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My Linn Axis runs 3.5% too fast (although at a very constant speed). The motor control board contains several ICs (e.g. LM324N, HEF4066BP etc), caps and output stages on a heat sink. I've read that the output stages generates two sines waves 90 degrees out of phase. The motor voltage is during upstart initially high, and goes down to 25-30 volt.How are the artificial sine waves generated? There are two pots, called 10K and 2.2K on the board. Are these for setting the speed, i.e. the frequency of the sine waves? Or for something else?
Anybody familiar with this motor control board?
Thanks in advance
Thomas
Follow Ups:
Yes! Those pots are speed controls for 33.3 and 45 respectively. Depending on the vintage of your Axis, early ones had a resistor that was under spec'd, causing them to run slightly fast. Later versions were bang on!
Even with my 33.3 pot all the way off, my Axis still runs slightly fast, but nothing to be annoying to me,even with great pitch perception. I have the resistor part number and correct value written down somewhere. Now where did I put it???!!!!! DAMN!
Dman
Which pot is the 33.3 RPM? Also, which direction (clockwise or counterclockwise) decreases the speed?Thomas
The 33.3 RPM pot was the left one. Turned it counterclockwise almost half a turn. Speed is now 100 revolutions / 180 seconds, which should be very close to 33.3 RPM... I will record and measure the 300 test tone later also.Thanks again.
Thomas
Now I can start correcting the speed.I do have a later Axis (1990/1991) since the on/off switch is on the subchassis. I think the resistor is the input resistor; found this info on rec.audio.tech.
>
Check the resistor that is is series with the input. If it's over 27 ohms
replace it. (Early units were 47, later changed to 27.)Check the two large 15K resistors. They sometimes go high or open.
Near the rear of the board is a parallel row of about 8 resistors (with
values from 220k to 820k if memory serves) that are perpendicular to the
heatsink. Unsolder one end of each and check them out of the circuit. Many
of these are prone to failure.>
Thomas
I really haven't looked at the Axis motor control board, but if memory serves me it was attached right to the motor itself. I would suspect that there has to be some level of adjustment and those pots are it. An ociliscope would be the way to find out and dial them in correctly.Your other option would be to bypass that board all together and put in something more reliable and potentially better sounding. When I used to sell Linn we saw a lot of Axis tables with failed motor control boards. I would suspect that a viable alternative and one that could be easily retrofit would be a used Valhalla board or for that matter a Basik or Lingo. Also there are a number of DIY power supplies based on the Naim Armageddon that work really well like:
http://www.neilmcbride.co.uk/ttpsu.html
http://membres.lycos.fr/jadai/Geddon.html (french)
http://translate.google.com/translate?u=http://www.multimania.com/jadai/Geddon.html&langpair=fr|en&hl=en&prev=/language_tools (translated)
http://www.geocities.com/Hollywood/Hills/4133/lp12_faq.txt
Now I don't know because I'm assuming that like most linn products this motor is basically the same and just looking for a 50 Hz signal to generate 33 RPM. I know the pulley was different sized than either the 50 or 60 Hz LP12 but so was the innter platter diameter.
I think also there were different versions of the motor control board. The early ones were more prone to fail, and had problems with self-starting motors. I've had no such problems during the 12 years I've used my Axis (which is an Axis II).I was once thinking of using a different PSU for the motor. But as I've read, the motor control board of the Axis is really excellent, running at very low voltage (25-30 Volts) after the initial high voltage to start the platter. This was made because of the poorer isolation of the Axis' rubber suspension, and they needed a very quiet motor running. So I think I'll stick with my Axis control board.
There may be some components that have gone out of spec also (electrolytic caps). I try and see whether I can get hold of a oscilloscope. I don't know if I dare to touch the pots before I know what they are doing.
Are there any pots in Valhalla or Lingo PSUs?
Thomas
The Lingo has no pots to the best of my knowledge. The latest revision of the Valhalla had a single pot but I think earlier boards had one or two more. I once built an electronic speed control from an Audio Amature article by Gary Galo and it had a number of pots. One for gain, distortion, symetry, and speed adjustment. It took some effort to set up.
