Sansui G-7500 Fried Resistors F2980 Board

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

  1. Young46

    Young46 AK Subscriber Subscriber

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    I will rerun the tests tomorrow and post the results. The 34.48 VDC was measured using the instructions in the G-7500 service manual for adjusting bias current...Fig. 3.2 on Page 6 of the manual I downloaded from the digital docs. This is between R72 and R78 (emitter resistors). I set my DMM to the mv setting and got a 0 reading. I then set it to VDC and read 34.48. These are the only measurements I took after installing the temp resistors. According to the manual I should have read 10mV DC, plus or minus 1mV. Turning the dial on the trimmer either way did not change the reading. I did get a change in vDC when I turned the dial (DMM on vDC setting). That is when I noticed the bulb I described getting brighter or dimmer.

    The relay does not kick in.
     

     

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  2. LBPete

    LBPete Rolling Along Subscriber

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    Do you have your meter set properly to read amperage? You need to move the red probe to the amp jack on the meter. If you have done that and are reading 0 mA, you may have a blown fuse in your meter. Fluke meters typically use separate fuses for the Amp and milliamp functions. The easy way to check the fuse is to set the meter to ohms and use the positive probe to connect between the amp jack and the volt/ohm jack. It should read 0 ohms. If it reads OL, the fuse is open.

    - Pete
     
  3. Young46

    Young46 AK Subscriber Subscriber

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    I’m a bit confused. I have a mV setting and that is what I used. But in answer to your question, no I did not have my meter set, “properly” to read amperage. My confusion stems from, is the measurement supposed to be in mV or mA? The SM I have calls for the adjustment to be for mV. I know that mV can be converted to mA, but why would I need to?

    To be clear, I have not attempted to make any meaningful adjustments. I believe I followed the instructions to install the 100 ohm temporary resistors to see if anything happens. I believe I had the trimmers in the positions as instructed and when I powered up nothing happened except for the emitter resistor R72 got pretty hot. I then removed the temporary resistors and soldered in the permanent ones. I then powered it up (all power up on DBT) and nothing happened, BUT the R72 resistor stayed cool. Bad solder joint on the temporary resistor? I am now waiting on the next Instructions from Kevzip. I hope I am not confusing anyone. I’m certainly not trying to.

    Thank you for your help.
     
  4. LBPete

    LBPete Rolling Along Subscriber

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    Sorry, I confused your receiver with a 9090DB bias issue in another thread. The G series receivers measure bias current as a voltage drop so millivolts is the correct setting. No need to move the probe to the amp position on the meter. Where are you seeing 34v, at the speaker terminals or at the emitter resistors?

    - Pete
     
  5. Young46

    Young46 AK Subscriber Subscriber

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    No problem, whew, thought I was doing something else wrong. I’m seeing the 34 volts at the emitter resistors. Only on the right channel though. I checked the left channel and read 0v. I am not trying to make any meaningful adjustments yet. Right now it seems ok except for that high reading. Nothing seems to be getting real hot or smoking. I can’t hear relay click.
     
  6. ghazzer

    ghazzer Sansui addict Subscriber

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    Please clarify the resistors values that are currently installed for R72 and R78. Are they 100Ω,5W or 0.33Ω,5W?

    Are you reading ~34vdc across each resistor (~78vdc total) or ~34vdc across both resistors (~17vdc each)?

    If you ever read zero mV drop across a resistor it indicates that zero current is flowing through the resistor.

    One thing that you might want to verify is that the trimmer VR04 is functioning. With power OFF: connect your mini-grabbers to the lead of R38 that connects to VR04 and the lead of C26,C28 or C40 that connects to VR04 (whichever is most accessible) and measure the resistance as you adjust VR04.With the mini-grabbers attached to adjacent components you will be reading the resistance through the solder joints, and validating them as well.

    If you measure resistance between jumper holes 58 and 60 you should see the sum of R72+R78, and validate all four solder joints of the two emitter resistors.
     

     

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  7. Young46

    Young46 AK Subscriber Subscriber

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    Thanks, see my answers to your questions in the quote.
     
  8. kevzep

    kevzep Its all about the Music Subscriber

    What's the voltage drop across those resistors with the 0.33Ω resistors in?
    I am thinking of you are seeing that much voltage even across the 100Ω then you have a collector to Emitter short somewhere there....
     
