Washing machine repair: a terrible pain in the diodes

[Nick Leverton]

The main motor on our washing machine is no longer working - neither on wash nor spin cycles. It runs through the programmes OK apart from that, so I suspect the motor controller.

Machine is a Bosch WAE28467UK/14. On looking at the power side of the controller board, there is what I think is a diode with suspicious carbonisation around one end:

is more carbon on the board off to the right of the pic, which has left a clear shadow around the legs of the mini-transformer making it fairly clear that the source is that component with the blackened end).

I'm assuming it's a diode cos I don't know what else it looks like. Electrically it seems odd though (I'm no electronics engineer I have to admit). I only have a cheap ten-quid digital multimeter, which starts off showing a middling resistance (100 ohm or so) but rises within a second or so to open circuit - same whichever way round I test it. The behaviour is more like a capacitor, but I've never seen one like that. Both the diode-like thingys visible in the pic behave the same.

The mini transformer seems OK I think, both primary and 2ndary coils have a small but non-zero DC resistance. The bigger caps just visible look OK, no bulging or distortion. The other visible diode also looks OK, as do the two resistors and the power transistor, which is about all there is on this side of the board. Nothing on the other, surface mount logic, side of the board is discoloured.

So - am I right this is a diode ? If it's blown, why does it show the same resistance characteristics as the apparently good one ? What type am I likely to need to replace it with ? Any help would be appreciated, as I don't want to have to pay 60 to 100 quid for a new board if it's only got a pain in the diodes down its left side ...

Thanks

Nick

[Graham.]

You can't use the resistance range of a digital multimeter to measure a good diode or transistor (but it might prove you have a faulty one) Even a cheap meter should have a diode test range. The readout is in milivolts, not ohms if good a diode will rad about 700mV or 0.7V in one direction and much higher or unreadable in the other. Your pic isn't quite clear enough but it looks knackered and will probably fall apart if you unsolder it, so read all the markings first so we can find a replacement. The diode might not be the only casualty though.

[Lee]

What he said, but if that is a "fast" diode check that cap near it as well.

Lee

[BluntChisel]

Apologies for asking the obvious, but have you checked the brushes on the motor? I failed to check the brushes on a Hotpoint with a non-functioning motor once.

[Brian Gaff]

Often though the soldering or the copper track itslef does get warm when its quite old and may well be a red herring. I assume you have proven the motor is OK in some fashion?

Brian

[Dave Plowman (News)]

Most DVMs have a diode test function. If they have, there will be a diode symbol indicating it.

Disconnect one end of the diode and connect the DVM. Leaving it in circuit can give a false reading. The end of the diode with a bar is the cathode, which connects to the black lead on the DVM. You should get a reading of approx 0.7 volts. This is the forward voltage drop of a working diode.

[Andrew Gabriel]

The dirt on the board/diode doesn't look to me like it came from the diode. When components let go, the scorch marks radiate outwards. The dirt on that diode looks like it came through the gap between the capacitor and inductor. It came through slowly enough to be deposited as a bow-wave, so it's probably just dropped out of the airflow during normal operation of the machine. (Dirt is generate by motor brushes wearing, and the belt wearing on the pullies, and airflow generated by the various rotating parts.) Also, an exploding semiconductor doesn't leave a bow-wave type deposit - the force is too high. So, without a more detailed examination, I would suggest that dirt is a red herring, but I would want a more detailed examination, eg around the other side of the diode and up the sides of the capacitor and inductor.

I would also suggest motor brushes, and also the possibility of a broken conductor in the wiring loom to the motor. I've had motor drive triacs go (caused by failing motor brush), but in my case that didn't result in any visible damage as it just went open-circuit. I diagnosed the issues by checking for voltage at the motor, and tracing back, but this is not necessarily very safe to do, depending on how easy it is to access the parts, and if there are any exposed live parts.

[Peter Parry]

While you are looking at the control board look carefully at the top of all the capacitors, they should be flat. If any are bulging upwards they have probably failed (or at least should be renewed while you have the chance).

[Jules Richardson]

I don't remember ever seeing a diode fail where it's emitted stuff, either. Died with no visible damage, yes, and blown completely apart, yes, but not like that.

If that discolouration on the leg were due to severe overheat then I'd expect more visible damage to the board, too (and doubtless the associated PCB trace on the underside).

That, or it's a component failure elsewhere.

cheers

Jules

[NT]

Yes its a diode, and blackening of diode legs is normal and common. You cant measure a diode while its in circuit, the resulting readings are meaningless. The reliable approach to troubleshooting is to get the circuit diagram and a multimeter, and find out where things are working ok and where they're not.

NT

[tony sayer]

In article , Nick Leverton scribeth thus

Check the brushes I'd go for that first thing most likely fault;!...

Don't bother with that check to olde carbons;)..