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SWR Workingman's 15- Voltages- DC on Speaker

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  • #31
    Q2 is saturated, note the base is half a volt more negative than the emitter, turning it on. In any case, it has teh +55 on the collector to power the circuit, which is its job. That Q1,2 circuit is working.
    Education is what you're left with after you have forgotten what you have learned.

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    • #32
      Hi everyone,

      Sorry it took so long. I replaced both Q6 & Q7 (KTA-1268). I was trying to rush to get the voltages around Q6 & Q7. It must be the time of day but my voltages were changing. I don't have it on a variac so it's probably everyone getting home. The numbers did not change much overall. Still have 31Vdc on speaker jack. I'll go read everyone's posts, but here is the new voltages. Thanks everyone for helping.

      I'll start tomorrow with a new schematic and sharp pencil and take voltages again.
      Attached Files
      Last edited by BStringThumper; 01-21-2020, 12:43 AM.

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      • #33
        Hey guys, maybe I’m off base here, but it seems like the circuit as drawn can’t do the job. I suspect a schematic typo. Makes troubleshooting difficult, no? Here’s how I see it. Q8 is incapable of sinking enough current to swing Q3/Q9 base. They need to go from -50 to plus 50 volts approx. At best, Q8 can sink 6 mA (as glbert noted) when driven from Q6/Q7 difference amp. R23 and R24 are the pull-up resistors for Q8 current. 6mA is not enough to swing that kind of voltage across R23/24. Suspect one or more value typos: R23, R24, R30, R39a, or R37.
        With 6 mA Q8 collector current, OP’s original results are not surprising. Gotta figure out what’s what before we can help the OP.
        Respectfully, Jcon.

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        • #34
          I would say that the latest set of voltages make more sense. Having -57V at the south end of R39 means more current is flowing through R39 than R39A, which would be coming from Q6 and Q7/R37. That makes more sense as it no longer looks like current is going into the collector of Q6. That being said I still can't reconcile the Q6 base current with the BE voltages.

          Can we get the voltage on the north and south ends of R30? Trying to figure out how the emitter voltages on Q6,Q7 can be so close when one is forward biased and one is reversed. The voltage across R37 implies at least 2.8mA of current coming from Q7 collector. 2.8mA of current coming into the emitter of Q7 would cause a 0.6V drop across R30a. Since Q6 is reversed biased there should be no current going into its emitter, so it's emitter should be at least 0.6V higher then the emitter of Q7 (no drop across R30b).
          I'm wondering if C12 could be leaking and dragging down the voltage leading into Q7 and Q6.

          edit: had typos and was calling Q6 Q8 and calling everything a collector for some reason.
          Last edited by glebert; 01-21-2020, 02:07 PM.

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          • #35
            Thank you all for the help. The voltages on the board are changing as it heats up. When I go back to double check it's different. Here are the newest readings. I will test or replace C12 if I have one. There are some typos on the board. R36 & R39 values have been swapped, but it matches the schematic, just not the value it says on the board. I'll put up a picture or two.
            Thanks for any suggestions.
            Attached Files
            Last edited by BStringThumper; 01-21-2020, 05:59 PM.

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            • #36
              To clarify....the resistor swap matches the schematic.

              Update: C12 tested good but was replaced while I was there. No change to original condition.
              Attached Files
              Last edited by BStringThumper; 01-21-2020, 09:58 PM.

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              • #37
                Since you have 56V at C12 I think it is fine.

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                • #38
                  Hi Glebert, yes, C12 was fine.

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                  • #39
                    Update: Q5 is not a 2N6474, it is a substitute part MJE15028G recommended by Newark.
                    Attached Files
                    Last edited by BStringThumper; 01-21-2020, 10:20 PM.

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                    • #40
                      Have you checked R28? I don't see that marked OK on the drawings, but maybe I missed one.

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                      • #41
                        For what it is worth, I see everything from Q5 and to the right as working. I see for whatever reason Q8 not turning on enough, so that just shoves all the Q5 and so on off to +30. Looking at the bias string, I see R38, R24, R23 all conducting about 7.5ma. And indeed across R39/39A I see about 9ma. I'd expect we need to triple that current. That would pull Q5 et al down to roughly zero DC.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #42
                          Yep, R28 is ok Fluke reading 99.4 ohms.

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                          • #43
                            Hi Enzo, thanks for taking another look at this. I've been checking and rechecking components. I'm ready to try anything.
                            I'll have to attack it tomorrow though. Thanks again everyone for all the suggestion of things to check or try. Have a nice evening.

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                            • #44
                              Originally posted by Enzo View Post
                              For what it is worth, I see everything from Q5 and to the right as working. I see for whatever reason Q8 not turning on enough, so that just shoves all the Q5 and so on off to +30. Looking at the bias string, I see R38, R24, R23 all conducting about 7.5ma. And indeed across R39/39A I see about 9ma. I'd expect we need to triple that current. That would pull Q5 et al down to roughly zero DC.
                              Is the current through Q8 set by the difference in collector voltages of Q6 and Q7? The collector voltage of Q7 goes to the base of Q8, the collector of Q6 goes between R39 and R39A. So I think the equation would be Vc(Q7) - Vc(Q6)=Vbe(Q8)+V(R39a) = 0.5 + I(R39a)*220. I think to get a 3x change in current through Q8 you would need to have about 5.3V between the Q7 and Q6 collectors. Both voltages would be higher (less negative) as the increase in current through Q8 is going to increase voltage across R39.

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                              • #45
                                I know you said the voltages are changing as you are probing, but these numbers don't make sense. To get the the 55-ish volt drop across R30 you have to have 2.5mA of current, and Q6 is not conducting because it is reverse biased (and no voltage drop across R30-B) so the 3mA is going through Q7 but that should cause a 0.5V drop across R30-A, which is not there.

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