode - any power line surge causes these oven boards to collapse and then cook the transformer. I'm still puzzled about what causes the transformer to overheat. There is an additional primary fuse F1 so I would expect that have protected the transformer but the design fails here F1 should only be around 1/2A max. it must be way over that.

I would say if you can't get a replacement thermal fuse, it is acceptable to add a (proper) normal fuse on the transformer's primary for F1. I use around 1/4A to 1/2A slow blow.

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

I hope you got fast acting fuse not time delay fuses they are very different

The amp rating is more important but also weather or not it is fast acting or a time delay type of fuse

It doesn't say either

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

[sam_sam_sam]

Re: Dead Oven Control Board, Possibly Transformer?

I hope you got fast acting fuse not time delay fuses they are very different

The amp rating is more important but also weather or not it is fast acting or a time delay type of fuse

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

So neither of the manufacturers have responded. Applianceboardrepair.com responded, but were of no help, only sending me links to irrelevant search results on mouser. Their last response was that they did not have a transformer, but if I sent the board in they would "patch it" for me. Not sure what they would "patch" it with considering the useless search results they sent me. Oh well.

Anyway, I couldn't find an exact match for the fuse and didn't want to wait on AliExpress, so I ended up buying some 2.5A 126C fuses from Amazon for $10. I figured the amp rating was more important than the temp limit. Fingers crossed.

[sam_sam_sam]

Re: Dead Oven Control Board, Possibly Transformer?

Here is a link that has 150*c

https://a.aliexpress.com/_mtc5gC5

[stj]

Re: Dead Oven Control Board, Possibly Transformer?

search ali for 169' thermal fuses - if your too specific the seach falls over

[R_J]

Re: Dead Oven Control Board, Possibly Transformer?

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.

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

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?

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

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.

[sam_sam_sam]

Re: Dead Oven Control Board, Possibly Transformer?

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

[budm]

Re: Dead Oven Control Board, Possibly Transformer?

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.

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

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]

Re: Dead Oven Control Board, Possibly Transformer?

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

That will be thermal fuse.

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

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

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

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

[Bobby78]

Re: Dead Oven Control Board, Possibly Transformer?

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.

[R_J]

Re: Dead Oven Control Board, Possibly Transformer?

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.