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Old 01-09-2010, 10:58 PM   #1
bluto
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Angry 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.
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Last edited by bluto; 01-09-2010 at 11:03 PM..
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Old 01-09-2010, 11:07 PM   #2
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Default Re: range hood insanity (mot microcontroller)

As long as you didnt short anything, the MCU is probably OK. The resonator may be failing as you suggested.

Also, carefully remove and reseat the MCU as it appears to eb socketed....a poor connection here could cause all sorts of maladies.

Its still possible the circuit may be failing when warm....put the freeze spray away and just monitor the +5 rail to the MCU....see if it fluctuates when it dies. (or since its dead...is it even there?)
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Old 01-09-2010, 11:18 PM   #3
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Default Re: range hood insanity (mot microcontroller)

Thanks for the reply. I was in the process of monitoring the +5v rail when I screwed it up. Had the unit rigged up on the bench and I had the my meter set up to monitor min/max voltage on the +5v rail. I let it sit overnight and no malfunction was apparent. The +5 voltage fluctuated less than 1/10 of a volt the whole time. Using the heat gun also produced voltage fluctuations of the same magnitude. But again I was unable to reproduce the malfunction on the bench.

After I got done probing the chip (and subsequently stopping it from working), the +5v rail was still unchanged. Also when I did the fatal probe, there was no pop, crackle or magic smoke released. So hopefully, the MCU is ok as you suggest.
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Old 01-09-2010, 11:37 PM   #4
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Default Re: range hood insanity (mot microcontroller)

Seems I was wrong about it being dead. I just went down and messed with it some more and it came back to life. Apparently a cooling off period was in order. But a great relief that it still works. I then removed the MCU and sanded the pins very lightly. Put it back in and it came right up again. I set the meter up to monitor the +5v rail again and will leave it running over night.

I suppose if I am unable to re-create the problem on the bench I should put it back in the hood with some leads coming out to monitor the +5v rail.

Would re-flowing all the solder joints be a good idea? Just to eliminate the possibility of a cold joint? It would not take that long on such a small PCB.
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Old 01-10-2010, 12:45 AM   #5
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Default Re: range hood insanity (mot microcontroller)

why not, just don't fry anything

jeez, you seem to be doing a lot of kitchen electronics. first a jenn-air wall oven, now this, what's next, a fridge with a bad lcd tv?
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Old 01-10-2010, 03:24 AM   #6
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Default Re: range hood insanity (mot microcontroller)

Rework the joints...especially on the timing resonator!

Also, check that big resistor. Replacing the socket may help if the resonator is OK and resoldering doesn't help; some are of mediocre quality.
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Old 01-10-2010, 12:23 PM   #7
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Default Re: range hood insanity (mot microcontroller)

Who etched that board?

It looks like something I could do at home with a bottle of etchant.

Did an employee at Broan do these in his basement ? 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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Old 01-10-2010, 12:23 PM   #8
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Default Re: range hood insanity (mot microcontroller)

Went ahead and re-flowed all the joints. Put it all back together with some leads to measure the +5v rail while it was running. The minimum observed was 5.413, the max was 5.470, and the average was 5.450. After a couple of hours the flickering came back. One observation was the if just the light was on and flickering, turning the fan on would 'quiet' the flickering a little bit.

I suppose that a new socket and resonator are about all that is left to change out. The big resistor as seen in the pic was my replacement (the original had a little bit of a burn mark on it).

Does anyone have any idea if it is possible to copy the code off of this MCU in order to burn it to a new one? Although finding a new MCU of this specific part number would probably be a challenge in itself.

Yes the kitchen appliances have been giving me fits lately. I also put a new impeller in the dishwasher, but that didn't meet the criteria for documenting it on this forum! I have done a few motherboard and lcd recaps, but none of those were unique situations that have not already been documented here by others.
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Old 01-10-2010, 12:27 PM   #9
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Default Re: range hood insanity (mot microcontroller)

The resonator seems like your last hope. Whenever a microcontroller seems wonky beyond reason the clock is the first thing to check.

I don't understand why they didn't just use a quartz crystal? Cost cutting again I guess.

Probably if you had a scope and scoped out the clock when it flickered you would be able to view a frequency instability if it was at fault.

As for copying the micro not only would you need the IC you would need the programmer if it can be found. Also there is no real way to convert machine language in the micro back to source code. Which traditionally I believe is what you need to write to the micro.

I'm not sure about copying directly though never messed with micros too much.

