Rega RB300 turntable...
 

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. 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.

Ian

Rega RB300 turntable...
 

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!

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.

Ian

Rega RB300 turntable...
 

Many turntables have a line resistor. It "does something" (that I don't
understand) to improve speed stability. Or torque. Or something. It's not to
reduce the line voltage.

Rega RB300 turntable...
 

William Sommerwerck wrote:
Many turntables have a line resistor. It "does something" (that I don't
understand) to improve speed stability. Or torque. Or something. It's not to
reduce the line voltage.


It's a while since I looked, and I could be wrong but:
The Rega planar motor is a pair of multi pole two phase synchronous
pancake motors sandwiched together with the poles interleaved.
The resistor and capacitor are there to change the phase (lag or lead?)
of one motor so that the assemlby turns in the right direction when it
is started.
Incidentally mine is a Rega 2 and the resistor burnt out too. A day
after a short friendly 'phone conversation with the company the postman
handed me an envelope containing a new resistor (with a higher power
rating).
--
Roger Thorpe

....Wait a minute, It's stopped raining/
Guys are swimming, guys are sailing.....

Rega RB300 turntable...
 

In article , Roger Thorpe
wrote:
William Sommerwerck wrote:
Many turntables have a line resistor. It "does something" (that I
don't understand) to improve speed stability. Or torque. Or something.
It's not to reduce the line voltage.


It's a while since I looked, and I could be wrong but: The Rega planar
motor is a pair of multi pole two phase synchronous pancake motors
sandwiched together with the poles interleaved. The resistor and
capacitor are there to change the phase (lag or lead?) of one motor so
that the assemlby turns in the right direction when it is started.

That chimes with my understanding, based purely on having had to 'fix' a
similar problem with a friend's Rega a few years ago. Haven't commented
previously as I don't know the full details. I just replaced the burnt out
resistor with one rated at a higher power. And have since forgotten the
value.

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.

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/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Rega RB300 turntable...
 

Roger Thorpe wrote:

William Sommerwerck wrote:
Many turntables have a line resistor. It "does something"
(that I don't
understand) to improve speed stability. Or torque. Or
something. It's not to
reduce the line voltage.


It's a while since I looked, and I could be wrong but:
The Rega planar motor is a pair of multi pole two phase
synchronous pancake motors sandwiched together with the
poles interleaved.
The resistor and capacitor are there to change the phase
(lag or lead?) of one motor so that the assemlby turns in
the right direction when it is started.
Incidentally mine is a Rega 2 and the resistor burnt out
too. A day after a short friendly 'phone conversation with
the company the postman handed me an envelope containing a
new resistor (with a higher power rating).
--

I read it that the resistor is in series with the whole
motor. How could that introduce a relative phase shift
between the two halves?

I looked at the site someone linked to, at the circuit for a
simple synchronous motor. When a resistor is used to drop
the supply voltage, the value of the recommended cap remains
the same. I didn't mention phase, therefore, not because I
know it's not significant, but that I have no evidence to
suggest that a resistor in series makes any difference.

Motors are something of a black art, AFAIK. Two motors in
parallel sharing a cap and resistor is about as black as art
can get. How can a symmetrical circuit favour one direction?
In what way is the circuit not symmetrical? Are you sure
that direction isn't determined by geometry?

Actually, this isn't a good place to explain such a thing.
If you have a name for the type of motor arrangement, maybe
I could look it up?

Ian

Rega RB300 turntable...
 

"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.

Rega RB300 turntable...
 

"Jim Lesurf" wrote in message
...
In article
, Roger
Thorpe
wrote:
William Sommerwerck wrote:
Many turntables have a line resistor. It "does
something" (that I
don't understand) to improve speed stability. Or
torque. Or something.
It's not to reduce the line voltage.


It's a while since I looked, and I could be wrong but:
The Rega planar
motor is a pair of multi pole two phase synchronous
pancake motors
sandwiched together with the poles interleaved. The
resistor and
capacitor are there to change the phase (lag or lead?) of
one motor so
that the assemlby turns in the right direction when it is
started.

That chimes with my understanding, based purely on having
had to 'fix' a
similar problem with a friend's Rega a few years ago.
Haven't commented
previously as I don't know the full details.

Now, since you have now commented, you *do* have the full
details, presumably?

I just replaced the burnt out
resistor with one rated at a higher power. And have since
forgotten the
value.

Oh...maybe not

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?

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.

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.

Ian

Rega RB300 turntable...
 

"Ian Iveson" wrote in message
...
Roger Thorpe wrote:

William Sommerwerck wrote:
Many turntables have a line resistor. It "does something" (that I don't
understand) to improve speed stability. Or torque. Or something. It's
not to
reduce the line voltage.


It's a while since I looked, and I could be wrong but:
The Rega planar motor is a pair of multi pole two phase synchronous
pancake motors sandwiched together with the poles interleaved.
The resistor and capacitor are there to change the phase (lag or lead?)
of one motor so that the assemlby turns in the right direction when it is
started.
Incidentally mine is a Rega 2 and the resistor burnt out too. A day after
a short friendly 'phone conversation with the company the postman handed
me an envelope containing a new resistor (with a higher power rating).
--

I read it that the resistor is in series with the whole motor. How could
that introduce a relative phase shift between the two halves?

I looked at the site someone linked to, at the circuit for a simple
synchronous motor. When a resistor is used to drop the supply voltage, the
value of the recommended cap remains the same. I didn't mention phase,
therefore, not because I know it's not significant, but that I have no
evidence to suggest that a resistor in series makes any difference.

Motors are something of a black art, AFAIK. Two motors in parallel sharing
a cap and resistor is about as black as art can get. How can a symmetrical
circuit favour one direction? In what way is the circuit not symmetrical?
Are you sure that direction isn't determined by geometry?

Actually, this isn't a good place to explain such a thing. If you have a
name for the type of motor arrangement, maybe I could look it up?

Ian



Ian. See my later post regarding the correct value, and how it all performed
when fitted. I think that I'm down the same road as Jim was when he did his
mate's one, and judging by what someone else said about Rega sending them a
shiny new resistor of increased power rating, that would seem to be the
right road, if their answer is also upping the rating from what they
originally designed in. Also, its primary function does seem to be that of a
simple dropper to get the 240v mains supply down to something closer to the
110v rating of the motor. My replacement R resulted in 240v in, 98v out
across the motor. I would say that the 0.22uF cap is there to introduce
phase shift between the two stacked windings, to ensure startup, and as
someone else suggested, correct rotational direction. I'm not sure that
stalling is a big current-draw issue for a synchronous motor like it would
be for AC or DC brushgear types. Anyone actually know ?

Arfa

Rega RB300 turntable...
 

In article , Ian Iveson
wrote:

"Jim Lesurf" wrote in message
...

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/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html