BOSCH MAXX Classic

[hexpope]

Hi all, new here to the forum. I have a bosch max classic WF0285SGB/01

It stopped turning spinning etc, like the usual when the carbon brushes have come to their end of life. So I went ahead and replaced the brushes as to be expected the brushes were not connecting to the motor commutator. I then went ahead and reinstalled the motor and put the belt back on. Tried a spin cycle and the motor started to make a hum noise and the lights dimmed a little while this was happening. I could hear the relays etc turn on and off.

So I thought I might have put the belt on wrong. So for testing purposes I removed the belt and tried the same without it. Same thing happened. So this time I decided to make sure all my connections were correct (Only one main and an earth) Tried the spin cycle again and same results.

 

I then decided to remove the motor and test it on my lab bench with a multimeter and I was getting 23.6ohms on a pair, 5.6 on another pair, and 1.6 on another pair. Having carried out the ohms check on the coils and commutator I decided to run the motor on 12VDC @ 2A and the motor worked fine in one direction and also in the other direction so this rules out that the motor is not at fault. (Note, it did nothing before the crushed were replaced)

 

So I move onto the control board and check around , can't see anything visually that's bad. I removed and checked the relays and they are operational. Checked the caps and some diodes and resistors and they all checked out fine.

 

Is there anything else that I am missing that I could check ?

Thanks

[Whitegoodshelp (Andy)]

If the motor hums and doesn't turn but clearly it's trying it sounds like not enough power is getting to it but if a motor hums and the lights dip it also sounds like when a motor is physically jammed. I couldn't rule out a motor fault when trying to turn a slow speeds but if it spins it does sound like it might be ok. Unfortunately this type of fault is a dead end for diy. If the pcb is suspected it's ok if you have one in the van to try, but buying one and hoping it fixes it is just too risky.

[hexpope]

Hi Andy, 

Thanks for your help and reply. In the end, it turned out to be a power mosfet switching IC on the controller board Top side. It turned out to be this component. http://ie.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&langId=353&urlRequestType=Base&partNumber=9921338&storeId=10163

 

It looked normal, but it just wasn't switching correctly. So I replaced it and now runs fine. I think the brushes becoming worn did caused some form of internal damage to the IC.

 

Thanks once again.

[Whitegoodshelp (Andy)]

Many thanks for the update. An appliance engineer would have replaced the whole PCB, which is what I suspected but advised against. It's lucky you have the knowledge to delve into those components. The PCBs are expensive and you can't return them if they don't fix the fault so I always advise against it for DIY in case it turns out to be something else.  Appliance engineers are never taught anything about the workings of the pcbs they just replace them.