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SWR Workingman's 15- Voltages- DC on Speaker

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  • #76
    My concern is this is the same circuit they have used in numerous models. PArts are essentially the same. The amps work.

    What base current would Q8 need to conduct say 20ma?
    Education is what you're left with after you have forgotten what you have learned.

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    • #77
      That’s not the best way to figure it out. Think of it this way: q7 collector sets up a voltage across r37. That same voltage is impressed across q8 base-emitter and r39a. The current through q8 is determined largely by the drop across r39a when you subtract Vbe (.6 for this transistor).
      To answer your question, to get q8 to give you 20ma, Q7 needs to source about 6ma. A bit more when you take into account losses due to base current.
      Enzo, I don’t doubt that the amps work. I just think we aren’t seeing everything in the schematic. I get a sense you’re seeing my point. Am I right? Are you seeing anything that I’m missing?
      Jcon

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      • #78
        What I am seeing is that neither of us - both with reasonable approaches - is not getting the right answer. I don't see what you are missing, and of course vice versa.


        Some posts ago, I determined the amount of current that would have to flow through that whole string to pull Q5 down to zero DC, and it was roughly three times what we measured at the time. So I wondered what would be different from what we see now to cause that extra current we need.

        I don't think the schematic itself is wrong, as they used this circuit and variants of it all over their product line. Plus we assume reasonably that this amp worked at some point with all these value parts.

        SO I don't know. if I had it in front of me, a lot of instinct could come into play that won't, due to the remote repair.
        Education is what you're left with after you have forgotten what you have learned.

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        • #79
          Respectfully, Enzo, can we really say that the schematic doesn’t have an error? 1.) it doesn’t seem like the circuit can do what it’s supposed to do, and 2.) the actual performance is exactly what the as drawn schematic would predict.
          Think of this: does it make sense that r23 should be 1 watt?
          Respectfully, Enzo I think I did try to show what you were missing. Thanks for the healthy conversation.
          Jcon

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          • #80
            Since this circuit seems to be stumping everyone, let me throw out two alternate theories.

            1.) in the first post the OP says he is concerned the transistors put in may be counterfeit. I think it may be good to know which have been replaced, by whom (OP or someone else) and where the components were sourced from.

            2.) OP says two resistor (R36, R38) values on the board silkscreen don't match the schematic, but the components do match the schematic. Are we sure this is the right schematic? SWR had a few iterations of this, and some of the changes are in this Q6, Q7, Q8 area. Why do we assume the silkscreen is wrong?

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            • #81
              I just checked out some other SWR schematics. Different in a big way. R39A is either very small, or zero. In that configuration, you could easily get the current in q8 to pull down R23. (Still think it’s overstressed). Why does OP’s schematic have 220 ohm to limit q8 current? Why is is called R39A? What’s the A for? Is it adjustable in some way?
              Jcon.

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              • #82
                Respectfully, Enzo, can we really say that the schematic doesn’t have an error? 1.) it doesn’t seem like the circuit can do what it’s supposed to do, and 2.) the actual performance is exactly what the as drawn schematic would predict.
                Think of this: does it make sense that r23 should be 1 watt?
                Respectfully, Enzo I think I did try to show what you were missing. Thanks for the healthy conversation.
                Jcon
                I cannot say that. Just pointing out that even with combined technical might we have been unable to solve this.

                I calculated quite a few posts ago that R23 is dissipating something like 0.8 watts or so. SO yes, 1W is underspec'd. On other models, the part is rated at 2.7k 2w instead of 1w.

                R36, R38? Well, R36 in series with a cap ought not upset the DC balance, whatever value is there. R38 says 470, to balance the 470 above the Q5 ckt. I'd believe it, but what was the silk screen, 270? Would that change the 30v offset?

                On other models I do find some differences in the Q6,7,8 area, but are only a few resistors. The one constant in all of them is the 2.7k up top.
                Education is what you're left with after you have forgotten what you have learned.

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                • #83
                  What’s the A for? Is it adjustable in some way?
                  NO, they split the resistor in two. Note above the diffy pair, R30, R30A, R30B. I THINK the As and Bs were added where originally had been just a wire, and they didn't feel like finding new part numbers.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #84
                    Originally posted by Enzo View Post
                    .

                    R36, R38? Well, R36 in series with a cap ought not upset the DC balance, whatever value is there. R38 says 470, to balance the 470 above the Q5 ckt. I'd believe it, but what was the silk screen, 270? Would that change the 30v offset?

                    On other models I do find some differences in the Q6,7,8 area, but are only a few resistors. The one constant in all of them is the 2.7k up top.
                    This got me thinking about the silkscreen value of those resistors. On R36 you can see it in the picture as 180. I can't see which is R38 or what the value is. But then I noticed that R36 was a higher power resistor, and sure enough the schematic calls out 2W. It took me a second to wonder why this needed to be so big, then realized, of course, it sees the full AC amplitude coming from the power transistors (through C17). This also explains why R39 needs to be 2W as well. This set off an even weirder thought than I usually have. Could this be a stability problem? Do we know these DC values we are seeing are pure DC? I didn't see any mention of an AC voltage measurement but may have missed it. Is there any chance the output is oscillating and what we are seeing is just the offset? OP said the speaker would hum when connected, and a speaker can hum a bit with DC but it makes me wonder if there is something else there too...or maybe I just watch too much crap on the History Channel and love a crazy conspiracy theory.
                    Last edited by glebert; 01-25-2020, 06:52 AM.