I have very poor knowledge of how these things work and also about electronics in general. To generate two artificial sine waves, there must be some kind of stable oscillator curcuit that later is amplified and then that signal drives the motor. How does the Valhalla and Lingo work to generate the sine waves? I've read the Lingo use some kind of crystal, but how does the Valhalla work?I really don't get how the Linn Axis PSU work to generate the artifical sine waves. There should be some similarity with the Valhalla, but I don't know whether the Valhalla decreases the voltage to drive the motor silently as the Linn Axis PSU does. Doesn't the Lingo decrease voltage fed to the motor after power-up?
Thomas
Well I cannot pretend to know a great deal more about how they work either but I do know both the Lingo and the Valhalla use crystals to generate their sine waves. The DIY one I build used an oscilator circuit for what it's worth.The Lingo is direct coupled and offers separate drives for both phases of the motor.
The Valhalla uses only one drive for both phases of the motor with a single capcitor to achieve close to 90 degrees phase shift between them.
I do not remember the Valhalla or Lingo changing voltages except for the Lingo in 45 RPM only - it took some time to get up to speed and change modes. Both also put out around 80 volts when measured during use. My experiments with similar circuits and motors suggest that you need at least 60 volts not only to get up to speed but to navigate difficult passages.
I don't know what the Axis motor control board does but it looks like a pretty simple circuit and I'd wonder if if even generates a proper sine wave or just makes do with a square or trangle wave which are far easier to achieve.
Maybe its Wein bridge oscillator? I'll measure with a oscilloscope next weekend, if I find one.http://sound.westhost.com/project22.htm
Thomas
Thomas, you almost make me laugh with nostalgia! ;-)
In 1974, I moved to Switzerland and I needed a 60 Hz source to run my Ampex tape recorder. I built it with an Xtal source that Ampex had made for motor drives. When I got my Linn, I got the 60Hz version and used the same source. In those days, Linn was bigtime into using the powerline only. In 1977, when back in the USA, I ran the Linn straight off the AC line, and Ivor (the terrible) came to visit one day. We discussed Xtal control and Wien bridges, and he said it was NOT the right thing to do. The powerline, to him at the time, as almost perfect. I begged to differ. Now, it would seem that he has changed his mind, and that virtually any artificial source is OK, so long as it still drives an AC motor. I think that either Xtal or Wien bridged oscillator could be OK in this case, as good as the AC line.
If you live in Europe, and have 50 Hz AC, then try this test, if you get a scope. Lock the scope to the line and monitor the artificial source going to your motor. Note if you see any phase shifting of the AC line. This is of course best seen and believed when you have a precision Xtal source, because you can see it weaving back and forth, because the 50 or 60 Hz AC is locked to the sweep and IT is weaving back and forth. It is an eye opener!
The motor initially starts with high voltage. It is 115 volt when measured between the right and leftmost connection. This drops to about 36 volts after a few seconds and the motor stops running. It starts again if you "help" the pulley by hand. So my understanding is that once the platter is up in speed, the movement of the platter itself that keeps the motor running via the belt. So wow/flutter control is mainly set by the platter spinning, while the motor only has power enough to keep the speed at 33.3 or 45 RPM constantly.I remember reading some quite expensive turntable designs have very weak motors and very heavy platters that required a manual start by turning the platter by hand.
Thomas
An XTAL is a crystal. Make sense now? The reason for low voltage is to keep the torque pulses from the motor, low . The scope comparison is not important here.
I see now. Thanks. :-)
Thomas
Well, I have no idea what an Xtal source is. Also, I will need to study the Wein bridge oscillator curcuit more to understand it. I am still curious how this Axis motor control board works. I found this on rec.audio.tech:"Motor is driven by two analog high voltage power stages,which provide two artifical generated sine waves with 90 degree phase shift.
So far I tracked down the circuitry. It consists of standard OPs,comparators and CMOS-analog-switches. A little bit tricky,as it incorporates what I call a mixed analog/digital feedback loop.Maybe a zero crossing detector on the motor-windings,which is coupled trough flip-flops and analog switches to the oscillator."Would this info ring a bell?
When I meaure, do you mean that I should measure both the AC line and the line going to the motor?
Thomas
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