Rega RB300 turntable...

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

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!

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.....

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

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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

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

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

Oh...maybe not

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

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

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.

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.

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

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.

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

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...