I have been using this Pioneer PL223 turntable for a few years, but last few months the speed has been fluctuating excessively. I have already tried re-lubricating the spindle shaft. Here is the schematic for the motor control.
It's a "DC Servo Control" system according the label on the turntable.
I am new to DIY audio and electronics, so although this turntable is not worth much I would like to fix it as a learning experience.
Will appreciate if anyone could point me to some information on this type of motor-control system and how it works (I noticed that unlike higher end turntables, there are no transistors or ICs involved, how is the servo control working here?) and any tips on how to go about troubleshooting this.
It's a "DC Servo Control" system according the label on the turntable.
I am new to DIY audio and electronics, so although this turntable is not worth much I would like to fix it as a learning experience.
Will appreciate if anyone could point me to some information on this type of motor-control system and how it works (I noticed that unlike higher end turntables, there are no transistors or ICs involved, how is the servo control working here?) and any tips on how to go about troubleshooting this.
The usual culprits in such a situation tend to be:
* worn-out belt (if synthetic, may be brought back in spec via boiling)
* dirty speed selector switch
* corroded speed adjustment pots (more a problem with pitch pots normally)
* fluctuating DC supply voltage e.g. due to worn-out mains filter capacitor
I don't think the motor bearings gumming up is a very common problem but I suppose is not out of the question (it does happen in some CD mechanism spindle and pickup motors, and keep in mind we are talking bread and butter level components). Turn the motor pulley by hand, it should spin easily. Also let it run for a while and check temps.
The main spindle bearing is, more often than not, the least of your worries due to the low RPMs involved. Recommended lubricants vary a bit, with a 20 weight fully synthetic turbine oil having been identified as a good candidate for Technics bearings (it's a plain brass bearing, making additives neither required nor desirable). Usually topping things off with some sewing machine oil is not going to do anything major, but it technically is too light (generally 5W). Spindle bearings can seize up if the lubricants were crap from the factory and the unit has been sitting for years. It happens in old Duals but this Pioneer isn't even that old.
* worn-out belt (if synthetic, may be brought back in spec via boiling)
* dirty speed selector switch
* corroded speed adjustment pots (more a problem with pitch pots normally)
* fluctuating DC supply voltage e.g. due to worn-out mains filter capacitor
I don't think the motor bearings gumming up is a very common problem but I suppose is not out of the question (it does happen in some CD mechanism spindle and pickup motors, and keep in mind we are talking bread and butter level components). Turn the motor pulley by hand, it should spin easily. Also let it run for a while and check temps.
The main spindle bearing is, more often than not, the least of your worries due to the low RPMs involved. Recommended lubricants vary a bit, with a 20 weight fully synthetic turbine oil having been identified as a good candidate for Technics bearings (it's a plain brass bearing, making additives neither required nor desirable). Usually topping things off with some sewing machine oil is not going to do anything major, but it technically is too light (generally 5W). Spindle bearings can seize up if the lubricants were crap from the factory and the unit has been sitting for years. It happens in old Duals but this Pioneer isn't even that old.
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These motors are quite simple and variations were seen in 95% of cassette decks in the 80' and 90's.
The speed vs load is regulated indirectly by means of the back EMF generated by a simple DC motor. Inside the motor is either a one or two transistor adjustable voltage regulator or a small IC (like a little power opamp).
The basic speed is set by altering the motor voltage.
The back EMF that a motor generates will vary as the motor is loaded and so by incorporating a low value 'sense' resistor we can wrap either the transistor or IC around this and use feedback to keep the motor speed constant.
What is Back EMF in a DC Motor? - Circuit Globe
As you load the motor the voltage produced by the regulator will increase and maintain the speed. The system relies on matching the low value sense resistor (RT in the diagram below) to the characteristic of the motor. Get the value wrong and the motr may speed up as it is loaded, or slow down as it is loaded.
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Had some time today so decided to spray some contact cleaner (as pointed out by Boydk) on the speed selector switch and also take a closer look inside... I had already cleaned the trim pots earlier.
The selector board and motor:
The motor seems to have been changed at some point by the previous owner, as evidenced by the extra, unconnected solder point. There is a small IC (I'm assuming) under the board. One quirk of this particular turntable, probably due to the changed motor, is that the speed selector switch's function is inverted, as only the 33rpm trim pot is able to get the motor fast enough for the 45 setting. :
I found a more complete schematic. I checked the voltages between the marked points and hooked up my oscilloscope. With the multi-meter on DC setting, voltage is fluctuates slowly from 14.4 to 14.5 volts. AC setting shows 0.33.
This is what I got with scope, on 5ms settings. Looks like a capacitor discharging but maybe it's the effect of adding the scope to the circuit?
Cleaning the speed switch did not have much effect. Speed variation is around 0.5% Note that it is not a constant varation but happens slowly, so for example the speed would be hover around 33.9-34 for a while, then in a minute or so start moving between 33.7-33.8.
Thanks! That's what I was looking for. I will try to find the data sheet for the motor that's in there currently.
