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From: Jim Lesurf on 20 May 2010 03:59
In article <SbZIn.6128$ea4.2910(a)newsfe29.ams2>, Ian Iveson
<IanIveson.home(a)blueyonder.co.uk> wrote:

> "Jim Lesurf" <noise(a)audiomisc.co.uk> wrote in message
> news:5119e3ce32noise(a)audiomisc.co.uk...

> > Must admit I was less than impressed with the circuit at the time.
> > From this thread it does look like this resistor is prone to burning.
> > Although perhaps that is made worse by the risk of the motor being
> > stalled or loaded.

> Perhaps? Certainly, surely?

No, I can't say for *sure* since I don't now know the rest of the circuit
details or the motor characteristics, fusing, etc. Lacking details I can
only say "perhaps" in the above.

> Is it really likely, do you think, that the manufacturer would continue
> to fit an inappropriate resistor for so long? It's not like resistor
> power rating is complicated or arcane. Cost difference can't be that
> significant, and there's no shortage of space.

Afraid I have no data to base a general value of what is "likely". I can
only note that:

1) I found that the resistor was burned out and seemed to me to be rather
small (hence probably low power rating) and replaced it with a higher rated
one.

2) That others here are commenting in a way that supports this idea.

> It would be interesting to know how the winding resistance compares
> with the impedance of an up-to-speed motor. If the difference is great
> enough, it could be that all those burned-out resistors are a result of
> stalled motors or increased drag, or even the fitting of heavy
> platters. It's also possible that every burned-out resistor has saved a
> motor winding, in which case a higher-rated resistor would be a
> liability.

That is possible. All kinds of speculations might sometimes return "true".
:-) However in the case of the resistor I replaced the turntable seems to
have worked fine since. The owner seems quite happy with it some years
later.

However, joining you in speculation, the problem with fitting components
with (relatively) low power ratings is that the component tends to run hot
and this can shorten life or lead to other problems. Given the cost of the
turntable it might be better to use a better design for the PSU. Or even
use a fuse that is user-replacable without a soldering iron. Or ensure that
the system is rated to remain undamaged even with a prolonged stall.

I agree that low power resistors are cheap, though. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
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Audio Misc http://www.audiomisc.co.uk/index.html

From: PeterD on 20 May 2010 06:40
On Wed, 19 May 2010 22:44:37 +0100, "Ian Iveson"
<IanIveson.home(a)blueyonder.co.uk> wrote:


>
>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.

IIRC the setup was that the motor was a 120 volt motor used on a 240
volt mains. Not a 220 volt motor on a 240 volt mains. If the desired
drop is only 12 to 15V then you are correct for the value. But that is
not what I understood.

>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.

I'd agree, given the conditions in my comment above, that a 120 ohm
resistor might in fact do 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
>

I thought he had, but I'd have to go see if there is a reply.
Something about using three or four larger resistors in parallel...
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