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-   -   Stove appeared to have bad circuit board (http://www.houserepairtalk.com/f11/stove-appeared-have-bad-circuit-board-14858/)

jjmartin1340 10-06-2012 09:23 AM

Stove appeared to have bad circuit board
 
About a year ago my stove would fail while baking, the oven would be off and the clock would be 12:xx, as if there was a power bump xx minutes ago. Since we had not had a power bump, I opened the stove up to check the logic board. No loose connectors or anything obvious, so I checked out the price of a new one. The local shop wanted $300, so I checked the internet. Found one place at $200 and one at $100. The stove stopped failing, so I didn't buy one. Glad I didn't. Recently it started failing again, but now the clock was completely out. Fiddled with the stove and found a) the 115v outlet on top still worked OK; b) if I turned a burner on it wouldn't heat, but the clock would light up for a second. Experience (I'm a retired computer/electronics tech) told me this indicated I was losing one side of the 240v input. Could be the electrical panel circuit breaker, wall outlet, or power cord. A voltmeter showed I had 240v at the outlet, so the circuit breaker was OK. Went out and bought a wall outlet $5 and power cord $25. When I removed the old wall outlet the problem was obvious: in three holes (ground, neutral, one power) I could see two heavy contacts pressing together. The other power hole had no visible contacts, either they had lost spring tension or broken. Installed the new outlet and all OK. Returned the unused power cord.

I may not be able to troubleshoot nearly as fast as I used to, but eventually I get there.

jeff1 10-06-2012 09:59 PM

Hi,

Quote:

but now the clock was completely out. if I turned a burner on it wouldn't heat, but the clock would light up for a second.
Feed back power...good catch!

jeff.

Wuzzat? 10-07-2012 02:20 PM

So your burner was running at half voltage and 1/4th of full power?

Why didn't the clock stay lit?

All's well that ends well.

jeff1 10-08-2012 07:32 AM

Quote:

So your burner was running at half voltage and 1/4th of full power?
With one of the two lines gone the 220 volt elements would normally be dead.

Quote:

Why didn't the clock stay lit?
EG: Red power line may be for the clock, but the red line is gone ( for what ever reason ) but when something was turned on/closed ( surface switch ) the power from the black line would feed over to power the clock. Once the switch was shut off ( open ) the clock would go out again.

Quote:

All's well that ends well.
Yup :)

jeff.

jjmartin1340 10-08-2012 12:26 PM

3 Attachment(s)
Why did the clock come on when a burner was turned on and then fade out? Because this stove has electronic continuously variable burner controls, instead of old-style mechanical switches. When this happened to me 30 years ago, the clock (mechanical) would run as long as a burner was turned on. With the electronic switch and clock, it’s a bit surprising it came on at all.
See the schematics below.
Let’s assume an instant in time when the R wire has +120v on it and the B wire has -120v. Now the kettle has 120v (R to W) and the clock has 120v (B to W), and the burner has 240v (R to B).
If the B wire breaks, there is still 120v to the kettle (R-W) but nothing to the burner or clock and they don’t work.
Now switch on a burner. There is a low impedance path from R to B. Current will flow from R through the burner to B then through the clock to W (red dotted line). There may be 5v across the burner and 115v across the clock, but that’s ok for the clock.
In this stove, the electronic controls for the burners are on the logic board with the electronic digital clock, so I am somewhat surprised it worked even for a second, but I’m glad it did.

Wuzzat? 10-08-2012 03:29 PM

Quote:

Originally Posted by jjmartin1340 (Post 78039)
See the schematics below.

You da' man!
I used to post schematics using Photobucket but I've gotten lazy. :(

And one time I used dropping resistors to run a 120v analog clock on 240v. That was when I found out these types of clocks are not so much a resistive as inductive load.
I wanted to find out how many hours overnight my elec. water heater ran (five minutes out of five hours for a 4500w element) so I'd know how much energy I was losing through the insulating jacket.
After I added external insulation it was 5 minutes/7 hours.
I think now I could have run the clock between the neutral and one hot lead.


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