Speed problems (BIX) - diyAudio
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Old 17th July 2013, 08:26 PM   #1
Akapaz is offline Akapaz  Norway
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Default Speed problems (BIX)

Hello fellow DIY'ers. I have a BIX turntable were the speed is impossible to keep at 33 1/3. It seems like the speed is always increasing. Anyone have any experience with this problem? I'm suspecting the motor to be the problem.

- Motor is a Premotec 9904 120 18105.

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Old 27th July 2013, 11:16 PM   #2
Akapaz is offline Akapaz  Norway
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Anybody? Also, my string is not original. It's a little thicker than the original nylon thread with a knot. Does that make any problems? Would it be better to try with the thinnest thread I can find? The TT is in level and lubricated.

Mikal
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Old 28th July 2013, 12:55 AM   #3
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It's a DC motor so it is either a steadily increasing voltage as it warms up or the lube in the motor and or platter spindle is a bit gummy but loosens up as it warms up.

It looks like you can operate the motor out of its housing. If you have a digital multimeter you can measure the DC at the motor leads when cold. Compare it to the voltage after the unit is plenty warmed up to see if it has increased. If the voltage is increasing I'd look to the voltage regulator or the screw speed adjustment pots.

I'd look to the belt if the speed was consistent but incorrect. Or if there was some flutter or wow. Not likely with that size platter but you never know.

Good luck.
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Old 28th July 2013, 02:36 PM   #4
Akapaz is offline Akapaz  Norway
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Thanks for your reply. The PSU gives out 12,15 - 12,16 Volts. The motor have been going for about six hours now and startet at 3,87V. After about ten minutes it increased to 3,97V and from then on been staying between 3,98V and 4V.

This was without load from the platter.
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Old 28th July 2013, 04:03 PM   #5
kevinkr is offline kevinkr  United States
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This motor is running open loop, there is no feedback control of motor speed and even things like the motor windings heating up during initial operation are going to result in speed drift.

A potential alternative that might allow you to salvage the existing motor set up would be to design a simple servo using a wheatstone bridge topology..

Here is what I was referring to specifically. Why TI changes their internal link structure and makes things impossible to find I'll never know, but here it is for now:
http://www.ti.com/lit/an/sboa043/sboa043.pdf
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Old 28th July 2013, 04:04 PM   #6
Akapaz is offline Akapaz  Norway
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Now at 4,03 Volts. 45 RPM-setting at 3,67 Volts. I'm gonna try leaving the 45 RPM go for a while. BTW. hasn't calibrated the 45 RPM-setting to actually 45 RPM's yet.
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Old 28th July 2013, 06:50 PM   #7
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Kevinkr makes an excellent point. Without a servo control the motor speed will slowly drift as it warms up. Nothing to fix as it is design. You can upgrade it as kevinkr suggests or get a strobe so that you can adjust the speed every time you change an album.

Cudo's to kevin.
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Old 9th September 2013, 03:55 PM   #8
Akapaz is offline Akapaz  Norway
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Thanks for the replies guys. I've been working a lot lately, so my turntable have been completly forgotten. Used the weekend to assembly a little shopping list, so maybe I can start up this project again Ordered myself a Okki Nokki this morning just to give myself a reason to continue with this project.

If this works, the next project will be modifying the Rega arm.
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Old 10th November 2013, 11:46 AM   #9
Akapaz is offline Akapaz  Norway
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Here is the datasheet for the OPamp. Can't figure out exactly where to get the control voltage (V -In & V +In).

https://www1.elfa.se/data1/wwwroot/a...elOPA548_e.pdf

PS. I don't have any real experience with electronics at this level, but this is good practice
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Old 10th November 2013, 12:12 PM   #10
Mooly is offline Mooly  United Kingdom
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Your control voltage is just taken from the wiper of a pot wired across a stable voltage. In other words a variable DC voltage but one that is stable once set.

(the type of speed control in Kevin's link was really common on cassette deck motors. It senses the back EMF from the motor which changes under load. You could even rip the back off an old motor and probably use the PCB. You would have to alter and select the "sense" resistor to suit your motor. If the value is wrong then the motor will either speed up under load or reduce speed depending on the resistor error)
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