On Sat, 22 Oct 2016 11:29:56 +0100, news wrote:

On Sat, 22 Oct 2016 01:34:05 GMT, Johnny B Good
wrote:

From your description, my inference is that the PCB in question is in
the vacuum cleaner itself rather than in the motor head.

No the PCB is actually in the head, it seems to be supplied with DC via
a 4 position switch in the handle which I have not been able to open up
to investigate. The first position is all off, the second is vacuum
only , the third is vacuum and motor head brush which gives a red light
on the motor head PCB and the fourth is supposed to just rotate the
head slowly. I took one input terminal off the PCB (and broke the push
on crimp in doing so) and tested the voltage in positions 3 and 4. Three
powers up the main vacuum motor and supplies only a few tens of VDC, 4
gives about 90VDC, this suggests a problem in the switch.






If you repeat your test with your moving coil multimeter using a
suitable AC voltage range (say 5 to 6 hundred volt scale) and take
readings with the test probes applied each way around, you should see
only a very low reverse reading when applied the wrong way round and an
ac voltage reading close to *double* the expected mains voltage when the
rectifier is working as a fullwave rectifier[1]. If the rectifier has an
open circuit diode as your voltage reading suggests, it will be acting
as a halfwave rectifier and the meter will then read just slightly less
than the AC voltage feeding the rectifier[2].

As you say it reads over 200V on an AC scale with terminals one way and
zero the other.

Looking at the forum Mike suggested the is a common problem with this
switch but no power supply components are mentioned.

I just need to get it apart and look.


There's a good chance that the switch feeds the power to a bridge
rectifier in positions 3 & 4. Using a 2 pole 4 way switch will allow one
of the brush head motor wires to be transferred from a DC output terminal
(+ve or -ve) on the rectifier to one of its ac input terminals (~),
effectively turning the circuit into a half-wave rectifier cutting the
unsmoothed averaged DC voltage in half[1]. It's a cheap and crude (but
effective) form of DC motor speed control.

The problem could be a switch or rectifier fault or just simply a broken
or disconnected wire between the switch and the brush head motor circuit.
You'll have to gain access to the switch wiring if you want to pursue
this fault finding exercise any further.

[1] This slow speed circuit might include a dropper resistor if half
speed isn't quite slow enough on its own account. It's also worth bearing
in mind that another possible way to feed that dc motor with half voltage
is to switch the rectifier's ac input terminal from full mains voltage to
half mains voltage available at the join between the two field windings
on the main universal motor.

Indeed, if the desired motor speeds are full and quarter speed, the two
methods can be combined on the fourth position if a three pole 4 way
switch is used. I can't think of a way of doing this with just a 2 pole 4
way switch off the top of my head but if there's a way, you can bet your
life that Dyson would have thought of it. :-)

--
Johnny B Good