g-7700

Discussion in 'Exclusively Sansui' started by tim in pa, Mar 18, 2017.

  1. tim in pa

    tim in pa Active Member

    Messages:
    156
    Location:
    north central pa
    decided to try to get my receiver back into working condition - "operate safely" led not on. really miss it. have a photo copied manual, somewhat difficult to read.

    able to perform the dc offset adjustment. but when attempting the bias current adjust, the test points for 87 and 88 are empty holes on my 2980 ckt board. see that it actually a reading across R601, TR601 emitter resistor. is that also on the 2980 or elsewhere. i have read that the manual was incorrect and that 3.3mv is the goal.

    any help with the bias adjustment points would be greatly appreciated. thanks in advance!
     
  2. tim in pa

    tim in pa Active Member

    Messages:
    156
    Location:
    north central pa
    found the R601 on the 3058 amp ckt board, and was able to decipher the test point for the bias adjustment. bad thing is after the dc offset and the bias adjustments, the safely operate led/speaker relay still don't energize. found 45.4 vdc on the control circuit for the relay, all the way, r601, r604, & zener, no voltage drops happening. Im guessing there is no sinking ground (pin 1 of the IC601??).
     
  3. moefuzz

    moefuzz Active Member

    Messages:
    222
    Location:
    Santartica
    Where did you get the service manual from?
    ?late production models got updates and/in which case the schem is just a guide

    last but not least, in the fine print of the manuals,
    Many many years ago I'm sure I remember reading somewhere that it stated in very, very fine print,
    "an informed Sansui user should never drink the Pioneer Kool-Aid "


    -Not that there's anything wrong with that.


    Cordiail,
    moe


    .
     
    Last edited: Mar 19, 2017
    tim in pa likes this.
  4. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,525
    Location:
    Sykesville, MD
    If you got your manual at www.HiFiEngine.com it should be "current".

    Bias current should be set at ~10mA. That is reading 3.3mv across the 0.33Ω emitter resistor (½ of R601) between TP87 &TP89 in the left channel and TP88 & TP90 in the right channel.

    The F-2980 is a very finicky board, with a hair trigger. The bias circuit can be off to the races in a heartbeat. Most people who have dealt with this board will suggest that the first thing you should do is to replace the single turn 100Ω trimmer pots (VR01-04) with Bourne type multi-turn trimmers. The originals don't age gracefully and you can lose a bank of output transistors if the bias trimmer opens.

    My 94 year old Mother is in better shape than the tracks on the 3058 boards, and that is being generous. You will save some aggravation if you can avoid dealing with either of these boards. That said, I have considered having modern replacement boards made with double-sided copper clad boards and a new layout, so let me know if you are interested. The same PCBs are used in the G-6700 & G-7700.

    There are many pics, hints and drawings scattered thru my thread at:
    http://www.audiokarma.org/forums/in...t-receiver-restoration-attempt-g-6700.480777/
    They are focused on the G-6700 so some of the components are different. The "X-ray" pics should be useful for locating components from the back side.

    Good Luck - - -
     
    tim in pa likes this.
  5. tim in pa

    tim in pa Active Member

    Messages:
    156
    Location:
    north central pa
    thanks for the info. found the bias 3.3mv to be relatively easy to dial in, but the dc offset 0v seemed to be very difficult, and drifted somewhat when i come back to check it. guess the bias and offset adjustments affect each other. so i see where the new trimmers could help for sure.

    i haven't layed the soldering tip to the board yet, but thanks for the heads up, i will hold it down to 25w.

    honestly don't know where i downloaded the schematics from, it was a number of years back.

    seems that with the 2 adjustments made, the relay will still not energize. i measured applied voltage (45.5vdc) is all along the circuit to the HAI2002 input pin 1, so no voltage drop across the zener. i think that suggest an open at pin 1?? or bad IC? will verify these readings today. thanks again.


    fyi... my rig bought in japan in 1980
    sansui G-7700
    sansui SE7
    Pioneer CTF-950
    Pioneer PL-630
    Bose 901 Ser IV
     
  6. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,144
    Location:
    Long Beach, CA
    Please post the serial number for the database.

    - Pete
     
  7. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,525
    Location:
    Sykesville, MD
    Hyperion likes this.
  8. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,144
    Location:
    Long Beach, CA
    A link is a good idea. I'll include it in my reminders. Better yet, I'll put it in my signature.

    - Pete
     
  9. tim in pa

    tim in pa Active Member

    Messages:
    156
    Location:
    north central pa
    found my first physical signs of a problem, brown under a host of capacitors. looks like the sm parts list for the 2980 board doesn't list all of the caps. any recommendations for cap replacement process? other than mark polarization, desolder, read pn and order?
     
  10. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,525
    Location:
    Sykesville, MD
    I can send you a parts list - I'll look for it tomorrow. I don't punch a clock anymore, and I'm not very good at keeping one either.
     
  11. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,525
    Location:
    Sykesville, MD
    Great solution! Now all people have to do is RTFM.
     
  12. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,144
    Location:
    Long Beach, CA
    . The brown stains under the caps is probably the notorious Sansui glue. It is not an indication of a failed cap.

    Don't take anything off the board before having the new parts on hand. Make a parts list by reading the values on the actual caps while they are still on the board. Take a picture or make a sketch of the board and mark the polarity of each of the caps. Then remove and replace the caps one at a time. it's the best way to ensure you get everything back in the right position with the correct polarity.

    - Pete
     
  13. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,525
    Location:
    Sykesville, MD
    The F-2980 bd can be worked on without removing any jumpers or other boards. If the receiver is working when you start, is it advisable to check operation after every replacement?

    Just wondering.
     
  14. tim in pa

    tim in pa Active Member

    Messages:
    156
    Location:
    north central pa
    what are the recommended sources for capacitors nowadays? thanks in advance
     

Share This Page