How does turntable AC speed controller work?

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I have a turntable using AC speed controller for 8 years. Recently, the 45 rpm is not stable but the 33.33 rpm runs well. The manufacturer is not going to repair it for me since; as they said, the model is outdated.

My limited knowledge on AC speed controller cannot support me to repair it. Can anyone give me block diagram on how AC speed controller works?
 
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An AC speed controller uses an accurate oscillator to drive the motor. If the motor is a 110/230 volt type then the circuit may consist of an oscillator driving a power amplifier (just like an audio amplifier) which then drives a suitable step up transformer to provide the voltage.

Alternatively an AC motor may be a low voltage type that can be driven directly from a low voltage circuit and drive amplifier.

This is low voltage type:
Sansui SR525 doesn't turn
 
My turntable and speed controller are from Amari - a manufacturer in China. They use Hansse for export models. In my experience, this is a very good turntable that outperforms my previous Projects RMP-9, Clearaudio Champion and Linn LP12.

There is an IC power amp in the circuit. I changed the capacitors to FC and regulators before the power amp to Dexa three years ago. Everything works fine with a better stability but the 45 rpm ran into trouble recently with very unstable speed.

I don't remember there is a transformer in the circuit box but I will open it and take pictures in these two days since I have just moved and the controller is still in the box. I will also measure the output voltage and frequency from the controller too.

Can anyone give me a block diagram so that I can understand how does the circuit work? What is the work of the power amp? How makes a better controller? etc. etc. etc.

Thanks
 
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As Mooly said, for an AC controller, there will be a stable sinewave generator to create an accurate and stable signal source. AC synchronous motor speed is determined by frequency, not voltage. The output of the oscillator is low voltage and low power. The amplifier boosts the voltage to drive the motor directly (low voltage motor) or to drive a step up transformer (high voltage motor), and provide the power needed by the motor. Some controllers for high voltage motors may have a HV amp that can create a 115VAC signal directly without a step up transformer (Linn, Phoenix Falcon). Be very careful when working on this type of controller as the DC supply to the amplifier can be in excess of 350VDC.

A better controller would have a synthesized sinewave generator so the speed is adjustable, but still crystal controlled (an adjustable analog oscillator will have limited stability compared to a crystal controlled circuit). An AC synch motor has 2 windings that are meant to be driven by signals that are 90° apart in phase. In a simple scheme, only one drive signal is used and a capacitor on the motor creates the necessary phase shift for the second winding, but it is not always optimal. On more sophisticated controllers, there will be 2 signals (along with 2 amps & step up transformers) with adjustable phase to drive the motor windings directly. The motor will have lower vibration and higher torque when driven this way.
 

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I took out the controller and opened it in the middle of the night (1:27 am in Hong Kong now).

The input voltage is 238V 50Hz.
At 33rpm, output voltage is 240V 50Hz
At 45rpm, output voltage is 240V but the frequency jumps between 60Hz, 70Hz and sometimes 240Hz.
There are two AC motors connected to this control box.
(The above output voltage is measured without loading and I think it is lower than than 238V when it is loaded. The consumption, if my memory is correct, is only about 3W)

My poor memory is wrong. There are two transformers in the box. Both transformers are 115V X 2 primary, 12V X 2 secondary at 7.5VA each. There are two voltage selectors, both set at 230V.

There is no crystal in the circuit board. Power amp is TDA2050 feed with + - 12V. There is a push selector to select between 33 and 45 rpm. There are two LM833N in the circuit and two 10K VR to fine tune the rpm at 33 and 45 rpm but the fine tuning is always a difficult job because one can only turn the VR with minimum turn because a slightly turn a bit more on the VR has big changes in turntable speed. One C33725 is next to one LM833N while there is a Fairchild D238 245A transistor next to the other LM833N. A TL084CN is on the circuit board too. There is also a 7805 regulator on board next to the transformer, is this for TL084CN? Is this critical?

I can't post picture at this moment and is trying to find a server to host them. Will provide photo later.

Any clue on where should I try to sort out the problem?
 
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If there are no crystals, it is probably using the LM833 opamps as two Wien bridge oscillators as the sinewave source. Look at the output pins on each LM833 (there are two on each IC); on at least one per chip, you should see a stable sinewave of 50Hz (33 RPM) and 67.5Hz (45 RPM). If the 45 RPM oscillator is unstable, check the 10K pot used for adjustment for scratchy operation. If the output is stable at both 33 and 45 ICs, check the selector switch.

Look for any obvious problems, such as broken components or leads, bad solder joints, or burnt/damaged components.
 
Thank you very much. I need to find my scope from the Bermuda Triangle before I can proceed. Will keep you post.

By the way, the motor of my turntable is SMR 300-100, Made in Czech Republic. Is it a good motor? Any better suggestion?
 
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Hi Pyramid,

The opamps are giving out the 50Hz and 67.5Hz therefore I re-soldered the legs of the switch and the 10k trimmer - bingo! The 45rpm works properly now. Thank you for your information.

The 33rpm is running very steadily between 33.305 and 33.37 with most of the time stay at 33.31. I can't fine tune the 10K trimmer to make it 33.33x because a slight turn may change the speed dramatically.

The 45rpm is not running very steady. It runs between 44.955 and 45.188 while most of the time fluctuates. Is there anything that I can do to make it more steady?

I just noticed you have another thread about DIY 4 Phase Sinewave Generator for Turntable Motor Drive. Is it possible to drive my SMR 300-100 motor?
 
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If the drive signal is steady, but the platter speed fluctuates, it may be indicative of other problems with the table such as bearing lubrication, belt contamination, worn belt, slippage etc. Changing to a different controller would not make much difference in this case.
 
If the drive signal is steady, but the platter speed fluctuates, it may be indicative of other problems with the table such as bearing lubrication, belt contamination, worn belt, slippage etc. Changing to a different controller would not make much difference in this case.

One thing about my controller is it uses a 10K VR to trim the speed that it is difficult to trim. Is there something ready made in the market like a digital trimmer to replace the 10K VR in the circuit?
 
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