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SL-1300 MK2 78 rpm mod

Discussion in 'Turntables' started by toups, Mar 10, 2015.

  1. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Technics SL-1300 MK2 78 rpm mod

    I've made an interesting discovery regarding modifying a Technics SL-1300 MK2 to add a 78 rpm mode. (I am referring to the MK2 not the original model, the original model mod is well known and involves an added resistor.) It may be some time before I get a chance to dig into one of my units, but here is the information I have found.

    The SL-1300 MK2 uses the DN860 chip to generate the reference frequencies needed for 33 and 45 rpm speeds. Note that this appears to be the same chip that is used in the SP-10 MK3 with the variable speed control that does 33, 45 and 78 rpm. From the service manual for the SP-10 MK3, it looks like pin 12 can be used to select the chip mode that generates the frequency needed for 78 rpm.

    The mod might be as simple as adding a switch from the chip side of R205 to ground (similar in function to S201 and S202).

    However, there are two nuances to consider.

    1) One is that on the SL-1300 MK2 the 33/45 selection modifies the auto-return time constant. Not sure the impact of 78 rpm on the auto-return function. (Although I assume no impact if manual return is used.)

    2) The other is that the SP-10 series uses different divider resistors for 33, 45 and 78 rpm selections on the AN-660 control circuit (IC203 on the SP-10 and IC202 on the SP-1300 MK2.) The SP-1300 MK2 uses a single resistor configuration for both 33 and 45 rpm. This divider may or may not be sufficient for stability or locking at 78 rpm also. Obviously, it may not be optimum at each frequency, but might be adequate. If so, no modifications may be needed to this part of the circuit.

    If anyone knows anything about points 1 or 2, I would appreciate some feedback.

    Although depending on the answers to 1 and 2, the mod may be as simple as adding a switch, a cleaner implication would involve the adding of a simple circuit to allow selection of the 78 mode by pressing both 33 and 45 buttons simultaneously. Also, with a little additional work, the display could display the correct number 78 when that mode is selected. I've worked out the details, but it may be a while before I can do it. The circuit details can be posted once I see if the mod works as desired.
     
    Last edited: Jul 30, 2015

     

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  2. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Update, opened up one of my SL1300 MK2s and removed the main circuit board and managed to hook it up to the motor by judiciously placing the various components near each other.

    Momentarily shorting pin 12 of IC201 (DN860) to ground which is conveniently located on pin 11, puts the turntable in 78 mode as verified using a strobe disk. Note the +/-9.99% speed variation still works as desired at 78 also as expected.

    Here is the patient on the workbench:
    [​IMG]

    Slow photo showing normal 33 mode:
    [​IMG]
    One can just make out that it is in 33 mode.

    Now photo showing "new" 78 mode:
    [​IMG]
    I was urprised I was able to get the strobe photos just by using a slow shutter speed and normal fluorescent lights.

    Note that without modification, the speed indicator will show an odd indication of
    [​IMG]

    Don't know if this might impact the auto functions.

    I've designed a simple circuit that will allow the pressing of both the 33 and 45 buttons to put it into 78 mode if they are simultaneously released. I also have figured out how to have it display 78 but that requires some cutting of traces and the adding of some jumpers and diode logic.

    Of course just adding a simple switch to momentarily short pin 12 to ground appears to be all you need if you don't want to get fancy.

    More later.
     
  3. Montycat

    Montycat AK Subscriber Subscriber

    Messages:
    17,401
    Location:
    Vancouver
    Very impressive! I wonder if this might work on some of the other models. I'm sure some share similar circuitry.
     
  4. kermit z

    kermit z Loud Music saves Lives!! Subscriber

    Messages:
    9,506
    Location:
    Denver CO
    Very interesting indeed. Thanks for the write up.
     
  5. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Completed full mod! Added a circuit board that allows releasing both the 33 and 45 buttons simultaneously to get it into 78 mode. Also cutting some traces and adding some diode logic allows the display to read 78 when in 78 mode. A bit of work, but well worth it! Hope to post more details later, maybe this weekend if I get chance. Need to shake it out, but don't expect any issues. Initial tests were great!
     

    Attached Files:

  6. KentTeffeteller

    KentTeffeteller Gimpus Stereophilus!

    Messages:
    25,986
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    Love this, thanks for sharing it. Appreciated.
     

     

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  7. stewy

    stewy Active Member

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    225
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    That's awesome, thanks for sharing!
     
