'83 Yamaha R-70 Restoration - Novice First Project

Discussion in 'Solid State' started by guy232, Nov 10, 2018.

  1. guy232

    guy232 New Member

    Messages:
    36
    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
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  2. Goldie99

    Goldie99 Well-Known Member

    Messages:
    808
    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.
     
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  3. guy232

    guy232 New Member

    Messages:
    36
    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
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  4. guy232

    guy232 New Member

    Messages:
    36
    Location:
    Melbourne, FL
    Was experiencing L channel being much lower than R. Swapped speaker wires and confirmed it wasn't the speakers.

    Pots and all switches functioning after proper application of deoxit.

    Searched forums, read about suggestion of using wood probe to poke at wires. Wires were good. Discovered that a very gentle tap on the relay box would cause the channels to go very low and distorted.

    Removed relay (pictured below) and popped off the cover. Deoxit soaked paper wiped through contacts to clean.

    Reinstalled relay; channels still imbalanced. Tapping on relay no longer produces any changes to channels though.

    Gently flexing the PCB ("FLAT AMP") that is connected to the volume pot causes the imbalanced left channel to restore to full. Going to investigate the solder connections.
     

    Attached Files:

    Last edited: Nov 16, 2018
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  5. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
    Fort Dodge, Iowa
    Your right. You don't.
     
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  6. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
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  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
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    Not a great idea.
     
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  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
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    This is your DC offset.
     
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  9. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
    Fort Dodge, Iowa
    How does the FM stereo sound?
     
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  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
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    I think this is the first Yamaha I've seen with a "heat pipe".That copper pipe has refrigerant in it. Probably R-12.
     
    Last edited: Nov 17, 2018
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  11. guy232

    guy232 New Member

    Messages:
    36
    Location:
    Melbourne, FL
    @avionic Thanks for the info, while browsing the forums I ran across alot of your older posts which were often helpful aswell.

    Will be adding that relay to my list, thanks.

    I don't have an antenna attached to the unit and only get static in FM mode, in AM mode I get a rapid pulsing tone if tuned to the lowest frequency ~500ish. I used the FM static and the AM pulsing tone to help verify that my left channel was low.

    I resoldered the connections on the flat amp vol/bal pot pcb and believe the L&R channels are now equal, need further testing just to be sure.

    Aye the heat pipe is rather unique based on such little info I could find online about it in regards to applying thermal paste. Ended up laying a bead down the channel it sits in and inserted the pipe + rotated to spread the compound. Yup, based on what i've read online this does have refrigerant in it, no idea on whether or not it is still filled to spec though. Unknown if microscopic leakage occurs over time.
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
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    Just clip and alligator clipped wire to one of the 300Ω terminals. Looks like you have quite a few FM stations in the local area. Won't need much of an antenna.
     
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  13. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,588
    Location:
    Fort Dodge, Iowa
    I'm curious how close your digital display is to the actual station freq.
     
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  14. guy232

    guy232 New Member

    Messages:
    36
    Location:
    Melbourne, FL
    I clipped a 10" piece of 18awg insulated wire onto one of the 300Ω terminals and dialed the digital tuner to a local FM station that I know of, with digital display reading 101.1, I was picking up the audio rather clear for such a small wire. Should it be extremely clear and undistorted in order to find out how close the display is to actual freq?

    I also noticed that my previously mentioned channel imbalance is still there. I read on the forums to disconnect both speakers, then I connected only the L speaker, the negative wire inserted into R + terminal, positive wire to L + terminal. Turned on unit to AM pulsing tone and moved balance quite a bit to the left in order to null out.

    I read conflicting info about whether or not this behavior is normal/expected. I shot a short 1 min video to help explain:

     
  15. guy232

    guy232 New Member

    Messages:
    36
    Location:
    Melbourne, FL
    Well after further testing the left channel is definitely acting strange. The overall sound of the amp still sounds very poor. Comparing it to another 80s "like new" virgin Fisher CA-39, the r-70 just sounds very poor in comparison. I imagine that both should atleast sound similar.

    Based on what I read, I've done all of the tests that I could with multimeter so far. I could be wrong but it may be time to buy an analog oscilloscope in order to be able to diagnose this problem accurately instead of trial & error pulling & testing / replacing components based on "assumptions".
     

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