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Topic: Correcting speed variations with turntable (Read 10417 times) previous topic - next topic
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Correcting speed variations with turntable

My Thorens TD 125 MK II is problematic for a variety of reasons, not the least being speed consistency.

I use a KAB Speed Strobe to verify speed, and I often find that is has changed during the course of a playing/recording session. This is after warm-up.

I've received various advice about this.

1. Motor spindle might be bent (it does wobble a bit, and the built-in strobe seems to reflect that wobble).
2. Belt (it's relatively new)
3. Dirty strobe electronics

and

4. Electrical source is inconsistent.

I don't have any way of measuring this, but given that I live in provincial Thailand where power outages are not all that uncommon (especially during the rainy season) this wouldn't surprise me.

I learned about the following product, which is designed for 120 VAC. I'm wondering if there might be a 220 version of this available. If I can figure out the product name, maybe I can find it in Bangkok.

Speed Box Mk II or III
http://www.soundofthewood.com/speedboxwhitepaper.html

Thanks,
gritto

Correcting speed variations with turntable

Reply #1 –
NOTE - The following is mostly a lot of speculation....

Quote
I use a KAB Speed Strobe to verify speed, and I often find that is has changed during the course of a playing/recording session. This is after warm-up.
  If you need to use an instrument to detect the problem... If you can't hear the variation...  You can simply ignore it.      If you can hear it and/or fixing the problem is very important to you, you might contact Thorens to see if they can evaluate and possibly repair it (assuming that you are also willing and able to pay for the shipping and repair).

Quote
1. Motor spindle might be bent (it does wobble a bit, and the built-in strobe seems to reflect that wobble).
  That would explain once-per-rotation "WOW", but it should not cause "speed-drift".  (The turntable's strobe usually runs off of the line-frequency.)

Quote
3. Dirty strobe electronics
  Except, your KAB strobe confirms the problem.... Both strobes (apparently) agree.    (The turntable's strobe usually runs off of the line-frequency.)


Quote
4. Electrical source is inconsistent.
  Most turntables have a regulated power supply, and an electronic speed control/reference making them fairly immune to slight power fluxuations.  Some turntables do use a synchronous motor.  With a synchronous motor, the speed is determined by the power line frequency and there is no speed control.  If your turntable has a speed control, the speed is not determined by the power line frequency. 

You might try running it from a UPS (an Uninterruptable Power Supply for a computer) that's disconnected from the wall-power.  If you get identical results with the UPS, the problems are not caused by the power-line variations.

Quote
learned about the following product, which is designed for 120 VAC. I'm wondering if there might be a 220 version of this available.
  I don't know, but it will only work if the Thorens uses a synchronous motor.

Correcting speed variations with turntable

Reply #2 –
Quote from: DVDdoug on
Quote
learned about the following product, which is designed for 120 VAC. I'm wondering if there might be a 220 version of this available.
  I don't know, but it will only work if the Thorens uses a synchronous motor.

The TD125mkII does use a synchronous motor, but it is not driven by the mains line frequency. Instead, there is a built-in oscillator which generates the required frequencies for the various speeds. I have no idea how this oscillator works, but it's possible that variations in the mains voltage might affect the accuracy of the generated frequencies. Or maybe the oscillator has some components that are drifting in value due to age (eg. old electrolytic capacitors can get a bit vague as they change temperature). Your best bet would be to contact Thorens direct for advice.

By the way, the TD125 was a really great turntable in its day, and is worth getting sorted. (The design of the famous (infamous?) Linn LP12 is obviously very similar - some people feel that the LP12 was a thinly disguised copy).

Correcting speed variations with turntable

Reply #3 –
Hey gents,

Thanks for the feedback. I tried to contact Thorens recently, but the email page sends me to a "local" Thorens rep in the country where I live. This is a problem, because they are a few hours away by bus, train or mail, so bringing the table in is hard. The other problem is communication. I tried to explain my issues over the phone, but they didn't really get it (language etc). Thorens is only a sideline for them.

I need to find a direct number to Thorens where I can speak to a technical person who can understand my questions in English. So far, it doesn't look like they want to help people like me.

