Rega RB300 turntable...
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From: Arfa Daily on 18 May 2010 21:09 "Meat Plow" <mhywatt(a)yahoo.com> wrote in message news:pan.2010.05.18.16.07.29(a)gmail.com... > On Tue, 18 May 2010 16:50:28 +0100, Arfa Daily wrote: > >> <snip> >> >> >> >>> Just another bit of info for what it is worth. The DC resistance of the >>> motor windings are grey to red = 4k49 >>> grey to blue = 4k49 >>> red to blue = 8k99 >>> grey to motor case greater than 20Meg as read on my oldish dvm >>> The 220nf cap should be o/c also >>> >>> Now I am curious as to what you find. If it is the motor, it might be >>> worth your customer getting the motor upgrade kit if it still >>> available. http://www.rega.co.uk/html/upgrade_kits.htm Cheers >>> DaveD >> >> >> Thanks for the link Dave. Do you know what the reasoning was behind the >> 'upgrade' ? Did they have a lot of trouble with motors failing ? I would >> say that this is the original motor, coming up on 25 years old. I fitted >> the 3 x 47k combination, and the motor runs normally. It has plenty >> enough torque to shift the heavy glass platter, and according to my >> strobe disc, the speed is spot on. Voltage across the motor measured at >> 98v, which would seem right, given that the 47k combo comes to 15.6k >> against the original value of 12k. In the circumstances, I'm prepared to >> call 98v on a 110v nominal motor, a draw. >> >> I s'pose it wouldn't be a bad idea to just check the winding resistances >> before it goes back, but I don't think I'll be expecting to find a >> problem, as it does run correctly, without anything catching fire. >> >> As to why the original resistor failed, anyone's guess. About 1 1/4 >> turns of the spiral element have charred, right in the middle. The >> resistor looks as though it has been pretty warm over its lifetime, but >> again, it is nearly 25 years old, so has worked hard. Could just be that >> it had started to go high and suffered a cumulative cascade failure. >> Might have been a voltage thing. Some of these resistors, particularly >> considering its age, are not rated especially well, working >> voltage-wise. I suppose that over it's lifetime, it's had around 130v >> RMS across it all the time that the motor was running. Could have been a >> slight manufacturing defect such as the spiral being 'nicked', that has >> degraded down the years, to the point where it became a failure. Endless >> possibilities ... >> > > I hate endless possibilities. :) > Fact of life in our business, though ... :-| Arfa
From: Ian Iveson on 19 May 2010 07:33 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
From: PeterD on 19 May 2010 08:12 On Wed, 19 May 2010 12:33:27 +0100, "Ian Iveson" <IanIveson.home(a)blueyonder.co.uk> 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 >
From: William Sommerwerck on 19 May 2010 08:14 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.
From: Roger Thorpe on 19 May 2010 09:05
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..... |