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Old 01-10-2010, 12:42 PM   #10
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Default Re: range hood insanity (mot microcontroller)

How could you tell that the board was un-coated just by looking at the top of it? It is in fact un-coated on the bottom. Is the lack of slik-screen on the top a giveaway? Now that you mention it, I could probably do a better job with a RatShack PCB etching kit.

Where would I connect the scope? Either of outer two resonator pins, I assume? I have an old scope meter if I can figure out how to use it. Would it need to be grounded to the ground pin on the MCU also?

I concede that it would be difficult to program a replacement. There is a schematic in the MCU's datasheet that details the programming circuit. The target code is put on an external eeprom and copied onto the MCU. I just wonder if it is even possible to pull the code off before I go try to hunt someone down with a programmer. Otherwise I have to find the guy that makes these in his basement and get the code from him
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Old 01-10-2010, 01:03 PM   #11
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Default Re: range hood insanity (mot microcontroller)

code is probably in one of the smaller chips on that pcb. probably the one near the mcu.
some googling should reveal it (type of flash etc.)...
if it's damaged you would need to find working one to transplant code from.

ponyprog is simple....
http://www.lancos.com/prog.html
programmers are cheap.

offcourse, that won't work if mcu is fried...
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Old 01-10-2010, 01:06 PM   #12
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Default Re: range hood insanity (mot microcontroller)

>How could you tell that the board was un-coated just by looking at the top of it?

he has x-ray eyes.
<wink>

but those fets(or transistors) and their cooler do look kinda shoddy...
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Old 01-10-2010, 01:16 PM   #13
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Default Re: range hood insanity (mot microcontroller)

They might be triacs.

Does the control panel lock up when the flickering begins? If not, the triacs may be at fault and the MCU may be good.
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Old 01-10-2010, 01:37 PM   #14
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Default Re: range hood insanity (mot microcontroller)

Hardwareguy:

He said he already replaced the triacs.

I made an educated guess the cheap light green see through plastic and no silkscreen is a dead giveaway to lowest bidder production. Plus I have etched a few Radioshack PCBs in my day and that one looks alarmingly like it .

I always laquered the bottoms though.

Anyhow just probe the crystals leads and power it up.

You should see a sine wave if your scope has the bandwidth to handle the crystals frequency. The 4.00 on the crystal could be 4 KHz or 4 MHz.

Here is a guide on 3 ways to test crystals:

http://www.electronicrepairguide.com...t-crystal.html

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Old 01-10-2010, 01:49 PM   #15
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Default Re: range hood insanity (mot microcontroller)

You want to connect to the OSC in and OUT pins on the resonator itself not the third pin that connects to ground. Just trace those two pins to the micro to find them.

If it's anything like checking a crystal that's what I do.

If you don't get a waveform go from OSC 1 to ground and OSC 2 to ground. Never had to check a resonator before only quartz crystals.

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Old 01-10-2010, 02:17 PM   #16
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Default Re: range hood insanity (mot microcontroller)

The other IC is an MC33161P which according to its datasheet is a "universal voltage monitor" with "two comparator channels". I think it uses this as a comparator to produce the output to fire the triacs at the appropriate time to create the dimming curve for the lights and fan.

I tried the scope meter and was able to see something that resembled a sine wave. It seemed like it could be unstable. But for some reason when i touched the probe to the device, the MCU would 'lock up' within a couple seconds. It required the power being removed for a few minutes to come back to life again. So I am now set to replace the resonator and see what happens. Otherwise I may have to buy a replacement control module for much more than I want to pay.
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Old 01-10-2010, 05:35 PM   #17
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Default Re: range hood insanity (mot microcontroller)

or you could add switches and a pot and hardwire the dumb thing. cheap and more reliable.
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Old 01-10-2010, 06:02 PM   #18
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Default Re: range hood insanity (mot microcontroller)

yes, some things are not worth the repair time esp. if their design has a big "OVERKILL" written all over them...heh...

mcu to do what exactly, turn on few lights and spin the fan?
umh...
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Old 01-10-2010, 07:20 PM   #19
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Default Re: range hood insanity (mot microcontroller)

I guess they used the micro due to that bar graph display.

They needed it to control both the light and the fan on one display. Plus retain a memory of the last known setting. The micro cut the part count down and simplified the design.

Without that bar graph the same thing could have been accomplished with a few extra transformer secondaries and a high / med / low switch if the fan had a universal motor.

But I guess they were trying to be advanced for 1995 .

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Old 01-10-2010, 08:25 PM   #20
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Default Re: range hood insanity (mot microcontroller)

maybe 2 pots with integrated on/off switches? then you add knobs, and sharpie the power range in relation to the pointer on the knobs.
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