Re: Darned capacitors again!


 

I have a similar tale to share about an oven controller. We've had this GE wall oven for about 10 years now. Several years ago, the temperature control started acting up, being erratic and complaining about the sense element in the oven. I went the the parts place and got a replacement for around 70 bucks. About a year later, the oven started doing the same thing, only this time a new sense element didn't fix it. The oven would start complaining about a sensor disconnect, even when the oven was not in use.

I priced a new control module at over $375. GE subcontracted this part to RobertShaw, a controls company. Being unwilling to pay what I refer to as 'appliance repair extortion money' I got the controller on the bench and started looking at it. I was able to power up the unit on the bench and reproduce the failure. Lacking a schematic, I was not able to pinpoint where the problem was.

I then decided to inspect the PCB for a cold solder joint Using a jeweler's loupe and a probe, I started pushing on components while looking at both sides of the PCB. I happened to touch the base of a small electrolytic that left a stringy, green gel on the end of my probe. This was a dime part at most. Tracing out the circuit revealed that this particular cap was responsible for decoupling the sensing bridge circuit, of which the oven sensor was a part. The cap was gradually failing, leaking, shifting the reference voltage and generating noise in the sensor circuitry. Previously I had to bias the temperature range about 40 degrees high, as the oven was too cold (the controller firmware had a feature for doing this). I replaced the offending cap (which was a 4.7uF 16V 105 deg C unit) with a junkbox part I had laying around. All the problems vanished, the oven now calibrates nominally with no bias offset necessary.

This episode reminded me of the subtle ways that electrolytic caps can cause problems that are not obvious, particularly the noise. If erratic behavior in an electronic device is a problem, there's a good chance that an electrolytic is at the bottom of it.

-Dan

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