Technics SL-P555 CD Player - Circa 1989 - MASH DAC - Restoration Thread

Discussion in 'Digital Sources' started by crooner, Dec 9, 2016.

  1. crooner

    crooner Tube Marantzed

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    Well, spent all yesterday evening working on this thing. Removed the motherboard and soldered in the "stealing points" after the R-C filter instead of directly out of the DAC.
    However, I noticed a much obvious thing and it's that the motherboard wasn't grounded. Duh!
    How did this happen?
    Well Technics grounds the motherboard only at the RCA jacks,and those I had completely removed in order to install my new ones. The circuit board doesn't have any other grounding points. The screw holes are not grounded.
    So, I went ahead and added a ground cable at the spot where the RCA output ground was and attached that to my star grounds.
    At this point it was almost midnight so I didn't test anything.

    So I'll fire it up again later today or tomorrow. Hopefully the noise will be gone!
     

     

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  2. Dandy

    Dandy Super Member

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    Amazingly good discipline, but it leaves us anxiously waiting... :)
     
  3. crooner

    crooner Tube Marantzed

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    LOL. I'll try to fire it up tonight. I have a feeling it will work this time. If not, I'll toss the whole darn thing in the trash. Perhaps save the turret boards and transformer for another project. o_O
     
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  4. crooner

    crooner Tube Marantzed

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    Here's an update, some good news and bad:

    - The Good news: grounding the motherboard to the chassis got rid of the digital hash noise.
    - The Bad news: gain is pretty low, so there is more attenuation that is needed in the signal path. I need to look at the schematic again and take out a series resistor in the signal path. Easiest way would be to "steal" the signal directly off the DAC chip again. I would still have the 47K and parallel cap, but those would be ahead of the signal and would not attenuate it.

    I listened to it a little bit in it's present state and the sound is rolled off, muffled, no highs and a lot of lows! So this thing will need a lot of tweaking and adjusting to get it to sound right.
    There is also more hum that I would like, and again this is probably ground related. Probably doesn't help the heaters are AC in the 6CG7. I used Lampizator's approach of tying the heater to 100 ohm resistors to ground, as way to cancel out hum. Perhaps I should twist the heater wires and move them away from the motherboard to reduce hum. Rectifying the heater voltage was an alternative I was considering but it would have increased the complexity of the power supply...
     
    Last edited: Feb 9, 2017
  5. Dandy

    Dandy Super Member

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    What have you done differently from Lampizator? Or does he just gloss over these issues?
     
  6. crooner

    crooner Tube Marantzed

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    The main difference is that I am using the player's internal de-emphasis and Low pass filter networks and then feeding the tube output stage. Lampizator simply takes the signal directly from the DAC.
    This player has an emphasis light on the front panel (a rare feature), so I just wanted it to be able to continue to apply emphasis as needed.
    This is an important feature for me to preserve as I collect early 1980's CDs, many of which were recorded with emphasis.
    The emphasis flag is encoded in an area of the CD that is virtually impossible to access by a computer CD ROM drive (it was not in the orange book specifications), so the player is invaluable in discovering discs recorded with emphasis and not documented anywhere.

    Another difference is that I am preserving the player's internal headphone amp circuit. So I am tapping the output from the tube stage and feeding the headphone amp with it.

    The best course of action would be by process of elimination to remove stuff from the signal path. I'll start with several caps and resistors right before the Low Pass Filter and also by eliminating a loop going back to the filter, which was probably used for feedback by the original "Class AA" buffer stage. I'll see if it cleans things up.

    I'll also work on re-routing all secondary wires carrying AC from the toroidal transformer. These can couple noise into nearby circuits. Specially the heater cables.
     
    Last edited: Feb 9, 2017
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  7. crooner

    crooner Tube Marantzed

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    Little update here:
    I connected the signal directly to the output legs to the DAC. I then snipped a series cap and resistor right before the emphasis/low pass filter and jumpered them, effectively taking them out of the circuit. The sound improved considerably, it opened up with more extension. However, it was rather distorted when the emphasis circuit was engaged by a disc recorded using it.
    I ended up removing the low pass filter from the circuit.

    I think the built in filtering was designed for the output impedance of the original OP-Amp stages, so they don't seem to work very well without them.

    The ideal solution is going to be to build a new filter network with discrete components on a separate board fed directly from the DAC output pins. I will need to devise a way for the trigger signal from the logic circuit to engage the emphasis filter (additional cap in parallel) when needed.

    When a disc recorded with emphasis is played the detected flag generates a voltage that feeds a transistor on each channel. These transistors will switch in the additional emphasis components for as long as that disc is played.
     
  8. Audioraven

    Audioraven Active Member

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    Any updates, Crooner?
     
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  9. Johnyboy

    Johnyboy New Member

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    Hello how did your project went?
     
  10. crooner

    crooner Tube Marantzed

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    This project is shelved for the time being. I am caught up in other things at the moment. I hope I can get to it this year!
     
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  11. Johnyboy

    Johnyboy New Member

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    Thanks for the update
     

     

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  12. Reamonnt

    Reamonnt New Member

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    Is grounding the motherboard to the chassis easy to do or a difficult task. I have this player and limited electronics skills.
     

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