Sansui g-5700 tuner bad

Discussion in 'Exclusively Sansui' started by Tomcow, Dec 31, 2018.

  1. Tomcow

    Tomcow Active Member

    Messages:
    117
    Hello.Anyway a sansui g-5700 stereo receiver’s digital display can be fixed?The quartz lock does not work,the numerical display is locked with a few segments missing,the tuning strength meter and indictator arrows still work.There is a black burn spot in the upper left corner of the display.
     

     

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  2. Tomcow

    Tomcow Active Member

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    The tuner board is the g-3000
     
  3. ghazzer

    ghazzer Senior Member Subscriber

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    F-3000
     
  4. Tomcow

    Tomcow Active Member

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    117
    F-3000.You are correct.Where can I find one?
     
  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Well, you do realise you can't buy one 'off the shelf' any more? So the only likely source would be a unit getting 'parted out' on ebay for example - but that isn't guaranteed to work, and you might have to wait months for one to appear.

    Several models used the F-3000 Digital Display Board, but there may have been a small number of component changes depending on exactly which model it was used in.

    The 'Black burn spot' you mention doesn't sound good, and might indicate that the display itself has some problems.
     
    Last edited: Dec 31, 2018
  6. Sansuiman

    Sansuiman Well-Known Member

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    I suspect what the OP is referring to as a burn spot in the corner of the display is the getter. This is normal, it is a chemical paste deposited in the glass envelope at manufacture and is present on all vacuum fluorescent displays. Its purpose is to remove any remaining air inside the tube once sealed, and is done by flashing it with heat to activate it. If the display tube is compromised (loss of vacuum) the getter turns white instead of a dark metallic color as it is normally.

    Aside from that, this is probably a bad MSM5540 IC, that is the chip that controls the digital display. Per a recent almost identical issue posted here, check the inter-board connector (should be at least one pluggable connector between F-3000 and other board, though the G-5700 may differ from.the larger models for all I know). If present, pull that plug and then re-seat this connector to be sure it is making good contact. If this doesn't resolve the issue (unlikely, admittedly) then the chip is probably at fault. Finding that part (or a complete F-3000 board to swap in) is not going to be so easy or cheap.

    If you do find the chip, it is not easy to install (not an operation for a novice in component level repair) due to being a 40 pin DIP and since a good desoldering rig is needed to pull it off a donor board without heat or mechanical damage, one best have a nice test bench setup for this kind of work.
     
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  7. Tomcow

    Tomcow Active Member

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    Interesting so that black spot that appears in the corner is normal.I would not have guessed that!Thankyou.I will try freshing up the connectors. Would a chip from a TA-80 tuner work?Just the chip,not the board.Regards,Tom
     
  8. nosirrah

    nosirrah Moderator Staff Member Moderator Subscriber

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    yes
     
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  9. Sansuiman

    Sansuiman Well-Known Member

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    Location:
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    [Sigh] yes the chip from a T-80 will work. And now we go on about decimating the supply of otherwise nice working T-80 tuners. It may not be a great tuner, but it is a damn shame so many are now getting stripped and made useless for this purpose.
     
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  10. ghazzer

    ghazzer Senior Member Subscriber

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    IF you find one, you should take it to someone you trust to be a good tech and DAMNED good at soldering, just to remove the chip. And also have them remove the chip in your F-3000 board, just I case.

    Have them install (and check) a 40 pin socket so that your replacement chip can be installed w/o any additional soldering. Why take a chance?

    Rotsa ruck - - -
     
  11. Tomcow

    Tomcow Active Member

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    117
    Need to know where to check the bias voltage on the sansui G-5700 receiver.I know it should be 3.3 mv but not sure where to place my mini clips on the power board.
     

     

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  12. ghazzer

    ghazzer Senior Member Subscriber

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    I would strongly advise that you replace the 100Ω trimmers before you set up the offset & bias. The single turn ones are ~40 years old and quite brittle. Failure could take out your output transistors. I use Bourn style multi-turn trimmers.

    Page 6 of the service manual tells you where and how to measure the bias voltage. The SM says it should be 5V, but everyone assumes that is a typo and it should be 5mv.

    Let us know if you do not have the SM and we will help you get a copy.
     