  9. Young46

    Young46 AK Subscriber Subscriber

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    ghazzer I connected to the lead of R38 and C40 and got 3.059 all the way ccw, and 3.064 all the way cw. The resistance reading I got between jumper holes 58 and 60 was 0.8 ohms.

    kevzep If what you are asking is the reading I got for bias across R72 and R78 with the 0.33 ohm emitters, it was 34.48. Perhaps I was reading it wrong. Give me a couple of days to collect my thoughts. Please.

    Apparently I did not pay enough attention to your instructions with the temporary 100 ohm resistors installed. I thought they were to just be installed and the unit powered up on the DBT and then removed. My mistake and I’m sorry that I misunderstood. Right now the 0.33 ohm resistors are installed. Do I need to backtrack?
     
  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Yes, the 100Ω resistors should 'stay in' until you have the circuit behaving itself, whether 'on DBT' or not. ;)
     
  11. ghazzer

    ghazzer Sansui addict Subscriber

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    Young46: After looking at my photos of the track side of the F-2980 I am beginning to think that the schematic in the G-7500 SM is wrong. Let me make some quick checks and I will be back shortly.
     

     

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  12. ghazzer

    ghazzer Sansui addict Subscriber

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    This is a schematic that agrees with the tracks on the F-2980 PWA in MY G-6700. It also agrees with the schematic for the G-7700 in the SM. You will need to check this carefully against the traces on your F-2980 bd --- Sansui was not known for their configuration control in those times. The traces should be correct but component values may differ.

    According to this you will need to take resistance readings between R38 and C18/C22/C24/C26 or C38 (on the VR04 side).

    Sorry for the confusion.

    G-6700 Right Bias Schematic-015B 2X.jpg
     
  13. Young46

    Young46 AK Subscriber Subscriber

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    Thanks, I printed the attachment and will check it against the board in my receiver.
     
  14. kevzep

    kevzep Its all about the Music Subscriber

    Bare in mind there are two versions of this board, so make sure you check 'em carefully ...
     
  15. Young46

    Young46 AK Subscriber Subscriber

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    Sorry it has taken so long to respond but I checked the schematic and it appears to agree with traces on the the board in my system.
     
  16. ghazzer

    ghazzer Sansui addict Subscriber

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    Not clear which schematic your board agrees with, the SM or my .jpg.

    kevzep said:
    Bare in mind there are two versions of this board . . .

    Maybe someone will chime in and tell us how to identify the two styles (or more???) of F-2980 boards.
     

     

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  17. Young46

    Young46 AK Subscriber Subscriber

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    It matches your .jpg
     
  18. tarior

    tarior Dirty pool, old man? Subscriber

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    I didn't read the whole thread, but if you haven't done so yet, you absolutely must replace the bias and DC offset trimmers.
    The way that Sansui biased this amp circuit, if the bias trimmer has even a minor scritch in it, it will smoke the output transistors, base resistors, and usually a driver or two.
    I've fixed several Sansui amps with this basic bias scheme (G-6000, 7000, 7500), and the bias trimmers were sketchy in every single one.
     
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  19. ghazzer

    ghazzer Sansui addict Subscriber

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    Yep, Spoken like a true veteran of the Trimmer wars!

    Rest assured that it was our first line of attack.
     
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  20. Young46

    Young46 AK Subscriber Subscriber

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    Sorry for the gaps in communication. Here is an update on where I am. I installed the 100 ohm resistors on the right channel and powered up on the DBT with a 72W halogen bulb. I let it warm up for about 15 minutes and then I took a few voltage readings around the board. Most of the readings were at the pins. Here are my findings so far.
    2-5 = 0, 6=.155, 8=.154, 10=26.08, 11=.029, 12=.029, 13=0, 15=0, 17=29.07, 18=29.07, 19=.001, 20=24.81, 22=24.81, 23=25.30, 24=24.83, 25=25.29, 26=24.84, 32=0, 35=0, 51=29.02, 52=28.95, 53=29.01, 54=28.98, 55=1.75, 56=28.58, 57=1.31, 58=16.06, 59=1.31, 60=17.14, 61=.88, 62=28.72, 63=29.02, 64=28.95, 65=29.01, 66=28.95. I also took some readings at components...TR10 base -27.13, emitter -27.13, TR06 base 25.90, TR08 emitter 25.89. I also measured bias across R78, R72 and it was 0mV. I tried adjusting VR04 ang got nothing. I will go around the board today and read some more voltages at points on the schematic. Any other test points I should do? Thanks
     

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