Announcement

Collapse
No announcement yet.

Strange switch behaviour

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Strange switch behaviour

    I'm looking at a Dr Z amp that runs four 6V6s, and has a Hi/Lo switch that switches a grid on each pair from either signal or 100K to ground. It was making a fair bit of noise in the Hi power position, so I got some deoxit in there. While I was investigating, I found the center tab measured a short to one side of the switch in one position, and 190K ohms to the same point in the other position. This true for all four cases, 190K to either side when in the off position, for both switches. Just to be sure I lifted the most accessable connection to the center lug, and the same is true. I realize switches go bad, but the same resistance in all four cases?

    Before I replace this switch, is there such a thing as a resistive DPDT switch? So strange.
    It's weird, because it WAS working fine.....

  • #2
    It seems to me that what you are reading is two grid resistors in series.

    Comment


    • #3
      "It seems to me that what you are reading is two grid resistors in series."

      No, I really don't thinks so. The grid resistors are 1.2K, and I don't see a path for any series connection. Plus, the center wiper, or common, whatever it is called, is disconected. Another thing I just realized, the two outside terminals of the switch measure 189K and 190K across each of them respectively.
      It's weird, because it WAS working fine.....

      Comment


      • #4
        Do you have a schematic so we can see what's going on in the circuit?
        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

        Comment


        • #5
          Perhaps it's like this?

          Click image for larger version

Name:	Switch.png
Views:	1
Size:	1.7 KB
ID:	856434

          Comment


          • #6
            Might as well just disconnect completely and measure.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #7
              It seems to me that what you are reading is two grid resistors in series.
              No, I really don't thinks so. The grid resistors are 1.2K,
              I THINK he meant two of the 100k resistors in series. As in perhaps the grid return resistors. I could imagine two resistors to ground, and when the switch opens measuring resistor to resistor yields 200k, 190k close enough, the fact that they go to ground in the middle doesn't change that. Possible?
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                This would make it 190k

                Click image for larger version

Name:	Switch2.png
Views:	1
Size:	1.9 KB
ID:	856435

                Comment


                • #9
                  It is wired like the one in post #5 if you change the leftmost 100K to 90K. It's a Dr Z so no schematic. I was reluctant to do much disconnecting given the immaculate wiring and red marker dots on every solder point, but once I did the switch acted normally, so it seems I actually have no problem here.

                  I still really am not seeing why I get 190K across the switch with it all connected.

                  edit: Oh wait, light bulb goes off, now I get it. The 100K and 90K are connected at ground putting them in series, but being grounded really doesn't have anything to do with it. DOH.
                  Attached Files
                  Last edited by Randall; 01-07-2020, 05:17 PM.
                  It's weird, because it WAS working fine.....

                  Comment


                  • #10
                    This may be the schematic (reverse engineered). I don't remember where I found it.

                    https://music-electronics-forum.com/...1&d=1578431216
                    Attached Files

                    Comment

                    Working...
                    X