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                    • #85
                      Well, a speaker hums a LOT when amps make DC. I tend to think it is stable and just offset, but it is simple to find out. Slap a scope on it.

                      I know we are throwing opinions around, so..I see three sets of data. One is the schematic, one is the silk screen, and one is the parts installed. I see the parts and schematic in agreement, silk screen is odd man out. Two out of three. SO my vote is the schematic is right. I could be wrong.

                      Without rereading all 80 posts here, I don't recall the source of installed transistors, nor the substitutions made. Certainly transistor fakes could be involved.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #86
                        Gentlemen, you are men of science. Trust your eyes. Trust your analyses. Don't let preconceptions ("The schematic has to be right") cloud you conclusion. Let’s test our assertions.
                        I ran spice simulations. As drawn, with +/- 60 volt supplies, Spice predicts a dc output of 33.2V. OP measured 32.7. Spice says the Junction of R23/24 is 38.0. OP measures 37.6. Two take-aways: the circuit is doing exactly what we should expect. It cannot work as drawn.
                        I ran some scenarios to show where a possible typo may be:
                        Test1: Change R23 to 27K. **Spice says output goes to 0.0025 V.** Diffy pair Q6/7 are in good equilibrium at 1.306 and 1.320 mA respectively. Works as predicted.
                        Test2: Change R39A to 22 ohms. **Spice says output goes to 0.037V**. Diffy pair is a little off at 1.365/1.260mA, but it probably works, as predicted.
                        Test 3: do I need a test 3? We could re-configure all day to make it work.
                        If OP would take a 22 ohm resistor and parallel R39A, I bet he would get some new data for us to evaluate. I kinda think it still wouldn't work cause I think we blew up Q7 in back-bias condition.
                        Jcon
                        Last edited by log1982; 01-25-2020, 04:00 PM.

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                        • #87
                          Originally posted by log1982 View Post
                          Gentlemen, you are men of science. Trust your eyes. Trust your analyses. Don't let preconceptions ("The schematic has to be right") cloud you conclusion. Let’s test our assertions.
                          I ran spice simulations. As drawn, with +/- 60 volt supplies, Spice predicts a dc output of 33.2V. OP measured 32.7. Spice says the Junction of R23/24 is 38.0. OP measures 37.6. Two take-aways: the circuit is doing exactly what we should expect. It cannot work as drawn.
                          I ran some scenarios to show where a possible typo may be:
                          Test1: Change R23 to 27K. **Spice says output goes to 0.0025 V.** Diffy pair Q6/7 are in good equilibrium at 1.306 and 1.320 mA respectively. Works as predicted.
                          Test2: Change R39A to 22 ohms. **Spice says output goes to 0.037V**. Diffy pair is a little off at 1.365/1.260mA, but it probably works, as predicted.
                          Test 3: do I need a test 3? We could re-configure all day to make it work.
                          If OP would take a 22 ohm resistor and parallel R39A, I bet he would get some new data for us to evaluate. I kinda think it still wouldn't work cause I think we blew up Q7 in back-bias condition.
                          Jcon
                          Wow! Thank you all for taking the time to give this some serious analysis. I really appreciate the explanations of the circuit. I'm willing to try anything at this point. I could just shot gun everything but I really want to understand why this thing is not working. Here are some answers to a couple questions brought up.

                          I was given the amp because it was not working. I found the output transistors missing. So I'm assuming someone tried to repair it and couldn't and pulled their new parts back out or burnt them up with the DC voltage on the circuit and pulled them anyway. Luckily the speaker is still good.

                          I sourced all the parts through Newark. I think the census is Q5 to the right (as Enzo said) is working. I think it's got to be in Q6,7,8, and resistor values.

                          I did at one point pull all the op-amps out of circuit to kill any signal, no change, put them back in. I will scope and probe for AC.

                          I will be back with some new numbers and resistor change values or a burnt smell.

                          I can't thank you all enough for all the help with this head scratcher.

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                          • #88
                            THE AMP WORKS!

                            I still have to do the amp biasing, but I only have 34mA of DC on a 8 ohm speaker load.

                            When log1982 recommended replacing R39A with a 22 ohm, instead of the 220 ohm stated on the schematic, I remembered that I had to replace that resistor. I initially replaced R30A & R30B because they were reading 21 & 22 ohms, not 220 ohms like the schematic. They were actually correct!! So the schematic is wrong in those three locations. Thank you log1982 and everyone that helped with this amp!! You saved me and an old SWR.

                            log1982, can you please tell me what splice program you are using. That was very impressive!

                            This is a great site! Thanks again!

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                            • #89
                              Someone should really notify Fender about those schematic errors. Maybe they care enough to edit the schematics they have on their servers?
                              When the going gets weird... The weird turn pro!

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                              • #90
                                Ah, so more like the LA15 schematic then?
                                Attached Files
                                Education is what you're left with after you have forgotten what you have learned.

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