My first receiver restoration attempt: G-6700.

Discussion in 'Exclusively Sansui' started by ghazzer, Nov 12, 2012.

  1. Firstblood

    Firstblood New Member

    Messages:
    13
    Location:
    N.W. Wisconsin
    Thanks ghazzer for the info. I looked really hard at the board schematics and there is a different trace path from vr03 to vr04. Here's my problem pretty much everything has been gutted on the dreaded f2980 board along with both f3058's except the 1/4 watt resistors which have been pulled and tested. When I slowly bring up the power on the variac relay kicks in after 5.3 seconds at full song that I can live with. The problem is that right channel is perfect on dc and bias but, the most I can get out of the trimmer is 0.61 mv while dc on vr01 is dead on at 0.00 mv after 3 or 4 minute warm up. So what I did after reading your post was reposition the trimmer and had no reading at all. I then replaced the trimmer with a new one no difference. Now keep in mind this 6700 was a dead horse when I started on it and last week when I started checking voltages OOPS!. Well I'm getting old and for some reason my arthritic hand decided to let go of the probe and bang! it hit terminal 26 and took out half of the power supply. No problem replaced a couple D313's and 2 zeners back up and running. The right output sounds beautiful the left sounds distorted. I tested pretty much everything on the left side including the 2581's and 1106's. all the board traces are great I did find some corroded leads from the sui glui. Any Ideas are surely welcome.
    I really love restoring these vintage machines. I had an uncle that was an audiophile back in the 60's and 70's and of course he had all the top stuff. Sansui, Teac/Tascam. etc. I learned what I could form him unfortunately he passed away 2 years ago just as I was getting serious about saving this stuff. Right now I'm up to 15 decent receivers and at least 12 Teac, Tascam and a couple of Tandberg Td 20a's.
    I have been following your posts as well as the other Sansui addicts and I want to personally thank all of you for all the advice and help you have given people like me. The day is coming when it will get almost impossible to keep these vintage units running. Smelly boards and rosin smoke forever Gotta love it!
     
  2. Firstblood

    Firstblood New Member

    Messages:
    13
    Location:
    N.W. Wisconsin
    Oops sorry after I proof read my post I forgot to mention that the bias problem is on the left channel and that problem could have been there before I repaired everything else
     
  3. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,593
    Location:
    Sykesville, MD
    Hello Firstblood! Welcome to AudioKarma, where there is always room for one more old timer. I have about a decade on you, but don't rush to catch up - the arthritis doesn't get better.

    The driver circuits on the F-2980 can play with your mind, so watch carefully. Some circuit tracks are a mirror reflection between the left channel and the right channel, but not all, and you need to be aware of which are, and which are not. Looking at the track side of the board (see pic in post #27) you can see that VR01 & VR02 are mirrored but VR03 & VR04 are not. The wiper must be tied to one of the other pins so there are only two ways to install the trimmer. One way will increase the bias when the trimmer is turned CW and the other will increase the bias when turned CCW. I need to check this, but I think that you want to install them so that you read ~1 or 2Ω with the trimmer fully CCW. Turning the trimmer CW will increase the resistance.

    I'm not real sure what you are saying about your trimmer adjustments. Are you saying that you can adjust left DC Offset down to 0.0mv with VR01 but can only get the right down to 0.61mv with VR02? Than should be AOK, but I would give it 20-30 min to warm up and stabilize. Double check the settings on your DMM to make certain that it is on the milli-volt scale. 0.61 VOLTS would be a little high.

    What are you L/R readings for bias voltage? Check to make sure that you are reading mv across one of the 0.33Ω emitter resistors. Left ch would be jumper points 87 to 89 and right ch would be jump points 88 to 90. You have a problem if either of them is 0.0 or does not change with the trimmers. My last event like this was a cold solder joint that cracked loose and created an intermittent open at one lead of the emitter resistors. Set the volume at zero and check the voltages wrt ground at successive points along the circuit and compare Left ch to Right ch.
     
  4. Firstblood

    Firstblood New Member

    Messages:
    13
    Location:
    N.W. Wisconsin
    I've been trying to understand why I can't get the Left channel bias {vr03} on a G6700 that I have been working on to go above 0.61 mv. The right channel or vr04 is at 3.3 mv at around 55 ohms on the trimmer. The left trimmer is at 99 ohms. ghazzer gave me some direction as where to look but, does anybody have a schematic for the newer style board with the upa68h fets like this one has. My manual for the g57,67,7700 has the old style with the 2sk129a style fets and the trace paths are definitely different than anything I have to compare to. I tried tying the sweeper #2 on the trimmer to either 1 or 3 post and it didn't do anything just 0.00 mv. Plus I'm wondering about the voltages on the old style 2980 board if they are the same on the newer style. I know one thing all my other Sansui's that I have all had there original schematics still fastened underneath. This one didn't.
     
