Washing machine Motor Control Unit repair

[Adler]

Not your usual repair...

So I've got a Maytag Maxima clothes washer and it's got some problems, long story short I've narrowed it down to the units MCU (Motor Control Unit). It looks like they used traces on the board as fusible links and two of those links have vaporized. The question is, can I just install a jumper here or will I create a fire hazard?

[thm_w]

There will be upstream fusing to protect from major burning. I would investigate that power device on the right, to make sure its not failed short or driver has failed.
Then put your jumper links in, something of similar size (you could calculate the cross sectional area but I wouldn't bother).

Those solder joints look terrible from this side of the board.

[TheMG]

Before you do anything else, check for shorts on the IGBTs. Blown traces and/or fuses in these things usually indicate one or more of the IGBTs has decided to pack it in.

[JFJ]

The question is, can I just install a jumper here or will I create a fire hazard?

After correcting the cause of the problem, a better option would be to bridge each blown trace with a surface mount fuse:
http://www.littelfuse.com/products/fuses/surface-mount-fuses.aspx

[Adler]

There will be upstream fusing to protect from major burning. I would investigate that power device on the right, to make sure its not failed short or driver has failed.
Then put your jumper links in, something of similar size (you could calculate the cross sectional area but I wouldn't bother).

Those solder joints look terrible from this side of the board.

I think the power device is a bridge rectifier, is it normal to feed live into both AC pins of a bridge rectifier? I don't think I've seen that before... I will check to make sure its not shorted.

Before you do anything else, check for shorts on the IGBTs. Blown traces and/or fuses in these things usually indicate one or more of the IGBTs has decided to pack it in.

I don't see any IGBTs on the board unless you mean this large chip with 21 pins, it looks rather unrelated to the location of the short.

The question is, can I just install a jumper here or will I create a fire hazard?

After correcting the cause of the problem, a better option would be to bridge each blown trace with a surface mount fuse:
http://www.littelfuse.com/products/fuses/surface-mount-fuses.aspx

That's a great idea, any idea how to choose the rating for the fuse? The burnt traces are about 1mm x 5mm.

I was thinking the problem must have happened during a power surge or some other powerline issue.

[Adler]

Got around to testing the bridge rectifier and there is a short between AC and -. So that's definitely the problem. Now I need to see if I can find a comparable bridge rectifier in my parts bin and maybe some smd fuses or just some very thin bridge wires.

[Adler]

I am both proud and embarrassed by my repair job... I used some pretty thin wire that will hopefully protect the circuit somewhat. I found a matching bridge rectifier in my parts bin but the leads were too short so I did some thinks I'm not too proud of (just kidding, I'm proud). Stay tuned to find out if it works.

[Gyro]

You think you've got problems with fusible traces! 

Oops: https://www.eevblog.com/forum/repair/induction-hob-power-board-repair/

[mich41]

Don't even think about powering it up.

The current which vaporized those traces and rectifier had to go somewhere. Unless there was a short caused by some foreign body, this somewhere almost certainly was "through the motor driver IC and the motor itself". Which means that the motor driver (this 21 pin heatsinked IC) may be busted as well. The motor driver is then controlled by some MCU running proprietary firmware and if high voltage gets through failed motor driver to this MCU you can kiss your board goodbye.

Put red DMM probe on rectifier + and make sure that there is continuity to only one pin of the driver IC. Ditto for black on -.

And as you are there, check if this IC really is the motor driver. It should have a pair of pins connected to HV supply, 3 pins connected to motor phases, 3 pins connected to + of small capacitors whose - is connected to motor phases, 3 pins connected to some MCU somewhere. If not, I might be wrong.

[JFJ]

... there is a short between AC and - ...

Are both AC connections shorted to -ve?

If only one is shorted then you may have found the cause of the problem. A single diode breaking down could have allowed one of the big capacitors to discharge through the fusible links. The unusual configuration of the bridge rectifier's diodes, into two parallel pairs, makes it likely that a large fault current would have destroyed both diodes in a pair (unless the one that failed first protected its partner, by carrying all of the fault current until the links fused).

