"PeterD" wrote in message
...
On Wed, 19 May 2010 12:33:27 +0100, "Ian Iveson"
wrote:

Arfa Daily wrote:

Thanks Adam. It is indeed a Planar 3. I had in fact
already been on that site, and seen the reference to the
150 ohm resistor. I suppose it's possible that there
might
be more than one value having been fitted over the life
of
production, because given 40 odd years of looking at
burnt
and discoloured resistor stripes, I definitely would not
have said that the middle one was originally green, but
who knows ? (well, hopefully someone on one of these
groups does ! ) I was pretty sure that the first two
were
brown and red for 1- 2 - something. I was just concerned
that if the final band was red, or even orange, I didn't
want to be putting 120 ohms in there ...

What's the resistor for? Assuming the motor draws tens of
mA
when running normally, the resistor would need to be a few
k
if its purpose is to drop from 240 to 120V.

If it is to drop from UK mains to 220V, then 120 or 150
ohms
would be the right order of magnitude.

Well the turntable draws about 4 watts according the the
OP (or
someone else in this thread). So that's 30 MA. To drop 120
volts (240
to 120)at that current and power you would have to use a
resistor of
4K. But since half that power is the motor, and half is
the resistor,
twice the value (8K) is more appropriate. 120 ohms would
result in a
current draw at 120 volts of hundreds of watts!

I've read this a few times but....perhaps you misunderstood.
I was entertaining the theory that the 120 ohm resistor
drops the difference between 220V (or whatever the motor
hopes for) and UK mains, i.e. 10 or 15V, whatever it was
when the deck was made. If that difference has increased by,
say 25% from 12V to 15V, then the change that has been
mentioned, from 120 to 150 ohms, makes sense, roughly.
Everything that Arthur has said seems to fit this theory.
It's simple, too.

Anyway, as I understand it, Arthur's worried because if it's
supposed to be a much higher value, he might do some damage.
That's why I homed in on the voltage dropping issue.
Whatever else it might do, a resistor in series will
certainly drop voltage. If the value is much too low, then
it could result in a burnout. This is true whatever its
effect on direction, speed, torque and lots of other stuff I
don't know about but probably everyone else does but didn't
fancy saying so until now.

I wonder if he's tried it yet?

Ian

If an increase in
value is advisable, then that should reflect a
proportional
increase in the difference between 220 and UK mains, so an
increase from 120 to 150 ohms would be reasonable.

It's common for motor drivers to use fuse resistors to
save
the motor from burning out if it stalls or jams. It could
be
that, in your case, the choice of resistor power rating
suitable for protecting the motor is such that the
resistor
runs habitually hot, and fails occasionally. I guess a
higher power rating, in combination with a higher
temperature coefficient, might protect the motor equally
well, whilst not burning out so quick under normal
conditions.