Hello,
I need some help concerning extreme wow/flutter on my WTT which turned out after using a test record during re-calibration of the device. Pls apologize for the long post; it describes what I found out yet. Anyone who can help right away may not need to read the details below.
I would like to refurb this record player but this would be silly if the wow/flutter cannot be resolved. The player sounds very good but could do much better. I have little knowledge of electronics, e.g. I can do easy soldering if someone tells me what to put where, but don't know much about circuitry. I have a standard multimeter and a simple soldering iron.
Summary & Questions- The motor show serious wow/flutter (+/- 0.4%) using a test LP tone, as well as a strobe tachometer app. That's too much for a high end model.
- The reason may be the (simple) circuitry, which has been added to suit the German 240V, or a wrongly placed part (capacitor)
- The root cause may also be the motor, as it was dismantled to check for mechanical/corrosion problems inside. Re-assembling was done according to the motor vendor's directions, but with no calibration.
- It seems that the 2 coils inside (with their 2 iron stator shields each) drive the permanent magnet rotor shaft slightly asynchronous.
Q: How can I get wow/flutter to an acceptable value (e.g. 0.05% or so)
Q: Does anyone has the same problem?
Q: What purpose has the phase-shifting capacitor soldered between the power resistors into the power cable?
Q: What wow/flutter is expected?
Q: What parts are in the power cable in original WTT classic models (mine was modified by whoever for German 240V) ?
Q: Do the motor stator sheets need an assembly calibration beyond eyeballing their position (all 4 must match at the square notch, the 2 with rings must be the inner ones)
Q: Do the 2 motor coils need the same phase or a phase shifted signal between them?
Q: What type of capacitor is soldered into the cable at an original WTT classic? (must be a non-polarized one, due to A/C used)
Q: What kind of bearing is at the bottom of the rotor shaft - just the large allen screw with its metal tip? I'd expected some kind of plastic/bronze etc. bearing material.
IntroductionI am in Germany and I own a WTT classic. It is an early 1990s model, the one with the square lead motor base. I like the audio quality, however It never calibrated it. In 1994, while being in US, I bought a replacement motor from Well Tempered or Stanalog (don't remember exactly) to put it on stock. In contrast to the original motor,
- it was filled with a soft black grease and
- has a pointed end at the bottom (as if it should dip into some kind of bearing)
My current system is a Goldring Elite and pre-pre is a modified Cambridge Audio Azur 640P (recapped, cap & op-amp upgrade).
I recently bought a test LP and an exact weighing scale to calibrate it exactly the 1st time (better than just using the protractor). Also, I mounted the "new" motor. However, the old motor has the same problem, so it may be the circuitry.
The Problem - 0.4% wow/flutterMy test record has a 3150 Hz test tone to check wow & flutter. I metered it using an app called "platter speed" (MotoG smart phone). The app shows the actual frequency (e.g. 3170 Hz means the record is too fast) and draws wow/flutter diagrams.
It turned out that the record has serious wow/flutter of about +/- 0.4%. Even without the app, I can hear the tone oscillating. It sounds (and the app graph looks) like it does one oscillating cycle at each full record rotation. Therefore I thought it may be the belt or platter bearing. However, it turned out to be the motor (see below Appendix A).
The Motor: DetailsIt is an "Hurst Syncronous 115V/ 500 RPM PA-B 10W" model (as shown on the bottom cap of the replacement motor) . It is still available today at Nidec, which is the owner of the Hurst brand today. This is the motor spec sheet:
http://www.hurst-motors.com/documents/PA_PB_Synch_DD.pdfIn Germany, we have 50Hz and 240 Volt. Therefore, the motor runs at 500 RPM, which is 50Hz*60 seconds=3000 divided by 6 pole pairs. The same motor would run at 600 RPM in the US (60Hz). I believe that the RPM difference was equalized by Well Tempered by a different pulley diameter for US and Germany.
Inside, it has a permanent magnet drum rotor with the shaft attached to it, sticking out of for the pulley. Around the rotor drum, there are 2 horizontal coils and 4 iron stator sheets; one on top and at the bottom of each coil. Each stator sheet looks like an iron disc from which 6 spiky tips evolve along the inner circumference 90 degrees to the top. As a result, each coil is enclosed by 6 tips from above and 6 from below. The tips are almost evenly distributed along the inner perimeter of each coil (about 3mm gaps, varying a couple of 1/10th mm). Top/bottom sets of tips are slightly shifted compared to the other. I assume these make up the 6 pole pairs that bring any such snchronous motor to 500 RPM (any 1-pole pair snychronous motor would run at 3000 RPM at 50 Hz*60sec).
The "circuitry"My model had some electronic parts soldered in one of the 2 pole main cables.
- series circuit: 600 Ohm resistor --> 1 myF capacitor --> 600 Ohm resistor
- parallel circuit: 1 myF capacitor between the red and black cable, as indicated in the motor spec sheet linked above.
The resistors obviously transform the 240V to 115 (or 120 V I assume), and they got very hot. The capacitor between the red/black cable is the one required in the motor spec sheet. Red/black are the connectors for coil 1 and 2 inside. The other poles of each coil are the blue cables. They are both connected to one power cable pole, without no curcuitry in between or attached.
I'm not sure about the capacitor between the resistors - it obviously shifts the phase, but I have no clue why. The pase shift applies to both coils.
Some time after buying it, I rebuilt the circuitry because the resistors were very hot and i didn't like the bulge in the cable. So I split the 2x600 ohm capacitors into 2+2x330 Ohms and replaced the caps by radial film caps. Same connections as before.
Checks made to isolate the source of the problem I checked the following; as a result I isolated the motor, and most likely the circuitry as the source of problem. Would be great if someone can confirm this and advise.
1. replaced the belt (rubber, not polyurethane like the original one) --> no change
2. slipped the belt off during playing the test tone --> platter & record got slower; the app recorded the resulting frequency as being absolutely linear with no more wow/flutter until 0 RPM --> platter bearing is ok.
3. Checked the motor RPM with a stroboscope app "Strobily" (also on the Moto G). I glued a slim bar of white paper to the pulley, sticking out at the middle of the pulley on one side using it as a stroboscope "pointer". Result: at 8.3333 Hz (=500 RPM) the paper bar (optically) stands still, but flickers plus/minus its static position:
- a constant flickering, so the paper bar looks broader than it actually is
- a larger flickering, when it jumps far off and back (approx. 30 degrees) to its position every few seconds
4. Disconnected Coil 1 and Coil 2, so that only one coil was working at a time --> both coils caused the wow/flutter, so it's not their relative position. Torque was less of course,but it worked.
It would be great if someone could answer the above questions or give me a hint what is wrong here. New motors are available, but if it's a wrong circuitry, this may not help.
