Hi all,
great forum (and "Hi Dave" if by some chance you're reading this
). This is quite a long post because I've tried to include all relevant info; if you read it all then many thanks, if you don't have time then thanks for looking at least!
I've got a problem with a Dyson AM08 fan and have been trying to track it down but without luck so far. I would be grateful for some guidance on how to go about troubleshooting and finding the problem - as much for my education as to fix the fan really. If anyone recognises the problem and can tell me exactly what to do then great, but I would also like to try to understand what happened - I'm curious like that...
I have some experience with electronics but nothing like some of you gurus. I'm getting into the hobby after many years of fixing bits and bobs (mostly on cars before) and developed a real fascination for it. I already had a Fluke 123 Scopemeter and some cheapie DMMs, but recently added a DE-5000 LCR meter (based on the reviews on this forum) and a FY6800 function generator to play around with. At Christmas my wife treated me to my first 'proper' scope - I had an old Hameg analogue scope but didn't use it much, preferring the portability of the Fluke 123 - which is a Siglent SDS-1104X-E. I've really enjoyed playing around and looking at waveforms of capacitors and inductors, etc. I'm a doctor, so (hopefully!) reasonably bright and quick to learn
The fan had a squeaky bearing in the motor and eventually wouldn't turn on - showing just an F4 fault. Dyson are no use here, saying to just send it back but it's well out of warranty so I "took it apaaart" and found the BLDC motor was sticky. I carefully injected small amounts of motor oil (after some research) using a fine needle and freed it up so it rotated beautifully, then plugged it back in. It did spin a couple of times, but then stopped and after this, the whole unit was dead - nothing at all on the LCD display which even before had always shown something. There was also a funny, high-pitched whining from the PCB but I couldn't pinpoint the exact component.
Using my shiny new DE-5000, I measured the inductance across the BLDC coils and found one was much lower than the other two - I assume the oil damaged the varnish and shorted some turns, leading to some damage on a PCB component. I have access to a couple of good boards and compared various resistances. I noted that the resistance between ground and the 5V pin of the voltage regulator and the microcontroller VCC pin was only about a steady 40ohm, as opposed to a rising value in kiloohms on the good boards. I took this to mean a capacitor was no longer being charged by the resistance test and was shorted. However, I've inspected the board under my (fairly decent) 10x stereoscopic microscope and checked ESR and resistances around the PCB but found nothing that enlightens me - just lots of low resistances in the digital section. My Fluke only measures to 0.1 ohm, so resistance tracing isn't really going to work. I can't get a 121GW past my wife yet. The DE-5000 can measure to 1 mOhm and I've done the lead upgrade, but they're still too big to get on small pins, etc. The tweezers are good but only open so far.
I did notice that when I power the board the voltage regulator gets very hot very fast (ask me how I found that out...
). I also only meaure about 240mV to ground instead of the 4.98V on a good board. I desoldered the VR and injected 5V from my power supply (current-limited at first, but it topped out around 100mA anyway - about what the KY5050 gives out normally). I didn't find any sudden voltage drops suggesting a short, but may have missed areas. The LCD did light up like this but the segments were random and didn't show anything sensible.
I was able to get hold of a burnt out PCB and desolder everything from it so I could put it on the scanner and follow traces under chips etc. I also used a trick I saw Dave use of inverting one side and positioning it on top of the other side with variable transparency to line up vias etc. It seems to be a 4-layer board.
My questions are:
- Can anyone explain how the low inductance damaged the PCB - which component(s) in particular would be affected?
- Does my capacitor theory sound right or would an IC input burning out show up like this too (worried about the MCU as I can't replace the firmware on that!)
- Is the hot VR a problem in that component or a sign that it's feeding a short - replaced it with a similar spec one but that got hot too...
- Is there an obvious test I'm missing which would move me a step closer - I'm out of ideas now...
I've attached various scans and photos if anyone's interested! I haven't included the LCD/power button daughter board as it doesn't seem to change resistances etc when connected.
The main components are:
Varistors: 07K300 (blue) and BC1382 (yellow)
Optocoupler: MOC3083
MOSFETs: FDD5N50NZ (six)
MCU: 104BAA
Buck converter: LNK304DN
Hex inverter: SN74HC14
Flip flop: SN54HC74
Half bridge drive: 73833 (three)
As I said, although a quick solution would be nice if it helped me fix the fan, I'm really interested in learning about what went wrong after I oiled the fan bearing and how that led to the problems.
If you got this far, congratulations and thank you!! I would be very happy to get further readings/pictures if anyone is interested.