C-70 right channel issues?

Discussion in 'Yamaha' started by Rogersjapan, Apr 25, 2018.

  1. Rogersjapan

    Rogersjapan New Member

    Messages:
    27
    Location:
    Tucson, AZ
    Recently picked up a C-70 per-amplifier in pretty beat up shape cosmetically. It did power up and that's as far as i got to test it. I've cleaned it up inside and out with compressed air and sanded, primed and painted outside covers with some black automotive paint and now looks very nice. On to testing it. Right channel is very weak in all sources. Para tone control on or off right channel very weak. looks like some glue near the power supply area might be corroding some parts as I see some green slime on a couple resistors. Is this usually a cause for channels dropping? When I have time I want to pull out that board and give a good once over but thought I would ask the experts who have seen this before.
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

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    The corrosion may be part of the issue.There are 8 little yellow/orange reed relays. One of those may have a fouled contact. Rather common for this pre-amp.

    Matsushita HB2E-DC24V
     
    Last edited: Apr 25, 2018
  3. pdm4606

    pdm4606 Super Member

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    Welcome to the world of Yamaha rotten relay design. You will be blessed with tons of work forever from now on. Just keeping the relays working and clean. Also if they are anything like the C-50 there are a couple of flat cable activated switches located in the low signal level amp area. Just to the front of the phono inputs. Any dust within a mile of your house and it will find a way. Now I am experimenting with pieces of clear tape taped to the top of the switches. Just a guess for now.
     
  4. avionic

    avionic " Black Knights " Subscriber

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    I have a couple C-70,s that are flawlessly working and sounding awesome. MM/MC direct ---> outstanding.. Inverted output is a must have for me.
     
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  5. avionic

    avionic " Black Knights " Subscriber

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    :bs:
     
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  6. Rogersjapan

    Rogersjapan New Member

    Messages:
    27
    Location:
    Tucson, AZ
    Just an update. Removed all that corrosive glue and replaced all the resistors and diodes that started corroding. Replaced all Eight of HB2E-DC24V relays as I got a good deal on them. Still right channel is very soft and have to turn balance almost all the way right to balance it out. This is all modes -Phono/aux/tuner/tape. with parabolic off or on. Swapped l and R outs and it follows. How do you get into the volume Pot area as it seems to be a sealed box?
     

     

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  7. avionic

    avionic " Black Knights " Subscriber

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    Can be disassembled. Must remove from the board and chassis. There are two long screws that hold the segments together. Remove the screws and very carefully slightly pull the segments apart just enough to get some deoxit on the ceramic plates that have the carbon resistance tracks.
     
  8. mbz

    mbz AK Subscriber Subscriber

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    Location:
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    Haven't worked on a C-70, hopefully picking one up off the bay, the schematic shows a maze of electronics, separate opamps are used for L&R channels.
    Cause could be anything. Rather than spend time disassembling/cleaning vol pot, it would be nice to confirm that it is the problem. Do you have any test
    equipment (oscilloscope, audio probe...)
     
  9. amr2

    amr2 Active Member

    Messages:
    480
    Check out R349-350, they are used for gain adjustment.
     
  10. Rogersjapan

    Rogersjapan New Member

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    Location:
    Tucson, AZ
    I have all the test equip. Will try to to chase signal and see where its dropping off.
     
  11. mbz

    mbz AK Subscriber Subscriber

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    The hifiengine schematic shows the audio path highlighted. Maube divide the amp in half and test there, the vol or balance pot wiper would be an easy test point.
    Compare L&R channels, use a low input sine wave, say 100mV, xxxHz
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

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    Trivia :This is called the half - split method for trouble-shooting.:thumbsup:

    This is the method used for the testing/trouble shooting software on ATE for military avionics.

    ATE - automatic test equipment

    My airforce specialty was
    AN/ALM-173 (TISS) TEWS Intermediate Support System.
    TEWS - Tactical Electronic Warfare Systems
     
    Last edited: May 10, 2018
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  13. Rogersjapan

    Rogersjapan New Member

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    27
    Location:
    Tucson, AZ
    Good test tone patterns coming into Vol pot--Low level coming out RH side. Pulled it off and opened it up and cleaned it with Deoxit. Same results -Compared both sides and RH side has 700 ohms + more when cycled through. Have new Alps 20k pot coming but cant find with long shaft. So got Pot shaft extender coming also. This should fix it I hope.
     
  14. amr2

    amr2 Active Member

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    480
    The pot is in the negative feedback loop (Varigain volume control system), and the tracking error is always a problem with this topology.
     
  15. Electone

    Electone Well-Known Member

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    Location:
    London, Ontario, Canada
    Sorry to hi-jack, but just wanted to say I hope you get your C-70 sorted out. I recently had a channel imbalance as well, but once I replaced all the relays, it was cured. My C-70 even managed to replace my beloved C-4 in my main system. Just an outstanding pre-amp.
     
  16. Rogersjapan

    Rogersjapan New Member

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    27
    Location:
    Tucson, AZ
    Received new Vol Pot and installed. "BINGO" right channel restored. Now to play with the shaft length right so the VOL knob will sit good. Thanks for all the tips. Another learning experience.
     

     

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  17. amr2

    amr2 Active Member

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  18. mbz

    mbz AK Subscriber Subscriber

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    Location:
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    Picked up a C-70 with intermittent right channel. While I have not ruled out the relays as contributing/cause this example has many dry solder joints.
    Working through the tone control 1 board, with switches and pots being the main culprit then transistors... see below. I know it goes without say that
    you should check the solder joints but this one was extra bad.

    upload_2018-5-24_20-8-49.png
     

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