On Fri, 21 Oct 2016 11:31:26 +0100, news wrote:

I inherited this a year ago and it has been a good vacuum cleaner with
it's own little electric motor to power the brush head.

It has stopped rotating the motorhead without warning and a red LED is
glowing when power is switched to the head.

I have stripped the head and it uses a Domel 230VDC motor via a PCB
which has the LED. This motor rotates slowly when 12VDC is applied.

Thus neatly proving that the motor is working ok. :-)


The two leads coming to the head measure 90VDC on a moving coil meter.

I'm thinking the head and PCB are probably OK but some diodes have gone
open circuit in the power supply.

Any thoughts?

Actually, there's every chance you've hit the nail right on the head!
Most likely (assuming there's no fancy speed controller included), the
"Power Supply" is nothing more than a bridge rectifier (a module or 4
discreet 1N4007 or similar diodes).

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

If this turns out to be the case, 1N4007 diodes or 1A mains bridge
rectifier modules are readily available and cheap. All you then need is a
soldering iron, some multicore solder, a cheap desoldering pump or small
flat bladed screwdriver, long nose pliers and the skill to use them to
remove the faulty component(s) and solder in the replacements.
Alternatively if you lack such tools, find someone who does have such kit
and the necessary skills. :-)

[1] This assumes that the rectified output voltage is not subjected to a
smoothing circuit such as a 400v rated capacitor placed across its DC
output terminals - a pretty safe bet in this case since the 230v DC motor
will work perfectly fine with an unsmoothed rectified supply without
incurring the needless expense of an extra failure prone component which
itself places additional stress on the rectifier.

[2] If one of the rectifier diodes has failed open circuit, you'll only
see the expected mains voltage reading. The reason for this behaviour
with the classic moving coil multimeter is due to the AC voltage ranges
relying on half wave rectification of the test voltage which halves the
average current, necessitating an approximate halving of the multiplier
resistances used for each ac voltage range (approximate because the meter
reads the average voltage but is calibrated to show the RMS value which
is about 10% higher, requiring a similar further 10% reduction in
multiplier resistance values to compensate.

A typical 20Kohms per DC volt multimeter will have a 10K ohm per AC volt
sensitivity (the moving coil meter circuit is either desensitised
slightly on its DC volts and amps range to make the half ohms per volt
sensitivity figure for the AC ranges true or else the given AC ohms per
volt figure is simply a rounded up approximation.

When you connect an old fashioned analogue multimeter to a dc test
voltage using one of its AC voltage ranges (say a 12v SLA measured on the
30 or 50 volt AC range) the reading will slightly double that which was
read on the DC volts range. If you test a halfwave rectified DC supply,
the AC volts reading will be only slightly less than the AC voltage
feeding the rectifier diode. Testing with a fullwave rectified DC voltage
will double this reading for pretty much the same reason that measuring
12vdc on a 30vAC range gives a reading of just over 25 volt.

--
Johnny B Good