Anyone happen to know the value of the resistor in the CR network connected in the synchronous motor feed ? Looks like brown - red - brown so 120 ohms, but it's well overheated and discoloured, with a nice burn mark right around the spiral element, right in the centre. Can't get a reading from either end to the burn point. Looks at a guess to be rated about 3 -5 watts. Anybody think of a reason *not* to use a wirewound rather than the original metal film type ?
No, but the RB300 is indeed a tonearm, rather than a turntable. There are PDF manuals on vinylengine.com for a bunch of Rega tonearms and turntables which might help identify what the deck actually is.
It's worth noting that the discussion on the page for the Planar 3 (which would be a plausible turntable to have an RB300 fitted) has some comments on replacement motor resistor values, with one comment suggesting 150 ohm 5%:
Well, sort of. I guess that the full deck is probably a Rega Planar 3, but definitely a Rega-original product. There is a "Grahams HiFi London" tested sticker on the bottom with a handwritten date of '86 on it. There is a manufacturer's label on the bottom also saying "rega Made in Britain" and "Rega Research Ltd"
To the best of my knowledge, it has always been used here in the UK.The motor is a synchronous type, 4 wire, and marked 110v 50Hz. Two grey wires go straight to mains neutral. Switched mains live comes back to a small (original looking) PCB in the motor compartment. It passes through the R in question to to the motor red wire. There is then a cap, 0.22u 400v polyester, between the red wire and the blue wire, so I guess that this RC combination is a phase shift / AC dropper so that the motor is happy at
240v. There is no indication on any of the labels as to the voltage specs, but the fact that it is a 50Hz synchronous motor single speed deck, and also that the mains lead is absolutely original and coloured blue and brown, would suggest that this was built for use in europe on 50Hz mains of at least 220v ??
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 ...
Hi Arfa Just took the cover off a Rega Planar 3 and the board has 5 components on it.
1 x .22uf 275v ac cap (220nf)
1 x 10k 5% 1-2 watt parallel (physically) to the cap (bn bk or -gold)
1 x 1M 1% 1/4 watt (br bk bk yell -bn)
1 x 3k3 1% 1/4 watt (or or bk bn -bn)
1 x 120nf 400v dc cap HTH DaveD
Hmmm. Well Dave, I've got to say that that bears no relationship to what's in this one, at all :-\
It definitely says "Rega Planar 3" at the top front right corner, although it's hard to see as the letters are just black 'shininess' within the black satin anodised finish of the cabinet. Underneath, directly under the motor, is a cover secured by two screws, When this is removed, the motor and a small PCB are revealed. The PCB has just two components on it - the burnt up R, which is a large 'film' type, I guess something like about 5mm diameter and 20mm or so long, and the 0.22u 400v cap, which is one of those green MKT monobloc types. These two components are mounted 'on the slant' on the board. The mains comes in, and the neutral is joined immediately to two grey wires from the motor. The live leaves the board again on a white wire which goes to the on / off switch at the front top left of the deck. Live returns to the board on another white, and goes to the motor red lead via the burnt up resistor. The cap is between the red lead and the remaining blue lead. Doesn't sound much like the example that you're looking at, eh ?
Mind you, dunno about you, but I find this all the time with 'high-end' gear. Guts that are nothing like what the the service manuals say they should be, poorly done mods involving extra components stuck across boards, bog-standard cheap as chips lasers fitted to CD players costing thousands, metalwork that fits where it touches, holes drilled in wrong places, inappropriate screw types and sizes - the list is endless.
I know that these things are often hand-built, which I guess is what attracts the people with more money than sense to them, but when I look at some items, I wonder how the companies that made them, and then stuck the multi-thousand pound price tags on them, sleep at night. I guess I've just got too much of a conscience ...
Anyways, I think tomorrow that I am going to fit a 5 watt 150 ohm wirewound that I have to hand, and then bring it up gently on the variac, and see what happens. I guess it's got two chances ... :-)
Yes, that's closer - although still not quite the same configuration. It's actually a different motor in yours, but the supply still goes to the red via the resistor, and the cap still appears to be between red and blue, and the same value, although a different type. Based on that, I think I'm going to start with 15k now, and see if the motor runs, and if it does, if it has enough torque to drive the platter.
Thanks very much for taking the trouble to post me the piccy. Appreciated.
Hi Arfa Further info. The data I supplied earlier was from the Planar 3 upgrade motor assembly. I have just pulled out the old motor and PCB. It has only 2 components on it.
1 x 220nf 250v ac X rated cap manufactured by Rifa.
1 x 12k 2-3 watt resistor, 8.5mm dia x 25mm long. The resistor is a mid brown body colour and the bands are quite hard to read but measurement confirms 12k 5% (as in brown red orange gold) The brown and orange are hard to differentiate The motor is a Philips 110v 50hz 250 rpm (12NC 990411131613) date of manufacture 1985 with the same colour leads I would suggest going with what you have in the 12 - 15k ~ 5 watt resistor. Check that the voltage to the motor is in the region of 110v under load and it should be good to go Cheers DaveD
Many thanks Dave. That's it exactly. I just had a look, and I haven't got a high power 12 or 15k in stock, but I have got some 47k 2 watt film resistors, so to save time and get the item back to the customer (he's already put up with me being away on holiday for 2 weeks !), I'm going to put three of those in parallel as there's plenty of room. Thanks again, all.
Is there a fuseable resistor function to the original ? ie 2 watts of dissipation from stalled motor and the R blows rather than the motor. Would not some 1/2 watt resistors together giving 12K be better?
I wouldn't have said that there was an 'implicit' fusible function to the resistor, but I suppose that you could argue it as a secondary function. The plate on the bottom of the deck claims that the power requirement is 4 watts, so given that the primary function of the resistor is that of a dropper for the 110v motor, you would have to say that this equates to around 2 watts dissipation in the motor, and the other two in the resistor. As the original resistor is rated at around 4 watts, that would sort of indicate that it had been dimensioned to be able to stand up to its job without burning out. I'm not actually sure what happens to the current that a synchronous motor draws if you stall it, but I wouldn't think that it ramps in anything like the same way as that of a DC motor, or any kind of brushgear AC motor ??
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.
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 ...