Yamaha C2A preamp: another thread about crackling and popping

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

  1. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    Greetings,

    I've found a few threads here and there regarding the same issue I'm having with a C2A, but none that I've seen have been resolved. I'm starting this post with the hope that I can, with your help as well, resolve the issue and help others as well. I have a good DMM and an ESR meter. I'm trying to track down a power cord for my HP 141A oscilloscope, though I don't think this is much of a 'scoping issue.

    I've preemptively ordered some 2SA1930 and 2SC5171 in case I need to replace the 2SA913 and 2SC1913 transistors.

    Here are some details about the problem:
    - The preamp plays fine, but is intermittently interrupted with a loud DC pop, followed by crackling and static noise. The sound then returns.
    - It happens on both left and right channels, BUT NOT AT THE SAME TIME :dunno:
    - It occurs with any input selected
    - Power supply voltage and idle current are stable. DC offset fluctuates, but only within about 25mV (the service manual specifies +/- 200mV).

    Here's a video (I apologize to those who have to witness an NS-500 suffering this unfair torture).


    I work a long and varied schedule, so there may be long gaps between posts, but I will post all progress here.

    Thank you for chiming in!

    Adam
     
    Last edited: Apr 3, 2017
  2. avionic

    avionic " Black Knights" Subscriber

    Messages:
    35,185
    Location:
    Fort Dodge, Iowa
    Does it pop without any source connected ?
    Rules out the power supply. If it was the PS it would pop in both channels at the same time.
     
  3. Nakdoc

    Nakdoc nakamichi spoken here

    Messages:
    2,952
    Location:
    Nashville, TN
    You can collect more info: do the tone controls affect the noise? The tone is in the middle circuit-wise, so you can move to just the input section if the tone has an effect.
    Looking at the manual I would suggest you replace D511, D512 a double diode VD1212 with a pair of single diodes (1SS1555, 1N4148 or just about any glass Si. diode). The VD1212 is a well known troublemaker.
     
    Last edited: Apr 3, 2017
    slimecity likes this.
  4. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    I haven't tried without a source connected. I will when I get home from work.

    Ditto on the power supply. I checked the capacitors with my in circuit ESR and they checked out OK. I know it is not an absolute measure of weather cap is good, but it has worked for me so far.
    I suspect there is some funny business going on with some semiconductors. There are 4 TO-220 packaged transistors without heatsinks on the tone control board that are my first suspects. They are grey in color, not the original black that they were. I'm not incredibly familiar with transistors ( I'm more of a tube guy), but my suspicion is that this color indicates a history of heat stress.
     
  5. slimecity

    slimecity Super Member

    Messages:
    1,579
    Location:
    New Zealand
    Can you post some photos of the PCB? I would be suspecting smaller-signal transistors failing simultaneously in the preamp stage somewhere.
     
  6. NAD80

    NAD80 Super Member

    Messages:
    1,158
    Location:
    Carlisle PA
    Hello lowredmoon. Welcome to AK. Checked the Service Manual on HiFi Engine.com. Just register and download for free if you haven't been there before. Saw the C2A has the 2SA798 transistors, one of the "Top Ten Worst Transistors" people have found. Check the thread. http://www.audiokarma.org/forums/in...ilure-prone-whatever-and-replacements.731653/. Nakdoc is right about the VD1212 diodes needing to be replaced. Hope this helps. Good luck. Question if I may ask where in central pa? As for the scope power cord, hardwired or plug into the rear of the scope?
     
    Last edited: Apr 3, 2017
    slimecity likes this.
  7. avionic

    avionic " Black Knights" Subscriber

    Messages:
    35,185
    Location:
    Fort Dodge, Iowa
    Check it over real good for annular ring cracks in the solder connections.
     
  8. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    Thanks for the welcome, NAD80. Coincidentally, I live in Mount Holly, 4 miles south of Carlisle.
    I ordered a hearty supply of 1N4148 diodes when I placed the order for the transistors, so I'll have those on hand shortly. I need to check my transistor supply before placing another order. I have no idea what I have on hand.
    The 'scope uses a plug-in cord. It's an odd one, with all three conductors on the same plane. I'll take a photo of it.

    Now, I'm back to work!
     
  9. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    Here's a photo of the HP scope's AC jack.
     

    Attached Files:

  10. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    Here are photos of the C2A's internals.
     

    Attached Files:

  11. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    I'll plug it in tomorrow and gather more info.
     
  12. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    Yes, it pops both with a source connected to any input, and also without a source connected.
     
  13. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    The preamp behaves the same with the tone controls in any position. I have 30 Fairchild 1N4148 diodes on the way.

    I gave it a once-over, looking for shorts caused by oxidation and broken traces. I'll give it another look.
     
  14. ConradH

    ConradH AK Subscriber Subscriber

    Messages:
    7,120
    Location:
    Canandaigua, NY
    See those stupid copper buss bars that distribute power and ground? They're prone to bad solder joints, probably right from the factory. Resolder each attachment point, using an iron with decent thermal mass, no tiny conical tips- use a big wedge and make sure the solder flows. Add flux if necessary. Might not be it, but the odds are pretty good.
     
  15. NAD80

    NAD80 Super Member

    Messages:
    1,158
    Location:
    Carlisle PA
    Hello lowredmoon, I might be able to get a AC cord that will work with the HP scope. I know where Mt. Holly is. Been there for bowling at Midway.
     
  16. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    That'd be great! I've had a hard time tracking down what type of connector that is -- makes it hard to locate one. I've seen similar plugs on Amazon, but they're listed for $50, with no guarantee the fit is correct. I thought I'd be able to find one from one of the main electronics suppliers, but without the standard name for the plug type, I haven't had success doing so.
    I bowl at Midway from time to time. Joining a league kind of sucked the fun out of it though.
     
  17. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    Thanks for the tip. I'll be sure to go over it.
     
  18. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    I apologize for the vertically oriented images. I uploaded them as horizontal images, but the forum is displaying them vertically. Anyone know how to fix it?
     
  19. NAD80

    NAD80 Super Member

    Messages:
    1,158
    Location:
    Carlisle PA
    Now to try to arrange a time to meet. Have the line cord.
     
  20. lowredmoon

    lowredmoon Member

    Messages:
    80
    Location:
    Central Pennsylvania
    V1212 diodes: why are they typically covered in heat shrink insulation?

    IF: The purpose is to electrically insulate the exposed leads, THEN why aren't they mounted against the surface like most other diodes on the board?
    IF the answer to the above is "they are mounted away from the board because they dissipate significant heat," then isn't the insulating tubing counter productive?

    My logic keeps looping around. Please help me see something I'm overlooking, if that's the case.

    I'm replacing them with 1N4148's. I want to do what's relevant to the original design.

    Thanks!
     

Share This Page