Poltergeist?

[Jeff Layman]

In mid-April I posted a message with the subject "Great service" about very quickly receiving a replacement remote-control receiver for a DC

6-speed and reverse fan which was not working properly. The fan barely turned, jogging forward and sometimes stopping for a few seconds. It only turned at one speed and in one direction. Unfortunately, fitting the new receiver did not cure the fault. I took the fan down again to check the connections and refitted it. It still had the fault. I looked for a new fan, and just before I ordered it a week later decided to see if the fan worked. It worked perfectly!

That was in early May. It's been on every day since then, working without problem. That was until yesterday morning, when it did not work at all! I tried it a few times without success, even trying the remote control when I was rotating the fan by hand in case it required some sort of feedback. I started looking at a replacement fan again.

This morning, I thought it worth trying the fan again. Unsurprisingly, it worked perfectly!

I've heard of intermittent faults, but this makes no sense at all.

[The Natural Philosopher]

If it is *only* controlled by the remote, there might be a clue there, that the remote signal is getting interference. But failing capacitors etc etc can bring kit slowly to the stage where it is inoperative after being 'unreliable' first. I have a toilet fan that steadily failed to come on for a longer and longer time before finally failing to come on at all. (Big Clive dismantled an identically faulty one. Its a capacitor, but to get to it involves removing the ceiling [again]).

Another strange thing that I learnt as a boy sailing my electric home made tugboat around, is that (zinc carbon) batteries stop working, but if left for a couple of weeks, start working again.

In short poltergeists and Murphy's law are in strict Occamish style, not necessary as explanations.

I took a TV back to curries, because the power supply shut down, but only if it was plugged into Ethernet.

They said they couldn't change it, because they couldn't reproduce the fault. I asked if they had an internet Ethernet connection. They said they hadn't. I said' I don't want my money back, I dont want to swap it for a different model TV, I want another TV exactly the same without this fault, if it didn't have this fault why would I bring it back?' They eventually agreed that was a reasonable point and swapped it. Its replacement is still going strong. Plugged into the Ethernet

I have no idea why it exhibited this behaviour. As a former electronics engineer, give me a bench full of test gear, a schematic, and I could probably find out in less than a week.

But in practice Curries will either have marked it down and sold it to someone without Ethernet, or sent it back to the manufacturers. Or simply written it off. Life is too short.

Sometimes its easier just to replace kit that behaves strangely.

[Brian Gaff]

Dodgy winding or joint in the fan itself. I can well remember back in 1976 or so, we had a Phillips VCR 1501, the one with the kitchen oven timer, and I have to say it gave very good service, so when I bought an N1700, which took the same tapes I decided to get the slow speed mod done. Two days before I sent it away, it started to go off for no apparent reason. I told the guy and he said he would look into it. Well, when it returned he said no sign of your fault, and as it worked fine I continues using it as a second recorder. I could see in those days of course. Then after about 6 months, it started to randomly shut down again. Shoving the meter on some of the supplies it was clear that one of the supply lines was going down to low voltage. Tracking back I ended up with four rectifiers wired as a bridge. What I found amazed me, Every one of these Mullard silicon diodes was intermittent, and I changed them one at a time to get the polarity right, though I used an assortment of left over ones, in the end, rather than trust more of the light blue ones, and it worked flawlessly for many years. The old ones actually could be seen to have wobbly leads as they emerged from the plastic. The internal wiring to the active component was obviously broken and they only worked due to pressure on the wires. It just goes to show that things that look perfect, are not always so. Even a washing machine we had made by Service had an intermittent fault, cured by resoldering all the pins of the contactor to its pcb. Brian

[Brian Gaff]

Yes those were the days. When we first got our Video, we had a Decca colour tv. the tops of all pictures bent over and wobbled, on talking to the shop the guy arrived with one resistor and a soldering iron, pulled out the time pcb, and took one resistor out and put the new one in and it worked fine. I asked him how he knew, he said that the flywheel sync on the Decca was known to be a bit 'soft', and this modification made it more robust when the timings of the pulses from a tape varied a bit.

It can be a bit of a black art, but as you say, nobody bothers to find the fault any more, mainly due to the fact that everything is surface mount and very hard to service. Even in the old days of TV, pcbs were often swapped out instead of anyone finding the actual component. Where I worked they used to send the boards back to us, we cleaned them repaired them, put in new components where the old ones showed signs of stress and tested and returned them to the rental centres. Brian

[Jeff Layman]

I think that is probably the only conclusion. The remote control receiver has been replaced, so the only parts left are the motor itself and the connecting wires. Quite why the possible dry joint behaves as it does is beyond me, as it does not seem related to, for example, the temperature of the motor. It has both started and failed to start when left overnight. As it is a brushless DC motor, there are no passive components in it to fail - just the motor windings. The operation of the motor controller seems rather complex; there are several ICs, some of which are not identifiable. One was was a serial 3-wire EEPROM, and another a custom(?) 32-pin QIL chip. The Taiwanese manufacturer of the motor also designed the controller, but it is not possible to access any information about either at the manufacturer's website.

[The Natural Philosopher]

Sensorless brushless motors can fail to start if the bearings are stiff or the fan too large. I know from experience on model planes - too big a prop and it wont 'kick' enough to give coil position feedback. But spinning the fan should help it start

[Jeff Layman]

I mentioned in my OP that I had tried starting it with the remote without success while spinning it by hand. When it works it has always had no problem starting even it if means a small amount of rotation in the wrong direction before it works out what way it should be turning!

[The Natural Philosopher]

Aha. That is valuable information.

That almost certainly puts the problem in the electronics, not in the motor, unless its lost a whole phase. If it still kicks, it could be a phase.

But failure to kick at all, sounds like the speed controller is majorly cattled, and that's beyond all but he most dedicated electronics enthusiast to fix

You may get lucky with an educated guess, and the 'parts cannon' - remove and replace all the electrolytic capacitors. And resolder all dry and or broken joints, or suspect ones.

Up to you and the value you place on your time.