The selector board and motor:
The motor seems to have been changed at some point by the previous owner, as evidenced by the extra, unconnected solder point. There is a small IC (I'm assuming) under the board. One quirk of this particular turntable, probably due to the changed motor, is that the speed selector switch's function is inverted, as only the 33rpm trim pot is able to get the motor fast enough for the 45 setting. :
I found a more complete schematic. I checked the voltages between the marked points and hooked up my oscilloscope. With the multi-meter on DC setting, voltage is fluctuates slowly from 14.4 to 14.5 volts. AC setting shows 0.33.
This is what I got with scope, on 5ms settings. Looks like a capacitor discharging but maybe it's the effect of adding the scope to the circuit?
Cleaning the speed switch did not have much effect. Speed variation is around 0.5% Note that it is not a constant varation but happens slowly, so for example the speed would be hover around 33.9-34 for a while, then in a minute or so start moving between 33.7-33.8.
Thanks! That's what I was looking for. I will try to find the data sheet for the motor that's in there currently.
The scope is just showing the 'ripple' on the power supply rail. The diode forms a half wave rectifier and so every time the AC voltage (which is a sine wave) goes positive it charges the cap (C4) to the peak of that sine voltage. As the sine drops down the cap then supplies the motor until the next peak occurs.
If you pick any point on the waveform and then look along for the next recurring point you will find it is exactly four divisions along. So the time period is 5ms multiplied by four which is 20ms.
Frequency is given by 1/T which is 1/0.02 which is 50 or 50Hz, the mains frequency.
If you pick any point on the waveform and then look along for the next recurring point you will find it is exactly four divisions along. So the time period is 5ms multiplied by four which is 20ms.
Frequency is given by 1/T which is 1/0.02 which is 50 or 50Hz, the mains frequency.
The motor may or may not be original, the unsoldered pad isn't really proof. It might be worth looking the motor number up (check the label on the side) as these were pretty common and often made by Mabuchi.
If you turn the power OFF it might be worth seeing if there are any presets behind those two black foam covered holes although its equally probably the motor case is just a generic one and the same as used on cassette decks etc.
Its also worth checking that the resistance you can obtain between '3' and '4' is alterable between the expected values. With both pots on minimum resistance there should be almost a short (zero ohms) between those points and with the presets on max it should be nearly 10k.
You will need to unsolder at least one wire to isolate it from the motor in order to get a true reading..
If you turn the power OFF it might be worth seeing if there are any presets behind those two black foam covered holes although its equally probably the motor case is just a generic one and the same as used on cassette decks etc.
Its also worth checking that the resistance you can obtain between '3' and '4' is alterable between the expected values. With both pots on minimum resistance there should be almost a short (zero ohms) between those points and with the presets on max it should be nearly 10k.
You will need to unsolder at least one wire to isolate it from the motor in order to get a true reading..
Thanks for your suggestions.
The number on the motor is EG-530SD-3F. This brings up generic 3 speed turntable motors on Google. The IC is probably AN6652 according to another forum post. Have attached the diagram from the datasheet.
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Any chance that is a brush type motor and it's simply wearing out? It doesn't
appear to have a lot of electronics.
G²
I'll add that IF there's any adjustment pots within the motor itself, that they might be in need of a shot of contact cleaner too.
Remembering the position of each one, and GENTLY working it back and forth to clean it usually helps.
Many people go into them and damage them with too much force.
A small drop of light oil on the pulley bearing is wise to do.
The motor could use any one of a number of different controllers, even down to using a couple of transistors.
What to do...
if you are now certain that the pots and switch are good, and a further test would be to unsolder them from the motor and tack a single preset in their place... then you have nothing to lose in opening the motor.
The cover is a press fit and should come off with careful prising with as large a screwdriver blade as you can fit in the gap. It should just pop off.
From there you can see what is inside, IC or not or whether there are pots in there or not.
If the commutator and brushes become worn/dirty/electrically noisy then that upsets the speed regulation. You can try unsoldering the motor from the board but beware as to much heat will melt the nylon supports holding the brushes.
You could also see if there is access to the motor at the brush end and try a squirt of cleaner.
If you unsolder the motor then it can be worth applying 9 volts (little battery) to the terminals and give it a good spin in either direction to clean the commutator up. The motor will absolutely fly around at that voltage. Make sure you mark which motor terminal goes where, if you get it back to front the motor will run the opposite way.
Any small electrolytic cap in their could also be suspect. As they age and dry out they allow the electrically noisy commutator to interfere with the regulator.
I couldn't turn up a suitable replacement from the number which is a pity because these things used to be ten a penny years ago. You also have to look out for CW or CCW (clockwise or counter clockwise) on any replacement.
Thought I'd update this thread.
I ended up taking the back cover off the motor and found another PCB with some small potentiometers. Thanks Mooly for pointing out that It is removable. Somehow it never occurred to me!
There was a LOT of mold on the PCB which I cleaned out with some contact cleaner.
The turntable seems to be keeping steady speed now, although I haven't been able to test it thoroughly yet, as all my amplifiers are either in sent out for repair, on are ongoing projects
I ended up taking the back cover off the motor and found another PCB with some small potentiometers. Thanks Mooly for pointing out that It is removable. Somehow it never occurred to me!
There was a LOT of mold on the PCB which I cleaned out with some contact cleaner.
The turntable seems to be keeping steady speed now, although I haven't been able to test it thoroughly yet, as all my amplifiers are either in sent out for repair, on are ongoing projects
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and it certainly looked a bit suspect from your images above.