Correcting speed variations with turntable

Reply #4 –
As the TD125 uses an oscillator driven synchronous motor, then the oscillator is not 100% stable - it drifts with temperature.

The likely culprits would then be:
* Dirty speed control pots
* Resistors in the feedback circuit that are exhibiting a greater temperature dependence than when first manufactured.

Capacitors may also have changed value, but they are less likely to have a temperature dependence. Electrolytic caps are rarely used in circuits with highly deterministic values, as they are not reliable in that sense. They just need to be "big enough".

I would start by cleaning the speed control pots with a typical spray cleaner. That's cheap and a harmless test. Otherwise, you need to write out the oscillator circuit and determine the points of greatest gain change per unit of resistance and replace those parts.

Correcting speed variations with turntable

Reply #5 –
BradPDX, great info, thanks!

My turntable is in a small office room with an air conditioner. Especially during the winter, I don't always use the air conditioner, and sometimes it is less than efficient (needs more refrigerant, maybe). Point is, I can see how the table and surrounding room temperature could play a roll, having read your post.

I wonder if I can clean the pots by spraying around the knob, instead of opening the table up. Not sure I can get to it from underneath, for that matter. Maybe I'll start with compressed air and see what happens.

-gritto

Correcting speed variations with turntable

Reply #6 –
Quote from: gritto on
I wonder if I can clean the pots by spraying around the knob, instead of opening the table up.

As I understand it, the knob is connected to some sort of arrangement of mechanical linkages which operate the switches on the circuit board "by remote". So spraying around the knob won't achieve anything.

Here are a couple of links that might be helpful:
http://www.retrohifi.co.uk/thorens_125.html
http://www.theanalogdept.com/td125_mkii_circuitry.htm

Correcting speed variations with turntable

Reply #7 –
Thanks, cliveb, good links. 

Thinking about BradPDX's suggestions I've noticed an interesting thing.

Some days, like today, the speed barely fluctuates at all (at least based on the times I check). Other days, it fluctuates a lot. I can't really correlate ambient temperature changes yet.

Going along with Brad's idea, maybe the the speed issue depends on where the pot is set (dead, dirty spot?).

I truly appreciate all the feedback I've gotten.

-gritto

Correcting speed variations with turntable

Reply #8 –
Looking at those pictures above really brought back memories.  In the mid-70s, I had a TD125MkII with SME3009 and AT "shibata" cartridge and it sounded great.  But, interestingly, I had a problem similar to yours that started about a year after I bought the table.  The motor would continually slow down over time such that I had to move the red speed control dial gradually to the right until it couldn't go any further.  Then, I'd have to disassemble the table from the bottom (after removing the arm board) and adjust one of the pots on the circuit board (I don't recall which one; I experimented to find the right one) so that the red speed control when placed at the extreme left would be at the correct speed.  This gave me the most time (a few months) between having to do this all over again.  Not a small effort, since breaking down the table, adjusting it, and reassembling took the better part of an afternoon.  Eventually, I decided to trade it in, but I always wished that I'd had it repaired instead.

I am unclear on the problem you are describing.  Is it that the speed as indicated by the strobe drifts a bit (up/down) during routine play?  Is it enough to be audible?  Is it happening from one album to another or during the same song or album side?  I recall other belt-driven tables I've owned with strobes where small variation from album to album was somewhat common; maybe owing to varying groove depth?  Never enough to bother me though I might have to tweak the speed up/down a bit, it always averaged around the same point.  I'd say this is typical unless the table has some sort of electronic "speed lock" which I don't recall was the case with the Thorens (more common with direct drive types).

Have you tried lightening up on your arm downforce?  Try it toward the bottom of the cartridge's recommended range.  That would lessen friction between the stylus and record surface and may reduce the speed variations.

