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'83 Yamaha R-70 Restoration - Novice First Project

Discussion in 'Solid State' started by guy232, Nov 10, 2018 at 4:28 AM.

  1. guy232

    guy232 New Member

    Messages:
    27
    Location:
    Melbourne, FL
    r70_face.jpg
    R-70 Service Manual: https://docdro.id/P9uWhrM

    Started this project with zero experience with electronics/repair. Did research via google, youtube & forum searches and bought the tools i'd need to re-cap the entire main PCB on this unit. A majority of the caps showed signs of leaking/discoloration so I thought it may be worth using this unit as my first project.

    Everything went pretty well, spent a lot of time off & on learning various things. Completely disassembled the unit including the heat sink. Cleaned the boards up a bit, installed all of the new caps, installed new AC cord, re-assembled everything, all parts accounted for.

    Only main issue encountered was that the filter caps I bought were too large so I had to extend the leads. Used some heavy gauge insulated solid copper wire to extend leads, then fabricated up 2 rubber stands, applied adhesive to give the caps support.

    Plugged into DBT (150W bulb), very faint brief glow of filament which quickly went away. Unit was operational.

    Plugged into wall socket, powered it up and started taking voltages as suggested in the service manual, findings recorded below.

    I'm not too sure how complete the other service manual tests such as Gain, DC Offset, and Phono Amp but I am reading into it.

    I noticed that the L channel speaker terminals have higher mV when tested in both A & B speaker sets. The mV is still super low based on what I read, maybe I don't need to address the slight difference in mV?

    Any other suggestions on what to test for that the service manual may have not mentioned?


    Adjustments.jpg
    Power Supply Voltage Measurements, No Load:

    Blk probe grounded to chasis, Red probe touching each point on main board set to DC V 200 range:

    +B: 45.3V [+43V ±8V]

    -B: -45.3V [-43V ±8V]

    +16: 16.4V [+16V ±2V]

    -16: -16.9V [-16V ±2V]

    +12: 12.3V [+12.5V ±1V]

    +5: 5.7V [+5.5V ±0.5V]

    +8: 7.8V [+8V ±1V]

    -8: -7.7V [-8V ±1V]

    DC Voltage at speaker terminals, No Load
    B & R Probe inserted into + & - speaker terminals, set to DC V 200m range

    A/B (Both A & B had the exact same readings)
    L:
    -2.9 mV
    R: -1.2 mV
     

    Attached Files:

    Last edited: Nov 10, 2018 at 12:20 PM
    Goldie99 likes this.

     

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  2. Goldie99

    Goldie99 Well-Known Member

    Messages:
    785
    Location:
    The original Boston, UK
    For a "novice first project" - that looks, and by all accounts is, absolutely superb, congratulations :thumbsup:

    I don't know the Yamaha at all, and couldn't easily find a manual online, but the very small difference, and very low values, of the DC offset measurements at the speaker terminals can be completely ignored, they're already better than many amps will ever achieve.

    I'm sure one of the Yamaha guys will be along shortly to answer your other points, but essentially they call for an audio signal generator - you may be able to work around that using a mobile phone, or eventually a pc/laptop, but I'd wait for further advice on that.
     
    guy232 likes this.
  3. guy232

    guy232 New Member

    Messages:
    27
    Location:
    Melbourne, FL
    Just wanted to post a link to the service manual:

    https://docdro.id/P9uWhrM

    (mirror) https://drive.google.com/file/d/1OlaTRrRrmZUitdT3q_7rN202cvNJDK6V/view?usp=drivesdk

    I sent a copy to the AK database, once it's uploaded I will edit the above link.

    Tested the unit with some music on low. The left channel speaker got a bit scratchy at times when moving the volume pot, shot it with dexit & rotate, scratch is gone for now.

    When plugged into source, without playing music I turned the volume up and started hearing a consistent light static/buzz that became more aparent as vol increased. I don't think this is normal but could be wrong, will be searching around about this issue will report back.
     
    Last edited: Nov 10, 2018 at 9:39 AM

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