  8. revox-b77

    revox-b77 AK Subscriber Subscriber

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    Very nice work!
     
  9. jasonsong86

    jasonsong86 Super Member

    Messages:
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    Omaha, NE
    Excellent work.
     
  10. eb2jim

    eb2jim Super Member

    Messages:
    1,556
    Bravo!
     
  11. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Sorry, more details may need to wait a week or two. Had to fix the suspension for this particular table. http://www.audiokarma.org/forums/showthread.php?t=672162

    Just moved it to the primary system for further testing and use. Suspension and 78 mode seem great at the moment. Need to build a pre-amp with switchable equalization for the 78s.

    Have a SP10 that needs a new plinth, but since I have the 78 mod working that project may wait a while. After the repairs and mod (suspension, tone arm lift fix, and 78 Mod), the SL-1300MK2 remains one of my favorite turntables.

    Too many projects, too little time.:D
     

     

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  12. RCA guy

    RCA guy New Member

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    15
    Location:
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    Great sleuthing! How does one mod an SP-25 to add 78rpm?
     
  13. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Doesn't look like that chip set supports 78 so there is not a simple solution.
     
  14. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Whoops, guess I never posted the full mod that allows one to press both the 33 and 45 buttons to go to 78 mode and allows the display to display 78. Need to clean up the documentation and will post if there is some interest. Involves a small add on circuit board and some cut traces and added wires to the original board.

    Too many projects, too little time.
     
  15. pyp

    pyp New Member

    Messages:
    9
    Hello Toups
    I have just restored a SL1300 MKII technics (classic broken arm lift),
    and would now be very interested in the 78 rpm mode that you have developed successfully !
    If you could give details about the electronic modifications made (for simultaneous selection of both 33 + 45 commands)
    I would be very grateful…
    cordially
    Peter
     
  16. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    I'll see if I can dig the information up later this weekend and post it.
     

     

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  17. pyp

    pyp New Member

    Messages:
    9
    very kind of you!
    my main difficulties are in the treatment of 78 rpm mode :
    - command (45 + 33 simultaneous switch control)
    - speed display
    - pitch control
    - automatic arm return
    thank you for sharing your experience
    (so impatient to test it on my own turntable...)
     
  18. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Quick test: as mentioned in an earlier post: pin 12 of IC201 (DN860) to ground which is conveniently located on pin 11, puts the turntable in 78 mode. Display will be odd but this will verify operation.

    Will post more information later tonight at https://stereo.toups.info

    Will post back after info is uploaded.
     
  19. pyp

    pyp New Member

    Messages:
    9
    78rpm command.jpg 78rpm display.jpg 78rpm command.jpg 78rpm display.jpg Good evening Toups,

    What do you think of these proposals:

    1) For 78rpm control: simultaneous contact 33 + 45 rpm (on touch control)

    Integrated circuit divider IC201 (DN860): pin 14 (33rpm), pin 13 (45rpm), pin 12 (78rpm)

    2) For 78rpm display D203 (GL 7R204):

    7 = segments a, b, c, + g (in France!)

    Interest: does not require a change

    shunt pin 5,9,10,11,13,17 & 20 through R232 (68 ohm),
    shunt pin 4,12,18 through R210 (150 ohm),
    Only adding the shunt pin (14 & 19)? through R (100 ohm for 11mA per segment)

    I did not find the pinout of the integrated circuit display D203 (GL 7R204), assumptions made according to the display 33 & 45, but to check ...

    Thank you in advance for your enlightened opinion!
     

    Attached Files:

  20. toups

    toups Well-Known Member

    Messages:
    889
    Location:
    Greater Boston Area
    Looks like we are thinking in similar ways. My incomplete notes can be found at https://stereo.toups.info/SL1300MK2mod/

    Regarding the circuit, need to look at it carefully at your version but no more time tonight. However, I only wanted the lines to go low momentarily so that is why I used the circuit I did. The trick is that the 78 line stays low for a short period after the 33 and 45 switches are released.

    I hope to finish the document this weekend.

    Oh my version uses the "American" style 7 without the cross bar. However, I think that cost me an additional trace cut to implement. Somewhere I have more documentation on the displays but need to do some archeology on my files to find it.

    Need to also find the spreadsheet with the LED resistor calculations but I think the replacement values are correctly shown in red on the board layout version. (I want to make sure that is the final as-built design.)
     

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