Wall oven control knob backlight repair

Forums › Home Workshop DIY › Wall oven control knob backlight repair

stuartmac

43 posts

Geek
+1 received by user: 16

Topic # 236351 29-May-2018 14:31

I have a double wall-oven, which has a small indicator light behind each control knob. The lights shine through the instrument panel so you can read the selected temperature.

The light for one of the knobs for the main oven has failed, which means neither knob for the main oven lights up. It is not a showstopper - if you shine a torch from the front you can make out the settings - but it is a pita.

The oven is from around 2000. Although there are some parts stocked, that particular part is no longer available (and I have tried several sources, both here and in Australia where it was manufactured). So the service agents have stated it irrepairable.

I have not completely disassembled to see if the faulty unit is repairable without replacement.

My thinking is to at least swap the light-units for the lower oven and main oven (because the lower oven is used less frequently). It would also be nice to (safely) bypass the faulty light so the other light unit in series works.

The wiring diagram shows two 2W lights in series with a 40W ballast, which suggests it is not simply a miniature incandescent.

Does anyone have any useful insights on this? I have uploaded the wiring diagram.

1 | 2 | 3

1746 posts

Uber Geek
+1 received by user: 356

Trusted

Reply # 2026137 31-May-2018 11:16

That's kinda bizarre - both lamps are in series, so I would have thought that loosing one lamp would make both lamps seem to fail.

I think that there is some additional circuitry in relation to the light circuitry module - considering it is defined as a module - meaning that there is more to the lamp than just the lamp.

And a 40W ballast? Wasting 40W on lighting a couple of bulbs ? Or they could be miniature fluorescent lights or similar.

I would suggest removing the module, and see if can be repaired. If one module goes, at some stage the others will too.

My thoughts are no longer my own and is probably representative of our media-controlled government

stuartmac

43 posts

Geek
+1 received by user: 16

Reply # 2026142 31-May-2018 11:36

Yeah, being in series both lamps do not currently light. I was hoping if I could not repair that I could at least bypass the faulty one, so one of the lamps was restored. I am cautious - it being 240V and I do not know why the ballast is necessary.

Looks like a job for the long weekend - disassembling the panel fully so I can see what type of light it is.

gregmcc

1604 posts

Uber Geek
+1 received by user: 333

Lifetime subscriber

Reply # 2026147 31-May-2018 11:53

It could be one of the lamps failed, which one?? it could also be the ballast or the switch.

If you bypass one of the lamps you will kill the other lamp, series driven lamps are quite common.

elpenguino

695 posts

Ultimate Geek
+1 received by user: 249

Reply # 2026162 31-May-2018 12:22

Please name the brand so others can avoid it. Anybody who can't or won't support an oven after 18 years doesn't deserve to be in business IMHO.

Re the light, remember everything is fixable but not always economically. Since the bulbs are in series you need to test to discover whether the bulbs or the ballast is the problem. Don't forget it could also be the function switch.

You could theoretically replace the existing lighting system with individual bulbs of some other technology but mounting the bulbs reliably will be your biggest problem.

stuartmac

43 posts

Geek
+1 received by user: 16

Reply # 2026213 31-May-2018 13:22

It is a Westinghouse, now owned by the Electrolux group.

In their defense other parts are available. And the service agent would be willing to fix the oven if a part was available. My interpretation is Westinghouse can no longer obtain that part from their supplier.

It just turns out to be a poor design - if they just had simple visible numbers on the faceplate instead of a black faceplate which requires backlighting the problem would never arise. Or a simple incandescent replaceable bulb.

Unfortunately I do not think you can weigh all those factors up at time of purchase of whiteware to be certain of no issues 18 years later.

stuartmac

43 posts

Geek
+1 received by user: 16

Reply # 2026218 31-May-2018 13:33

gregmcc:

It could be one of the lamps failed, which one?? it could also be the ballast or the switch.

If you bypass one of the lamps you will kill the other lamp, series driven lamps are quite common.

The lights are inside a white plastic shroud around each knob. I can see a black mark on one of the shrouds near where the wires enter, so I am sure it is a single burnt out light. But as in series, this takes out both.

I will swap the lower and main oven light units this weekend. That moves the problem to the less frequently used lower oven. If no repair is obvious, I will tell my wife she will just have to live with it.

robjg63

2750 posts

Uber Geek
+1 received by user: 548

Reply # 2026229 31-May-2018 13:55

stuartmac:

Unfortunately I do not think you can weigh all those factors up at time of purchase of whiteware to be certain of no issues 18 years later.

