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12.4.185.2
Well, it seems that maybe one or two of you are actually following my dilemma here.
So the journey continues- I got the nerve up to actually measure things out on the Linn Axis PSU, and have discovered what I believe is the REAL problem!
The oscillator(s) is/are not set correctly. This may come as no surprise to those who have owned an early production Linn Axis table; Linn themselves owned up to the fact very early on (as I stated in a previous post) that they "goofed". They suggested some resistor value replacements. I did those many years ago; while it made SOME difference, certainly not enough to make things right (meaning CORRECT speed).
Ah, yes! The measurements! Here they are-
Spec states that for 33.3 rpm, the PSU should be outputting 50hz. As as measured at the output to the motor, I get 48.035Hz (0 phase), and 48.040Hz (90degree phase). Hmm... Not quite right (3.93% slow, if I did the math right!).
Spec for 45 rpm should be 67.5 (45rpm), but as measured at the output to the motor, the number is 66.100Hz (at both the 0 phase and 90degree phase). Again, not quite right (2.07% slow).
So, the tricky part. While I have a great respect for Electricity, some time to "play" with this thing, and of course a schematic, I am at a bit of a loss with this type of oscillator (a quadrature oscillator), and obviously there needs to be some changes made.
Anyone willing to help out?
Dman
Analog Junkie
Follow Ups:
Hi Dman,
Go ahead and post the schemo. I'll see what I can figure. What they must mean by quadrature oscillator is an oscillator which has two outputs 90 degrees apart.
By the way the frequencies of both outputs should be the same.
By the way one of the reasons for these devices is to be able to adjust the frequency to get the speed right. Did you adjust it using a strobe on the turntable.
Phil
While the voltages seem fine (I know, Synchronous motors rely on Freq for their speed first), the frequncies are off.
As stated previously, the spec states that for 33.3 rpm, the PSU should be outputting 50hz. As as measured at the output to the motor, I get 48.035Hz (0 phase), and 48.040Hz (90degree phase). Hmm... Not quite right (3.93% slow, if I did the math right!).
Spec for 45 rpm should be 67.5 (45rpm), but as measured at the output to the motor, the number is 66.100Hz (at both the 0 phase and 90degree phase). Again, not quite right (2.07% slow).
BTW- I can't reply back to your email to me, as it is from AA and not your email.
Dman
Analog Junkie
Hi Dman,
I would not worry over the exact frequency of the oscillator. The main reason to drive a synchronous motor with an adjustable oscillator is to be able to adjust speed. There are second order belt effects which make it difficult to get an exact ratio between the motor pulley and the turntable pulley. I have a R-O-K Rondine 2 which has a mains driven synchronous motor so I've become painfully aware of this. Driving the motor with an adjustable frequency oscillator gets you out of that pain as well as permitting electrical speed changes.
The two phases for each speed MUST have the same frequency and should have a fixed phase difference (usually 90 degrees) to produce the rotating magnetic field in the motor. If you are seeing different frequencies at the two phases it is a measurement error. In some philosophy class you may have heard that you can't observe anything without altering it. This is an example of that. I am sure that if you were to display the two phases on a dual trace scope you find them to be coherent (and hopefully 90 degrees apart).
I recommend that while observing the platter speed at 33 1/3 RPM you adjust R37 to get it right on. After the 33 1/3 speed is correct switch to 45 and adjust R 35 for the correct speed.
If every thing is quiet, you are done. If the motor hums, check the voltage at each phase. I would expect them to be equal. If you can measure phase, tweak the values of R33 and R 34 in 33 1/3 RPM to get the phase correct (90 degrees) or for the least hum from the motor. After that repeat with R 32 in 45 RPM.
Do be VERY careful. The power supply is not isolated from the mains. Ideally you should use a 1:1 isolation transformer on the power line. This is almost a must if you are going to connect a scope to the motor drive signals. If you are just listening to the motor and tweaking for minimum hum you can do it without the transformer if you are very careful. Just remember that non-isolated power supply can bite you.
Phil
Well, therein lies the issue- both the pots for 45 and 33.3 are adjusted full up for the fastest speed...
An iso-fmr I DO have! No motor hum (never has, actually)...
Weird (and somewhat frustrating) stuff...
Dman
Analog Junkie
Hi Dman,Not a big deal really. The 33 1/3 adjustment pot, R 37 is in series with R 38. Measure the maximum resistance of R 37. Subtract 1/2 the resistance of R 37 from the current value of R 38. Chose the next lower standard value and replace R 38 with it. Try to use a metal film resistor. Turn on the machine in 33 1/3 RPM and adjust the speed with R 37.
Switch to 45 RPM and adjust the speed with R35. If R 35 is reasonably centered for the correct speed, you are done. If it is still slow do the same drill as above with R 35 and R 36.
Let us know how it goes.
Phil
Edits: 04/16/15
Thanks, Coffee!
I'll get to it (hopefully this weekend, as D-wife is on her Sojourn to Canada for a week! Sigh...), and see how it pans out!
Dman
Analog Junkie
Hi Dman,
So, did you get to it? Is the machine on speed now?
Phil
Weirdness continues in this world! I thought I had posted the results a last week, but then my post magically (or would that be "majiK" being a Linn-like 'table?), but it worked out so well, I don't mind "re-sharing"...
I replaced the resistor as you suggested, and got 50.00 Hz (33.3rpm) bang on with the trimpot around the 9 o'clock position. Same thing with 67.5 Hz (45rpm, setting that trimpot around the 11 o'clock position).
The final test was making sure that the mechanical side of things equaled what the electrical was doing. Using the Cardas Sweep record, I cued up the 1 kHz tone, and as you can see from the pic, the accuracy is pretty good (same thing with 1 kHz played at 45 rpm, being 1350 Hz).
Thanks again for your help and suggestions! I should have thought of it, but sometimes it simply takes a second set of eyes to verify things!
Dman
Analog Junkie
I only have the schematic as a PDF, which is a pain. Drop me a line (email) and I can send it to you.
Otherwise, if you look at Vinyl Engine, it's in there (where I got it).
Thanks for the offer to help!
Dman
Analog Junkie
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