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Trace Bi Polar Bear added resistors/circuit help

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  • Trace Bi Polar Bear added resistors/circuit help

    Problem: There appear to be either post schematic or post manufacture circuit updates done to the bottom of my Bi Polar Bear circuit board (PC00026Sx7) 300 watt BPJ output board.
    They being the addition of 2 resistors.This amp had the wrong output transistors installed and they were replaced with the SM recommended replacement. 2SA1695 & 2SC4468 as well as the drivers and the bias transistor. The photos I originally took of the bottom of the board disappeared along with the hard drive of my recently deceased laptop! But I only lifted one leg of both resistors and I'm not sure about exactly where they were connected to.
    What I've got: I need to figure out the destination of 4.7K ohm resistor (blue) "originally"/currently connected between R24 (1K) & R25 (1K) and looks like, by length, is destened for the emitter of TR14. There is also a 22K ohm resistor (red) that is "originally"/currently connected between the collectors of TR14 and TR13 and by length destined for a point between R28 (0.22 ohm) & R34 (0.22 ohm) or the Base of TR17. I'm hoping that someone that knows a bit about circuits can tell me if either addition makes sense, is wrong or maybe should connect to another point near by. I've included a shot of the board with the current positions of the resistors and a schematic shot of the circuit in question. Any help at all would be greatly appreciated.
    Attached Files

  • #2
    This is the model called GP7 ? (service manual attached)
    Attached Files
    Last edited by g1; 09-14-2020, 12:04 AM.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • #3
      See that R37 in the fan circuit and the note about it? Sounds like the 22K you are describing.
      The 4k7 sounds like a tweak for the bias circuit, and I'd guess it could go across R24 (as you describes) or across R25.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      Comment


      • #4
        g1: Bingo! Right between the V+ and the base of TR17. Thanks. As for the 4.7K resistor, it's currently connected to where R24 and R25 meet at the base of TR11 in the bias portion of the circuit. You mention that resistor as possibly going across R24 or R25 but I believe it's "original" placement, on one side of the resistor at least, is at the point mentioned. Does it not make sense connecting this point to somewhere near by? I'm well aware that being able to "fix" something and completely understand how and why circuits work are rather different capabilities but I'm working on it : )
        Thanks again and for the SM as well. It's the one I'm working off of as well.
        M

        Comment


        • #5
          Originally posted by aavatech1 View Post
          As for the 4.7K resistor, it's currently connected to where R24 and R25 meet at the base of TR11 in the bias portion of the circuit. You mention that resistor as possibly going across R24 or R25 but I believe it's "original" placement, on one side of the resistor at least, is at the point mentioned.
          Yes, both our statements agree.
          If the 'original' connection is at R24/R25 junction, then I believe the other end goes to the far side of either R24 or R25. There is no reason for it to go to TR14 emitter, that would not make sense.
          I am speaking only from the schematic, electronically, but now am looking at the picture of the part & board. From what I can see, it should go straight up and down (as opposed to sideways like it appears in the pic). That will put it in parallel with R25 I believe, changing the R25 value to about 825 ohms.

          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #6
            Gotcha. Thanks for clearing that up. I'll give it a try. I fired it up on DBT with the 4.7K connected to the emitter of TR14, no excessive current draw but DC offset is pretty much not settable. (..for my bad English of the day!) So I'll give it a shot with the 4.7K tied up to R25 and see what that does. If it's still wonky I'll try R24.
            I'll be checking the bias current in progress since I believe it should affect it. I'm wondering if the person that put the "other" output transistors might have slung that in there to make the 1K bias resistor remain functional. The output transistors I took out were: 2SA1263 & 2SC2625. I'm also going to look closely at the circuit board again to see if there is any other possibility for a connection from where it is. The 4.7K is cut a bit long for the distance to R24 or R25 but hey, people do weird things. (..like not backing up their photos! Guilty : (
            Thanks again,
            M
            Last edited by aavatech1; 09-14-2020, 09:48 PM.

            Comment


            • #7
              Yes, it's possible that the 4k7 was added by the person who installed the substitute output transistors (for bias purposes). If that is the case it can just be removed.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment

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