In the end, it may be changes in your line voltage causing the problem.  Since you say local power is not particularly reliable, it's possible that the voltage coming off the wall goes up/down by 5-10% over the course of your listening session.  Due to power demands in your local area, it is likely this variance would change over the course of the day (in evening more people turning on TVs and household lights) or even from day-to-day (on hot days more people using air conditioners).  I doubt the oscillator in the TD can smoothly handle that kind of variation.  An external power conditioner may be able to tame it, but you'd want to check that it can adjust over a wide enough range.  I've seen meters you can plug into a power outlet that will visually indicate via LEDs whether the voltage is above/below nominal specs.  You may want to visit a local hardware/electronics store to see if you can find a similar tool.  I suppose a standard voltmeter would serve the same purpose.  Just be careful putting the prongs into your wall jacks!  At least that give you an indication whether the problem is in the mains or somewhere in your system.

Correcting speed variations with turntable

Reply #9 –
Quote from: Apesbrain on
Have you tried lightening up on your arm downforce?  Try it toward the bottom of the cartridge's recommended range.  That would lessen friction between the stylus and record surface and may reduce the speed variations.

I do not recommend this. The vast majority of cartridges track best at, or close to, the maximum specified downforce. Lowering the tracking force increases the danger of mistracking - which you REALLY want to avoid, as it can permanently damage the vinyl grooves.

Quote from: Apesbrain on
In the end, it may be changes in your line voltage causing the problem.  Since you say local power is not particularly reliable, it's possible that the voltage coming off the wall goes up/down by 5-10% over the course of your listening session.

This seems possible. I'm not an electronics expert, but looking at the picture of the circuit board, it seems clear that the power input is AC (presumably from the mains). I see a rectifier and some smoothing capacitors, but nothing that looks like a regulator (or any zeners that would do regulation). So it seems quite likely that the DC voltage driving the oscillator will vary along with the mains voltage. Whether that affects the frequency generated by the oscillator is presumably down to the design of the oscillator circuit. Are there any electronics wizards around here who can help?

Correcting speed variations with turntable

Reply #10 –
Quote from: cliveb on
I see a rectifier and some smoothing capacitors, but nothing that looks like a regulator (or any zeners that would do regulation). So it seems quite likely that the DC voltage driving the oscillator will vary along with the mains voltage.

There is Zener diode Z1 stabilizing VDD supplying oscillator which frequency is generated with quartz X1. Totally it should be more than enough.
But it seems that diagram doesn't correspond to layout photo (where we can find 9 potentiometers).

Correcting speed variations with turntable

Reply #11 –
Hey, all, interesting thoughts.

Apesbrain, you really did some work to try to fix your strobe!

I actually have two tables (got the second when I bought the used SME arm on eBay). If memory serves, the strobe was tricky on that one too, which is what motivated me to get a KAB strobe. It had a strange fuse plate and only ran on 120/60. I think this caused the motor to burn out when I ran it with a step-down transformer. At  any rate, I swapped arms, and now struggle with my original table.

The speed fluctuation is not noticeable as wow/flutter, but when I measure with the KAB every record or so, I sometimes find that the table is faster or slower, and the degree of change varies too. I'm not sure how to equate toe strobe bar movement relative to speed, so I can't say how much faster or slower the recording might be.

For casual listening, it's not a big deal, but I am archiving, so I'd like to get it right. Also, I'd hate to have a picky guest comment that the music is a tiny bit off key due to speed, or that the BPMs are off for certain kinds of music. I could always adjust the speed digitally, but that causes some digital distortion.

I should try to find out how subtle (or not) the measured difference is.


As far as testing with stylus drag, I find that I'm always using the same side of one old record (it's the only one I guess I don't care about ruining) several times a day. Knowing that records really shouldn't be played that often, I wonder if a test done on the same groove an hour later than the first test might be affected by the over-used vinyl. I doubt it, but I guess that would throw the measurement off if it did! 

At any rate, day or week, or time of day, doesn't seem to make a difference to stability of speed. Yesterday, a week day, the table seemed stable almost all day long, but today, another work day, it is fluctuating.


I'm hearing different opinions about my problem. Some say it's probably the hardware (dirty or old), some say it might be the electricity, and some posit temperature sensitivity. I'd like to contact Thorens directly about this because the local representative in my country did not respond to my email (via the Thorens site) and is too far away for me to just pick up the table and go there. I tried calling them, but communication was difficult.

So, can anyone help me with a phone number that I can use to reach a technician or knowledgeable person at Thorens?

Thanks,
-gritto