Sansui G-7500 Fried Resistors F2980 Board

Discussion in 'Exclusively Sansui' started by Young46, Oct 10, 2017.

  1. Young46

    Young46 AK Subscriber Subscriber

    Messages:
    54
    Location:
    Houston,Texas
    Hello, I am an AK member for approximately a year now. I have received help with repairs to Pioneer Receivers from AK members and this is my first plea for help with a Sansui. I have this beautiful G-7500 but it does not have any sound on any channel. After receiving it I powered it up on my DBT and the bulb (100W Halogen) glowed bright and then quickly dimmed. I opened it up and discovered and area of fried resistors on the F2980 Board. I am really seeking any help that I can get to get this fine piece of equipment playing again. I have several photographs and would like to start a new post for this unit. I have never posted photos before so I will most likely need help.l

    Thanks!

    Frank
     
  2. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,762
    Location:
    Long Beach, CA
    F-2980 is the power supply. Specifically which resistors are fried? To post picture, click on the link that says "Upload a File" in the lower right corner below the text entry box. It will allow you to search files on your computer to select an image. There are some file size limits so you may have to size the image first.

    Also, please post the serial number for the database. Click on the link in my signature block.

    - Pete
     
  3. Young46

    Young46 AK Subscriber Subscriber

    Messages:
    54
    Location:
    Houston,Texas
    R58, R70 look to be the specific ones that fried. R70 appears to be the main culprit however it is hard to tell from the condition. I can still get readings from R46, R56, and R68. TR20 checked ok. In the photo the resistors are located like the board is actually laid out. The layout in the service manual does not match the actual layout. Also the parts list for this board does not list these resistors unless I am missing something. See photos and thank you so much for your response.

    I am not a professional tech nor do I have an electrical or electronics background. I have done repair to boards (replacing caps, transistors, trimmers, resistors, etc) so I am not a complete dummy.
     

    Attached Files:

  4. Young46

    Young46 AK Subscriber Subscriber

    Messages:
    54
    Location:
    Houston,Texas
    Image of F2980 board from the service manual. Note the mismatch between actual and the manual...nothing unusual but worth noting.
     

    Attached Files:

  5. Willy6

    Willy6 AK Subscriber Subscriber

    Messages:
    1,023
    Location:
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    Check those transistors located near the fried resistors...
     
  6. kevzep

    kevzep Its all about the Music Subscriber

    TR20 is the DC protection transistor, the amp doesnt need that to operate, so you could leave that out whilst you troubleshoot.

    Things to look at, driver transistors, Bias transistor, every resistor in the driver and output section basically all the emitter resistors.
    You will also have bad output transistors

    These driver boards are horrifying to work on, and they are very temperamental and when things go wrong, boy do they go wrong, cascades of destruction....

    So work methodically through the whole thing, also install 100Ω 5 watt resistors in place of the 0.33Ω emitter resistors, this will protect the output section from running away on you when you first fire it up, once its stable then you can put the 0.33Ω resistors in.

    You'll need to replace the bias and DC offset trimmers, and recap it whilst you are in there, its a big job which requires a lot of patience.

    Let us know how you go...
     
  7. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    10,762
    Location:
    Long Beach, CA
    You need to check the output transistors as well as the driver transistors with that much damage in the driver circuits. The drivers are mounted on those steel heat sinks.

    By the mid to late 1970s, Sansui stopped listing common resistors and capacitors in the service manuals. They published a separate "Common Parts List." It's available in digital docs forum. It won't help though, it only lists values and part numbers. Short of having a board to compare it to, the best reference is the schematic.

    - Pete
     
  8. Young46

    Young46 AK Subscriber Subscriber

    Messages:
    54
    Location:
    Houston,Texas
    Thanks to all who have responded. Does anyone know if there is a parts list that has been created for this board that lists all the current parts. This would be like the one Mark the fixer has created for some of the pioneers. A lot of the parts on the board are obsolete and although I will attempt to find alternates I would appreciate help from those more knowledgeable.
     
  9. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,658
    Location:
    Sykesville, MD
    I have a parts list for the F-2980 board in a G-6700, but do not know how many (if any) are the same. Will check when I get back.
     
  10. Young46

    Young46 AK Subscriber Subscriber

    Messages:
    54
    Location:
    Houston,Texas
  11. rcs16

    rcs16 Super Member

    Messages:
    2,886
    Location:
    Baldwin, Ontario, Canada
    Check for burnt traces on the foil side. The o/p bjts can be replaced with 200W TO-3P devices such as
    NJW3281G (NPN)
    NJW1302G (PNP)
    the drivers can be
    MJE15028, MJE15030 (NPN)
    MJE15029, MJE15031 (PNP)
    the VAS section, TO-126 devices can be KSC3503/KSA1381
     
  12. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,658
    Location:
    Sykesville, MD
    Let me say from personal experience with this board: Going in to replace all (most) of the parts is a bad mistake!!!

    You need to identify the cause and fix that first. Once you get it working you can replace the electrolytics one at a time. I would not replace anything else unless it fails or is out of spec. This board has a hair trigger, and it wants to have a fine balance between its components.

    I'd replace the 100Ω single turn trimmer pots first, making sure that the replacements are orientated the same. That is, note which direction you turn them to increase the resistance.

    Then I'd put it on the DBT and check the P/S voltages. These will change with different bulbs in the DBT, so take good notes.
     
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