  5. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,593
    Location:
    Sykesville, MD
    Odd. I don't believe that I have ever seen a G-X700 schematic with an 2SKxxx FET. The µPA68H is a SIP of 7 pins, usually by NEC. The 2SK150 is a 6 pin "tin hat".
    Here is a pic I borrowed from Nicowico. It shows the µPA68Hs just outboard of VR01 & VR02.

    http://www.sansui-parts-shop.com/prodgfx/672.jpg

    Edit: Section 4.4 of the SM for the G-5/6/7700 parts layout for the F-2980 board shows the six pins for the 2SK129 or 150, not the seven in-line pins of a µPA68H. I don't know if these F-2980 boards have any rev or configuration markings on them, but all the PCBs I have seen have µPA68H chips installed. AFAIK the PCB for all three models should be the same, except for component values and more parts on the higher model numbers.
     
    Last edited: Jan 20, 2017
  6. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    38,669
    Location:
    Hertfordshire, UK
    Please don't do that again, if this was working ok before you did that, it wouldn't be now. ;)
     
  7. Firstblood

    Firstblood New Member

    Messages:
    13
    Location:
    N.W. Wisconsin
    It still works fine after I put it back to where I had it originally but I still only get 0.61 mv. with the trimmer maxed out. If I turn it the other way { sorry I didn't check if it was cw. or ccw.} it just sits at 0.00 mv. Ok ghazzer my downloaded manual that I have been using only shows the pc board of the 5700 of which does use the sk129's and if you check the parts list it shows that for the 6700 and 7700 it then uses the upa68h. the only reason I was trying to locate the schematic of the 6700 & 7700 was for future reference and to use to check voltages as I have to believe that there are some differences between the 2 board styles. Apparently there is a component issue or bad solder connection with this one that I'm working on now that I have yet to find but, I'll win this one.

    Thanks to everybody that's helping me with this one and if there is anything that stands out in your minds for me to look at it will greatly be appreciated.
     
  8. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    38,669
    Location:
    Hertfordshire, UK
    What I meant was, if you could set the bias OK - but shorted those trimmer legs for another reason, that would damage the power amp. ;)

    The existing fault protected the power amp from further damage when you shorted the trimmer, so you got away with it - this time.

    Hence, "please don't do that again" ;)
     
  9. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,593
    Location:
    Sykesville, MD
    Please clarify: Are you working on a Sansui G-5700 or a G-6700? I see both mentioned in your previous posts and it is important to know what we are dealing with.

    Since you are a new member here, let me offer a few suggestions (FEW???). If you plan to continue this effort I suggest that you consider starting your own thread. I'm not complaining, but this is an OLD thread and is probably skipped over by viewers who might be able to help you.

    Take the time to find a comfortable chair at a table and write down every salient point that you can remember about this receiver. Put them in chronological order. Now go back and fill in the holes as you memory improves. Make sure you include everything you have tried - and the results. I believe that you can never have too much information, and maybe it will answer an odd question for someone.

    Next you should write up a summary, including your objectives. Start a thread with that and add a cleaned up list of your observations.

    Have patience.
    If you are asking for people's help you need to do what they ask/suggest as long as they are helping you. Make sure that you understand what they ask before you do something. Don't take actions on your own initiative unless they approve. Hopefully they are following a logical progression, but it won't be as quick as you'd like. Remember that they have projects of their own. Ask as many questions as are needed for you to understand what is going on, but be clear, and ask good questions.

    You may get a flurry of suggestions that are not all headed in the same direction - slow down and wait for a cooler head (like John/Hyperion) to sort things out. It is sometimes easy to just follow instructions, but remember to think.

    Members who help you have a bad habit of not being able to explain everything they are thinking about or planning (as do we all!), but this is not their first rodeo.

    I have faith that your Sansui G-?700 can be cured, you will learn some more about it, and future members will be able to learn from your experience.

    Happy Trails - - -
     
    Overundr1 and Hyperion like this.
  10. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,593
    Location:
    Sykesville, MD
    I will post these "X-Rays" of the F-2980 board here and try to keep them updated, following the debacle with 3rd party hosting of pics by our friends at F-Bucket. These boards are populated for the G-6700, but the bare PCB is the same for the 5700,6700&7700.

    Main amp:
    F2980 XRay-006A.jpg

    Power Supply:
    F2980 PS XRay-006.jpg
    .
    Enjoy - - -
     
    Hyperion likes this.
  11. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,615
    Location:
    New Jersey
    I'm not working on one but nice pics. If i were those likely would be helpful. I'm a nut for taking a lot of pictures of everything when i work on them. I find i pick up details i might not have been looking for later. Also they're way better than my memory:thumbsup:
     
  12. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,593
    Location:
    Sykesville, MD
    It took me a significant number of hours in Photoshop Elements to add the component outlines and designators to the pics, but It certainly helped me troubleshoot this G-6700. The "X-Ray" figures in the SM I DL'd are poorly copied, and too blurred to be discerned. These pics are clear, easy to read, and have been very helpful to me. Maybe others will find them useful.
     
    Hyperion likes this.

Share This Page