  13. kevzep

    kevzep Its all about the Music Subscriber

    3.3mV Steve for these amps. 5mV is too much.
     
  14. ghazzer

    ghazzer Senior Member Subscriber

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    Thanks. We've had this conversation before, and I keep forgetting. I wish that I had access to that PDF file so I could make that correction. The schematic for the 5700 calls for 3.3mv, the 6700 calls for 10mA, and the 7700 just says "Bias Current". If the Bias Current is the critical parameter, then one needs to measure the resistance of the emitter resistor(s) and divide that into the desired current (10mA/.33Ω = 3.3mv).

    In this day & age I think it is important to know the value(s) of the emitter resistor(s) because they are rarely exactly the 0.33Ω listed in the schematic. Especially if they have ever been replaced.

    An AKer suggested that I could measure the resistance across both resistors (in this G- series) and divide by 10mA to see what the voltage drop should be across the two resistors ((0.33Ω + 0.33Ω) X 10mA = 6.6mV), or ~twice what you should expect across a single emitter resistor. This would give you an "average".

    I wonder how many enthusiasts measure the bias in both channels?
     
  15. kevzep

    kevzep Its all about the Music Subscriber

    Then there is the conditions for setting bias, and this is probably why so many of these things blow up.

    Turn unit on.
    Let unit idle for about 10mins.
    Set bias to 3.3mV.
    Then LEAVE IT ALONE!!
    Do NOT sit there as it idles and keep correcting the current over a long period. This is a fatal mistake.
    You will be winding up the current, so much so when you turn it on next, when its cold it will have dangerous levels of bias.

    I have a lot of experience with these amps, and this is what I have found from experience.
     
  16. TMaster

    TMaster Transistors for teeth? They do have roots... Subscriber

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    Location:
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    Thanks guys for clearing up setting the bias on the G-5700. I bought one that is coming next week and it has no sound output, but everything else works. I have been making a list of all the threads on here that have to do with the G-*700 series. I found Steve's giant thread "My first receiver restoration attempt" had have yet to go through it all :) I will most likely start a new thread on my restoration. Here is a better (clearer) version of the service manual for the G-5700,G-6700, G-7700. It is a LOT better than the one on hifiengine.

    High quality 42Mb version
    https://www.electronica-pt.com/esqu...k,22936874a310450e3514012493371831/no_html,1/
    The second to last page has the bias adjustment corrections, but it says to measure between the emitter resistor leads and not point A/85 and B/86.
     
    Last edited: Jan 16, 2019

     

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  17. Tomcow

    Tomcow Active Member

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    Here is a picture of my g-5700 receiver.Where to connect to set the bias settings?
     

    Attached Files:

  18. ghazzer

    ghazzer Senior Member Subscriber

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    3,104
    Location:
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    Looking at the schematics for all three you will note that you cannot access both sides of each 0.33Ω emitter resistor unless you have good access to the F-3058 bd or the F-3093&2981 bds for the G-5700. If not you will have to follow the schematic lines for access to points on the F-2980 bd.

    The points marked (B) {a circled B} and (B') are connected to the common connection of the two emitter resistors in each channel, and is also jumper pins 59 and 60. There are about 4 resistors common to this point, so pick the one that is most convenient for you and attach a mini-grabber to that lead. Touch the other lead to that jumper pin or one of the other resistors and check for 0.0Ω to confirm that you are on the desired side of the resistor.

    The other side of the emitter resistor (connected to the emitter of the NPN output) is connected to jumper pins 57 & 58, and routed on to R69 & R70. So connect your other mini-grabber to that resistor lead when you check bias in that channel.

    CAUTION: Sansui does NOT update their manuals to reflect design changes they may add later, so ALWAYS check the circuit(s) on your boards.
     
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  19. ghazzer

    ghazzer Senior Member Subscriber

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    I made standoffs out of twisted, soldered wire and short pieces of insulation, and installed them in jumper pin holes that were hard to get to. Now it is simple to connect to them with mini-grabbers.

    P1030585.JPG

    P1030588.JPG
     
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  20. Tomcow

    Tomcow Active Member

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    117
    So if I make pins for 87 and R71(left channel)and pins for 88 and R72(right channel)I can then set the bias for each channel?
     

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