... any idea how to choose the rating for the fuse?

You might determine a value for the fuse from the motor's power rating and its supply voltage. For example, if the motor is under 1 horse power (750W), a 3 Amp fuse should be suitable for use with a 230V supply, or a 6 Amp fuse with a 120V supply. The fuses would need to be 'Slo-Blo' (timed), because of the likely high inrush current during power up.

[Adler]

Update!

I tested all I could with all the advice provided and installed the board back in the machine. It's working fine and all is well with the world.

For anyone searching the internet with a similar problem. the machine is a Maytag Maxima "mhw9000yg" and instead of buying a $400 board you may be able to replace a $1 bridge rectifier.

... there is a short between AC and - ...

Are both AC connections shorted to -ve?

If only one is shorted then you may have found the cause of the problem. A single diode breaking down could have allowed one of the big capacitors to discharge through the fusible links. The unusual configuration of the bridge rectifier's diodes, into two parallel pairs, makes it likely that a large fault current would have destroyed both diodes in a pair (unless the one that failed first protected its partner, by carrying all of the fault current until the links fused).

I tested the failed rectifier out of the circuit and only one diode seemed to be blown.

... any idea how to choose the rating for the fuse?

You might determine a value for the fuse from the motor's power rating and its supply voltage. For example, if the motor is under 1 horse power (750W), a 3 Amp fuse should be suitable for use with a 230V supply, or a 6 Amp fuse with a 120V supply. The fuses would need to be 'Slo-Blo' (timed), because of the likely high inrush current during power up.

I sure hope my thin wires act as some kind of fuse if this ever happens again. Maybe I'm being irresponsible.

[TheMG]

Interesting. In my experience it's actually extremely rare for those bridge rectifiers to fail unless something else shorted and pulled excessive current through the bridge rectifier.

Consider yourself lucky!

[Adler]

Interesting. In my experience it's actually extremely rare for those bridge rectifiers to fail unless something else shorted and pulled excessive current through the bridge rectifier.

Consider yourself lucky!

Maybe Maytag (Whirlpool) bought a special batch of rectifiers designed to fail after 3 years so they could offer a 3 year warranty. Surprisingly plausible these days...

[Jeff5may]

Sorry for resurrecting an old thread, but I need help with one of these.

Same type of washer, same board, different failure. With my motor control, the fusible link traces near the mov had vaporized. Also, one of the 3 components leading from ground to the motor control has vaporized. Further troubleshoot revealed a shorted phase in the control chip, a low side transistor shorted source to ground.

I found the datasheet for the chip, and it looks like the component is a current sense shunt for a phase of the motor. They're pictured previously and are little rectangles with a Chevron in the middle.

My burning question is what are these components? I did a resistance check and the two that are good measure about 3k ohms and diode check both ways about 1.4 volts with my fluke meter.

[Hectrik]

I know this is an old thread but its all that comes up for actually repairing a MCB, and it answers your question. Its a R050 .05 ohms ThickFilm SMD resistor. Its 3/4 watts. I just ordered from digikey (Ohmite LVK20R050DER). Took me a while to find out why those "Chevron" resistors are green. Im sure the driver chip is fried but I was willing to gamble $10.

Again, sorry for resurrecting an old thread to answer a different question LOL.

[richy]

i have the exact same problem. Cant figure out which bridge rectifier i need to use, witch did you use?

[sandmon]

Hi I have the same board and was hoping someone can tell me what the resistor value is in the picture I attached.  This picture was posted in this thread above but it cut off the resistor that I am questioning. I have a blown resistor but can't read that value.

Thanks.

[amyk]

An old direct-drive washer with an induction motor and mechanical timer won't fail like this. They last for decades and can be easily repaired.

[sandmon]

I ended up ordering a new board and just wanted to share that that resistor is a 1R00 in case anyone needs this in the future. Attached is an image of my new board.