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Turntable running slow even though strobe dots are steady

Discussion in 'Turntables' started by Eastham, Dec 8, 2018 at 7:59 PM.

  1. Eastham

    Eastham More Class-A than ever!

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    So I recently got a test record to bench test turntables and during the 3KHz test tone for W&F measurement I noticed it was reading about 2.92KHz even though the dots on the strobe where steady, and I'm now playing Pink Floyd - Animals and I'm noticing Rogers voice sounds kinda slow so I sped it up to the point he sounded right and now the dots are advancing forward. Turntable is a Technics SL-1500 Mk1, I've checked my mains frequency and it's rock steady at 50Hz. Any ideas what could be going on? I'm gonna put my test record back on and set the speed for exactly 3KHz, is it possible my Technics' strobe isn't accurate?
     
    Last edited: Dec 8, 2018 at 8:10 PM

     

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  2. Eastham

    Eastham More Class-A than ever!

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    Got it adjusted to 3KHz and everything sounds right, my by ear adjustment wasn't to far off either 2.97KHz! Not to shabby if I do say so myself :naughty:. I've got another 'deck with a built in strobe inbound A Dual 601 so I can compare the two and see if it's limited to my Technics or if it is something to do with my mains freqency or perhaps even the strobes.
     
  3. spicer

    spicer Super Member

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    If I'm not mistaken those tables are electronic drive and largely independent of line frequency. I also believe the strobe lamp for the Dual is not line frequency driven but has a dedicated crystal controlled oscillator circuit specifically designed to drive the strobe lamp, with appropriately spaced lines on the indicator for the frequency of the drive circuit, not the mains. I agree.. good ear, it is very difficult to set speed by ear.. seems to always be too fast when doing it by ear. On the other hand, just a little bit slow and everybody notices.
     
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  4. Eastham

    Eastham More Class-A than ever!

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    Nope, my SL-1500 has a mains frequency neon strobe with the servo drive system, I believe it was the MK2s that got fancy strobes and quartz lock, as for the Dual, I'm not sure but the schem shows a neon with a resistor across the mains lines so I think it's got a frequency driven neon strobe too. Eh, I think I just listen to too much Pink Floyd and that's how I got it fairly close lol...
     
  5. Hajidub

    Hajidub Chihuahua/Pug = Chug Subscriber

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    It's light and distance, not sure how it could be incorrect.
     
  6. spicer

    spicer Super Member

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    Ahh... about the strobe circuit, I meant the Dual but the schematic tells the story. Back in the day I bench serviced hi fi units and the particulars get jumbled. I thought it was the 601 but obviously not... one of the high end Duals though. A neon with a resistor in series across the line is line frequency driven for sure. I hear you about being very familiar with certain music but it remains you have good ears.. we all had special records we used to evaluate repairs and none of us could get the speed right, always too fast.. we used to have competition among ourselves. We had a strobe disc we'd place on the platter under fluorescent lights to check speed whether the machine had it's own strobe or not.. just in case.. the two should agree. We also had Leader speed and Wow and Flutter meters but only used them for tape machines with test tapes. Tables got the strobe disc and listening test, and end of record and auto function test if applicable. Kind of laughing to myself.. if there was any variance, and there was with anything adjustable, it was always set a touch too fast... any variation must not go into the slow region, even if in 'spec' or it would come back. With single motor cassette decks, because of the take up clutch, speed would vary a bit depending on the tape pack and it was paramount it never go slow, into the minus area of the Leader, or below frequency on the counter... that's just the way things were.
     

     

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  7. Eastham

    Eastham More Class-A than ever!

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    Thanks. I have yet to get a strobe disc. I use a Hi-Fi sound record and I'm honestly not sure if the record is accurate though I'm sure it should be. As for wow and flutter I use an application on my PC that I found on the tape heads forum, my SL-1500 measures about .05% W&F which taking into account accuracy of the software and record I'd say is about right. My SL-1500 has a nasty habit of drifting forward in speed, I could set it so the dots would be still and at the end of a record they'd be advancing slowly, even after rebuilding the driver board and throughly cleaning the pitch pots. Must have been interesting working in a shop and stressful at times I'm sure, seems the days of fixing electronics are dying now.
     
