Bang, flash, what a failure (mode)

[John Rumm]

This one raised an eyebrow...

The time before last when I used my mains powered multimaster, there was a bang and a blue flash from the back of it as a capacitor in its mains inlet filter decided to let out the magic smoke. Oh well, never mind carry one.

Last week I was using it to cut some floorboards in situ, and another bang flash... oh well it seems like it had a pair of suppression caps in there. The surprise however was when I came to turn it off and found I could not - the switch was not longer effective.

So taking it apart it appeared that it was not the suppressor failing "short" in some way that managed to bridge the switch, but the switch itself had actually had its contacts welded. Quite impressive just from the failure of the suppressor!

A sharp rap with the back of the screwdriver sprung the switch back open, and its been fine since then.

[Brian Gaff]

Its not unusual. I had this happen to a Panasonic Vacuum cleaner some time ago. The point is though, why in the case of the vacuum did it not trip the breaker even though it blew the plug fuse?

The suppressor as i recall was a tiny little encapsulated lob thing with terminals on it. No idea what was inside it, but a triangle of1500volt .01 caps seemed to work, and I suspect its now been skipped since we took it to a charity shop due to its tendency to eat belts. Brian

[Dave Plowman (News)]

I had a Maplin Gold which did just that. The rotary switch arced over. Could have been fine metallic particles between the tracks.

I no longer ever use a multi-meter with rotary switch for mains. Have a dedicated mains only one for this.

[John Rumm]

Yup rotary switches are often a failure point once a little warn.

In my case, it was the not switch that caused the problem directly, but I am guessing there must have been a very quick short exhibited by the cap filter cap as it died. As it was wired directly across the output of the switch, it must have been a high enough current to weld the contacts. (the motor is only 180W, so I doubt the switch is rated for that much current).

Just debating whether its worth spending £7 to replace the filter, or just carry on without.

[tabbypurr]

I'm sure you'll find an RC snubber in the junkbox. If not those don't cost £7.

NT

[John Rumm]

Sure, although the proper part has a third contact "probe" that slides into a pocket in the stator laminations:

Like:

[Thomas Prufer]

ISTR it's a close fit, so I went for the exact replacement part.

Mine failed with a pop, taking out the circuit breaker -- Fein Multimaster, old one with the screw, not the star. Worked fine afterwards, probably because the caps are self-healing. But it happened again, 'bout a year later, and rather than having it pop a breaker at an inconvenient time, I got the replacement. Pretty much drop-in, after taking out a bunch of screws; also a change to blow out the inaccessible bits of wood dust.

Thomas Prufer

[tabbypurr]

3 pin ones also have 2 very small Y caps from L-case and N-case. Are they necessary? Not really, they just conduct away rf on the motor frame. A 2 pinner will keep the motor happy & get rid of most rfi, but not as much as a 3 pin one.

NT

[John Rumm]

Yup, same model - about 12 years old.

Yup easy enough to get at - three screws in the panel on the case, and a couple for the cord grip inside.

There was a fair amount of very fine masonry dust in mine - probably from when used cutting tiles in situ etc.

[jkn]

I powered up an old Tektronix oscilloscope after it had been in the garage (cold but dry) for a couple of years recently. It seemed to work fine but then there was a marked 'bang' and a load of stinking smoke from the rear. I powered it off in a hurry, fearing the worst.

After a fair bit of internet research it became clear that this was one of the X2 caps failing. Such is the predictability of this particular failure that people seemed to be able to tell, within the month, when they would blow ;-/

So luckily all I have to do is replace that cap (together with a load of others I might as well do at the same time) to have a fully working 4-channel 300MHz scope again ;-)