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Yamaha C2A preamp: another thread about crackling and popping

Discussion in 'Yamaha' started by lowredmoon, Apr 3, 2017.

  1. CT Jim

    CT Jim AK Subscriber Subscriber

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    I'm following this in the event it solves a similar issue I have with my CX-2 preamp, because I don't think it's the ST-70! However, mine is a one-time static burst shortly after turn-on, and no issues after that!
     

     

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  2. lowredmoon

    lowredmoon Active Member

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    I'll update once I make changes, which should be in the next couple days.
    Are you using a GZ34 / 5AR4 rectifier in your ST-70?
     
  3. CT Jim

    CT Jim AK Subscriber Subscriber

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    Looking at a file pic, it's a CP-122, there is another set of numbers under that but nothing that resembles what you asked.
    I will check it tomorrow and advise.
     
  4. lowredmoon

    lowredmoon Active Member

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    I was essentially asking whether you are using tube or solid state rectification. We're talking about a Dynaco ST-70, right?
     
  5. CT Jim

    CT Jim AK Subscriber Subscriber

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    3,120
    Location:
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    Correct, I have an upgraded and modded St-70 as performed by Wharfcreek a few years ago. My first check using a CD source without anything hooked up came up normal, no static burst.

    I'm wondering if my MR-78 is the culprit? I will be checking phono input on Thursday
     
  6. lowredmoon

    lowredmoon Active Member

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    148
    Location:
    Central Pennsylvania
    Update: I replaced each of the 4 VD1212 on the tone control board with 2 1N4148 in series. I replaced the 2 2SA913/2SC1913 pairs with 2SA1930/2SC5171.

    Now it doesn't pass sound at all.

    The DC offset won't budge from 35mV when turning either trimpot.

    The replacement transistors have the same specs as the originals, but the hfe is 140, vs. 100. I tested them.

    This is why it's wise to replace one thing at a time.

    I'll try again tomorrow.
     

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  7. Mr. Yamaha

    Mr. Yamaha Not so much Yamaha lately...

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    Have you tested the ECB pin-outs of the old and new transistors?
     
  8. lowredmoon

    lowredmoon Active Member

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    148
    Location:
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    I tested them with my transistor tester. The pinout is the same as the originals.
     
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  9. lowredmoon

    lowredmoon Active Member

    Messages:
    148
    Location:
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    I'm presently trying to figure out why I'm getting +35V (not mV) at TP3. I pulled and TR527 and tested it for a short, even replaced it -- still 35V at TP3. Given this voltage at that point, I don't want to turn it on for long, or at all, until I can reason it out. I pulled and checked C517, 19, and 21. All test good. I also looked closely for solder bridges and shorts. I'm too tired to go further for now. More tomorrow.
     

    Attached Files:

    Last edited: Apr 27, 2017
  10. avionic

    avionic " Black Knights " Subscriber

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    Either ,one of these outputs is shorting the rail to TP3. Or .. something , up stream, is holding one of the tone amplifier outputs (2SA1930/2SC5171) to rail.
     
  11. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    Location:
    France
    Do you have a cold spray ?
    If yes , put your voltmeter on TP3 and ground . Switch the power on , and use the spray area by aera untill you see the voltage change back to zero (or closer ) .
    I am pretty sure that your trouble is not coming from transistors !
     

     

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  12. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    Hello lowredmoon

    I know from where your trouble is coming from
    All you have to do is to replace every small flat resistor . This is a huge job to do but this is the only way to refix your unit . One (or more) resistor is cracked and make huge voltages variation to the TR ! here is a picture of one of these famous resistors cracked . If you watch closer the body of this resistor , you will see the cracked area :

    zoom-defect-resistor.JPG

    this was not the only one !

    C2A-defect-resistor-R.jpg

    A view of the pcb with one channel with original flat resistors :

    C2A-01-RR.jpg

    same PCB with other channel with all flat resistors replaced :

    C2A-resistors-replaced-RR.jpg

    Good luck because it is really time consuming job !
     
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  13. avionic

    avionic " Black Knights " Subscriber

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    Thats alot of resistors ..
    Within the entire amplifier or just that board ?
     
  14. lowredmoon

    lowredmoon Active Member

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    148
    Location:
    Central Pennsylvania
    Clinic-audio,
    Thank you for the tip! I will check all of them. I have finished touching up the solder. I have to put the potentiometers back in. I am still working on the preamp when I can, but I have had other responsibilities lately.
     
  15. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    just that board
    I never met this trouble on other boards but it is possible because these resistors goes bad by heat or age depending where they are
    Only one C2A did resist to my investigations after spending hours and hours and the owner take it back not refixed (wait too long ! )
     
  16. avionic

    avionic " Black Knights " Subscriber

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    Probably a good clue is the darkened PCB around those resistors perhaps.
     

     

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  17. lowredmoon

    lowredmoon Active Member

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    148
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    I completed resoldering all joints on the preamp today. I set all voltages and they remained stable for the 30 minutes that I was monitoring it. I plugged it in to an amplifier, and it sounds EXCELLENT. I only got to listen for about 10 minutes before I had to go to work, so I don't yet know if it's reliably stable.

    So far, this is the work I have done:
    - Re-solder all solder joints, including flush mounted power transistors, and cleaned all flux from the circuit boards
    - Cleaned the volume, bass, and treble potentiometers
    - Replaced filter capacitors (original capacitors measured lower than specified capacitance, although ESR measured OK)
    - Replaced all other electrolytic capacitors
    - Replaced pair of 2SA913 (Q) / 2SC1913 (Q) on tone control board with 2SA1930 (Q,M) / 2SC5171 (Q,M) - So far they work, but I haven't yet monitored it for very long

    Work yet to be done:
    - Inspect and replace cracked resistors on the tone control board (as suggested by Clinic-Audio)

    Adam
     
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  18. lowredmoon

    lowredmoon Active Member

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    148
    Location:
    Central Pennsylvania
    I really like those RN60D resistors. I have had very favorable results with them in the past.
     
  19. sonavor

    sonavor AK Subscriber Subscriber

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    Location:
    Texas
    Thanks for this thread Adam. I am following with interest as I have a couple of C2a units I need to get to this year. Where did you order your transistors from?
     
  20. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    Location:
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    Hello Sonavor
    Watch inside your 2 C2A before to suspect transistors .
    I have no doubt that your 2 items are sic and you will find cracked resistors inside .
    The first thing to do is replacing bad capacitors (most of case the green ones ) and then all adjustment pot . Once your C2A are adjusted you will see if there is (or not) defective transistors !
     

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