  8. spicer

    spicer Super Member

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    Yes, electronic service for consumer units has largely died. The W&F measurement you have seems good to me. We would measure it on high end units that had mechanical drives and listen to a bit of a record we were familiar with on any unit before it was shipped. The speed drift you observe was not too uncommon, one of the problems if the machine had a strobe as it would report to the owner speed variations they wouldn't notice otherwise... something I had to explain a time or two. I have a belt driven Dual 1245 that speeds up just a touch from a cold start to warm, a few minutes. I either ignore it or let it warm up a couple of minutes. It's not uncommon with the mechanical speed control of the Dual, noticed them on the bench... it's a small variation but it is there.. I keep it adjusted so it just crawls ahead when cold and when warm it's practically still. Quartz locked should be right on if the quartz hasn't drifted... but if it has, so long as it stays, the speed will lock to it, for good or bad. When quartz lock first came out it was a switchable function, but soon that was no longer standard and one could not tune the speed, at least not from the top... I'm sure the benefits outweigh any drawback but I always liked to be able to adjust the speed a bit. I wouldn't even begin to suggest a reason for the speed drift on your SL-1500 but I'm sure it's something to do with the temperature of some component in the circuit.. depending on how much it is drifting upward it could be doing that and be within it's specifications. One thing I might say is be sure the center spindle bushing is clean and lightly lubricated, sometimes old lubrication will change it's characteristics over a few minutes of use and affect speed. On a quartz locked table, they hardly ever drift... might be slow or fast but hardly ever drift.
     
  9. Eastham

    Eastham More Class-A than ever!

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    I did read in the SM for my 1500 that mains frequency drift can cause strobe drift aswell which would explain the small ammount of drift I have during playback but I think it would have to be pretty off to account for the strobe seemingly being slow. I recently had a Pioneer PL-300 that was quartz locked, solid 'deck but definitely built to a price, never thought to see what its speed was with my record... I re-lubed the main spindle wit some SAE-20 though maybe friction and heat could be a cause, as the spible heats up the oil becomes thinner causing less friction?
     
  10. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    In a regular non-quartz servo table, it's pretty common for the speed to drift upwards after the table has been on for a few minutes, or by the end of a side or two. Usually the drift will stop after a half hour or so when the internal temperature changes start to slow down as everything reaches some equilibrium.

    A typical mid-seventies servo drive uses a zener diode to set the reference voltage, and V_z is typically quite temperature dependent.

    Some fancy voltage reference ICs consist of little more than a zener diode in a tiny "oven" with a circuit that maintains a very tight temperature range at all times. I've always thought it'd be fun to try to modify a non-quartz lock table with a modern voltage reference circuit just to see how stable a speed you could achieve without quartz.
     
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  11. ripblade

    ripblade Super Member

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    I have a Micro Seiki DDX-1000 with the same problem. There's a strobe frequency calibration routine in the service manual to correct this. Perhaps the Technics has one as well?
     

     

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  12. spicer

    spicer Super Member

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    That was my experience too... upwards speed drift with non quartz tables. VCRs started using frequency generators as feedback built into the capstan motor to keep them on speed, seems like there were a few Japanese tables that had that too before quartz locked became the buzz word. Some VCRs back in their golden years had some eloquent and sophisticated motor control circuits.. Mitsubishi comes to mind, they had a high end unit with five motors all system servo locked to provide a noise free picture in any mode.. still, frame advance, forward, backward, whatever... with the bottom cover removed and in the still mode you could move the picture forwards or backwards a frame at a time by moving the capstan motor flywheel forward or back against it's synced position, it would electronically cog to the next frame with all the other motors following. Fascinating, and completely packed with IC loaded circuit boards, each board stuffed with parts.
     
  13. WaynerN

    WaynerN AK Subscriber Subscriber

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    Hopefully your reading the 50hz strobe dots, not the 60hz.........My Technics have two sets for 45 and 33 1/3, 50 and 60hz.

    I;m sure your reading the right one, just had to bring it up.
     
  14. spicer

    spicer Super Member

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    That table might have it's own strobe drive circuit operating at a frequency other than the mains... sounds like it. I assume they do that to make it independent of any external light sources. One of the uses for the generic strobe disc we used was to verify the table strobe, if the table strobe was adjustable it would be adjusted to the external disc. It was assumed the mains frequency under fluorescent lights was accurate.. which it is, the power company, the grid, has a real need to maintain line frequency accurate beyond concerns for the customer.
     
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  15. spicer

    spicer Super Member

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    Yes, the oil becoming thinner making it spin easier. I think I would use some lighter machine oil instead of the SAE-20 but not sure if that is a concern or not. It is true that the mains frequency drift would cause the strobe indication to drift but it is not reasonable to expect the mains frequency to drift.. at least in my opinion and experience. The power company must maintain accurate frequency or it would cause the grid and generation stations all manner of serious problems, for them it is critical.
     
  16. Eastham

    Eastham More Class-A than ever!

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    Yep, the 50Hz are the top ones right?

    I found a bottle of the original factory oil in a cut out under the platter of my 1500 and it felt pretty viscous and I read somewhere that SAE-20 is what Technics recommends. I do have some thinner SAE-5 on the way to lubricate the motor in my Dual when it gets here, I'll try some of that in the spindle of my 1500.
     
    Last edited: Dec 9, 2018 at 7:41 PM

     

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  17. WaynerN

    WaynerN AK Subscriber Subscriber

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    Correct.
     
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