Agreed - In fact if most of the functional repair parts are available at 18years - that's pretty good.

Nothing is impossible for the man who doesn't have to do it himself - A. H. Weiler

stuartmac

43 posts

Geek
+1 received by user: 16

Reply # 2026230 31-May-2018 13:56

gregmcc:

...

If you bypass one of the lamps you will kill the other lamp, series driven lamps are quite common.

Could I measure the resistance across the good light unit with a multimeter, then bypass the bad one including a resistor?

gregmcc

1604 posts

Uber Geek
+1 received by user: 333

Lifetime subscriber

Reply # 2026267 31-May-2018 14:11

You would be better retrofitting a couple of leds, don't forget to remove the ballast

elpenguino

695 posts

Ultimate Geek
+1 received by user: 249

Reply # 2026272 31-May-2018 14:17

stuartmac:

gregmcc:

...

If you bypass one of the lamps you will kill the other lamp, series driven lamps are quite common.

Could I measure the resistance across the good light unit with a multimeter, then bypass the bad one including a resistor?

No, cos the resistance changes when the bulb heats up,

You'd need to measure the current of a good bulb when running, or read any markings which may help to discover the power/voltage/current.

elpenguino

695 posts

Ultimate Geek
+1 received by user: 249

Reply # 2026274 31-May-2018 14:21

robjg63:

stuartmac:

Unfortunately I do not think you can weigh all those factors up at time of purchase of whiteware to be certain of no issues 18 years later.

Agreed - In fact if most of the functional repair parts are available at 18years - that's pretty good.

I didn't want to sound harsh but it's the little things that count.

One of your design choices when bringing a product to market is to source components with multiple options for supply. They've failed in this case.

18 years should not be the age at which you throw out a built in appliance ( not saying the OP was). We shouldn't accept the throw away society foisted on us.

stuartmac

43 posts

Geek
+1 received by user: 16

Reply # 2026358 31-May-2018 16:32

elpenguino:

No, cos the resistance changes when the bulb heats up,

You'd need to measure the current of a good bulb when running, or read any markings which may help to discover the power/voltage/current.

The wiring diagram has them marked at 2W , So 8.3 mA @240V ?

kryptonjohn

2523 posts

Uber Geek
+1 received by user: 970

Lifetime subscriber

Reply # 2026414 31-May-2018 17:41

gregmcc: You would be better retrofitting a couple of leds, don't forget to remove the ballast

Need a high voltage diode and calculate a high wattage resistor for the 220V though!

SepticSceptic

1746 posts

Uber Geek
+1 received by user: 356

Trusted

Reply # 2026416 31-May-2018 17:44

stuartmac:

elpenguino:

No, cos the resistance changes when the bulb heats up,

You'd need to measure the current of a good bulb when running, or read any markings which may help to discover the power/voltage/current.

The wiring diagram has them marked at 2W , So 8.3 mA @240V ?

I'd be careful about making assumptions like that - the light "module" could have other components on it.

The ballast could be a capacitor / resistor combo to drop the voltage, but either end could still be a mains potential.

The 8.3mA would be in the range for using a capacitor voltage drop - this is quite common to supply low mA from a mains supply without needing a transformer.

From https://www.electroschematics.com/5678/capacitor-power-supply/

For incandescent lamps, one could connect across were the ~ symbols are.

My thoughts are no longer my own and is probably representative of our media-controlled government

stuartmac

43 posts

Geek
+1 received by user: 16

Reply # 2027839 3-Jun-2018 16:36

Disassembled the front panel. It is definitely a blown bulb in a light "unit". I switched a light unit between the lower and upper oven, so have successfully moved the problem from the upper over to the lower oven. So - as well as making it a less significant pita - that has isolated the problem to only a light "unit".

Had a close look at the failed light "unit". This is a picture of the faulty light "unit".

Each light "unit" only has two capsule glass bulbs in them. There is no other electronics in them. I measured the voltage from the ballast and it is 230V i.e. it is not a lowered voltage.

If I can find replacement bulbs it is an easy fix - just a matter of soldering in replacements.

However I could not make out any identifying information on the bulbs. Both are blackened (one very much so). Any insights as what they are would be appreciated.

I did not attempt to bypass the faulty unit in an attempt to get the other light unit in series running.

I am open to ideas for re-engineering but not sure it is worth the extra effort of creating a low voltage power source. I have reservations re the suitability of LEDs as well, given it is above an oven.

1 | 2 | 3