range hood insanity (mot microcontroller)
I have been trying to fix my malfunctioning range hood for quite a while. I just backed myself into a corner so it is time to consult the experts.
The range hood seems simple enough (fan and lights), but inside it has a circuit board to control the works. And not just some plain old ICs, but a flipping motorola mc68hc705p9! Yes that's right, this range hood has a system on a chip complete with embedded software!
Pushing either button activates the fan or light. Holding the button brightens or dims the light or fan in measured steps. The bar graph led indicates the fan speed. The 'system' remembers the previous settings when either the fan or light is turned off and back on. Way overkill for range hood, but I digress.
The problem is that after a while the lights and fan will start to 'flicker'. Eventually, the whole thing just shuts down. As if it 'rebooted', thus all outputs revert to off. It will come back on immediately with a button push, but the flickering starts right up again.
So in my attempts to fix the thing, I have changed out the caps, some resistors, the small ic (comparator I think), the triacs, optoislolators, etc. Nothing seemed to help. I thus have been narrowing in on the cpu. I also tried the heat gun and freeze spray to see if that would trigger the malfunction, to no avail.
To make this already long post shorter, I am now looking at the ceramic resonator (a three pin yellow ceramic thing, pointed to by the pen in the picture) as a possible cause. it connects to the OSC1 & OSC2 pins on the chip (the clock input to the chip according to the datasheet). The resonator has "4.00" printed on it, which I assume means 4mhz. Could a resonator fail intermittently and cause 'flickering' and 'rebooting' symptoms on a micro controller chip like this?
To make matters a little worse, I decided to try probing voltages on all of the pins on the chip. It was all going dandy, until I touched one of the pins connected to the resonator. Doing this immediately killed the board (the light & fan went off and will not come back on). So either I fried the resonator (a $0.50 item) completely or fired the micro controller (an essentially un-attainable item). Or both.
At this point all I can think to do is replace the resonator and see if that revives the board. Beyond that I am not sure what else to do. I would kick the whole think to the curb, but replacing the whole range hood seems like not much fun either. I appreciate any input that this board may offer.
I will close by expressing my hate for the folks that designed this thing. Although I am not an expert, my eyes tell me that this thing does not even have a proper power supply, just a couple big resistors and a few diodes and caps to produce +5vdc out of 120 a/c. I can't ever remember seeing a microprocessor based device getting its dc voltage in this manner.
The range hood seems simple enough (fan and lights), but inside it has a circuit board to control the works. And not just some plain old ICs, but a flipping motorola mc68hc705p9! Yes that's right, this range hood has a system on a chip complete with embedded software!
Pushing either button activates the fan or light. Holding the button brightens or dims the light or fan in measured steps. The bar graph led indicates the fan speed. The 'system' remembers the previous settings when either the fan or light is turned off and back on. Way overkill for range hood, but I digress.
The problem is that after a while the lights and fan will start to 'flicker'. Eventually, the whole thing just shuts down. As if it 'rebooted', thus all outputs revert to off. It will come back on immediately with a button push, but the flickering starts right up again.
So in my attempts to fix the thing, I have changed out the caps, some resistors, the small ic (comparator I think), the triacs, optoislolators, etc. Nothing seemed to help. I thus have been narrowing in on the cpu. I also tried the heat gun and freeze spray to see if that would trigger the malfunction, to no avail.
To make this already long post shorter, I am now looking at the ceramic resonator (a three pin yellow ceramic thing, pointed to by the pen in the picture) as a possible cause. it connects to the OSC1 & OSC2 pins on the chip (the clock input to the chip according to the datasheet). The resonator has "4.00" printed on it, which I assume means 4mhz. Could a resonator fail intermittently and cause 'flickering' and 'rebooting' symptoms on a micro controller chip like this?
To make matters a little worse, I decided to try probing voltages on all of the pins on the chip. It was all going dandy, until I touched one of the pins connected to the resonator. Doing this immediately killed the board (the light & fan went off and will not come back on). So either I fried the resonator (a $0.50 item) completely or fired the micro controller (an essentially un-attainable item). Or both.
At this point all I can think to do is replace the resonator and see if that revives the board. Beyond that I am not sure what else to do. I would kick the whole think to the curb, but replacing the whole range hood seems like not much fun either. I appreciate any input that this board may offer.
I will close by expressing my hate for the folks that designed this thing. Although I am not an expert, my eyes tell me that this thing does not even have a proper power supply, just a couple big resistors and a few diodes and caps to produce +5vdc out of 120 a/c. I can't ever remember seeing a microprocessor based device getting its dc voltage in this manner.
Attached Files
? Definately doesn't look professionally done the cooper traces would be real smooth and coated in acrylic. So when you solder a trace the solder doesn't run down the lines.
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