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SWR Workingman's 15- Voltages- DC on Speaker

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  • #91
    Very much so, Enzo.
    Bstring, suggest you concentrate more on studying transistor circuits than depending on simulators. I know too many who use a simulator as a crutch. All of the analyses that we provided in this thread were done based on circuit knowledge. I only resorted to the simulator because I was having trouble convincing the community to take a leap of faith that the schematic was wrong. I can understand that. Maybe we can all take something out of it. On the plus side, despite differing perspectives, not one flame in this entire thread. Well done.
    Jcon

    BTW Bstring, any spice simulator can analyze this circuit... if you know what you're doing. In this case, I needed to generate x-sistor models, and modify a model to simulate back-biased leakage. It's not rocket science, but those who use spice as a crutch will frequently get the wrong answer. Without circuit knowledge, they will not realize it.
    Jcon

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    • #92
      I tip my hat, your sim told the tale.

      What continues to nag at my souls is how did this ever work. it wouldn't have passed QC with 30v on the speaker.
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #93
        Originally posted by Enzo View Post
        What continues to nag at my souls is how did this ever work. it wouldn't have passed QC with 30v on the speaker.
        See post #88, it had 22R stock, OP replaced with 220R (because of erroneous schematic )

        log1982, top notch work!
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #94
          Enzo, Maybe in the process of repairing it, someone replaced parts to match a schematic, not necessarily the right one. Probably never know...

          Comment


          • #95
            Originally posted by DrGonz78 View Post
            Someone should really notify Fender about those schematic errors. Maybe they care enough to edit the schematics they have on their servers?
            Well, it does say "Preliminary" across the top, and it is a legacy brand so Fender usually just says "this is what we were given."

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            • #96
              Fender usually just says "this is what we were given."
              Not only usually, that is exactly what they say. I attach the disclaimer from the Fender Service collection.
              Attached Files
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #97
                So in recap:

                Our schematic was wrong, the silk screening was wrong, the parts installed were wrong. It's a hat trick.

                general error being three 22 ohm resistors had 220 ohm resistor in their places.

                I am left with one dangler from my previous thoughts: how much current now flows through Q8 and all above it?
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #98
                  Originally posted by log1982 View Post
                  Enzo, Maybe in the process of repairing it, someone replaced parts to match a schematic, not necessarily the right one. Probably never know...
                  See post #88.
                  What I get from that is that BStringThumper had earlier replaced the 22R's with 220R's.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #99
                    Good morning,

                    Again, I can't thank you all enough!!! I learned a lot from this, mostly I know so little. Log1982, I will be studying transistor circuits and biasing. Thanks for using your knowledge, time, and splice to show us all and explain how to fix it. It was impressive how you all knew the circuit wouldn't work as drawn.
                    Enzo, thanks for putting up the FMIC disclaimer (wow) and that SWR LA15 schematic. That confirmed that Q6 & Q7 were in fact 22 ohm not 220 ohm as stated on the schematic. So recap. Schematic is wrong on the SWR Workingman's 15 bass amp schematic, R30A, R30B, R39A are 22 ohm resistors. Not 220 ohm.
                    DrGonz78, g1, Glebert, Strato56, Jazz P Bass, loudthud, thank you too for assisting. I will send an email to FMIC regarding the errors on the schematic.

                    We go through all this and I sure hope it sounds good! Thanks again!

                    Comment


                    • Bstring, I should remind you that we think R23 as a 1watt resistor is overstressed. SWR seems to think so too. In Enzo’s corrected schematic, it’s listed as 2w. Suggest you double check to see if it’s a 2 watter to avoid future repairs. Some numbers. At rest, R23 dissipates .92W. When the amp is pumping out 50 watts, R23 goes to 1.07. At 100, it’s 1.22, and at 200, it’s 1.5. I don’t even want to think about what it looks like in saturation.
                      Jcon

                      Comment


                      • UPDATE:***** AMP NOT FIXED****** I BLEW IT UP**

                        Hi Everyone,

                        In the process of checking and setting the bias, following the hand written instructions on the factory biasing page, Q3 went up in smoke. I forgot how bad a burnt up semiconductor can smell. I replaced Q3 and resistor R28 (100 ohm), all other components tested good. I powered it up on the bulb current limiter first, no glow, bias was stable. I powered down and hooked up an 8 ohm speaker and CD player to the input. Powered back up and had sound. So I powered down and got set up again with the 2 ohm load and input signal. I still had it on the current limiter just to see what it would do. As I was just about to check DC offset the bulb went full bright. I powered down. Q3 was blistering hot and shorted. This time I wasn't so lucky in regards to the components. It shorted both outputs and driver transistors. I'm out of parts. I'm wondering what I did wrong. Could that 2 ohm load be too low??? It didn't blow up when I was running the 8 ohm speaker load.

                        Anyway, until I order parts...………….or not.... I have a big paper weight.

                        Log1982, thank you for pointing out that under-rated resistor. I will put a two watter in there to be safe.

                        Thanks again to everyone who assisted on this. I'll report back if/when I have parts. Happy soldering!

                        Comment


                        • Very likely that Q4 is dead and Q3 died trying to carry his load.
                          2 Ohm vs 8? Large difference. For example, with a 20V p-p signal, Q4 on average dissipates 20 Watts @ 8 ohms, and 80 Watts at @ ohms. With a 40v p-p signal, Q4 dissipates 35 watts @ 8 and 140 @ 2 ohms. I suspect you damaged Q4 (and Q10 for that matter) with a 2 ohm load, and Q3 died soon thereafter. In case you're wondering, peak dissipation is several hundred watts @ 2 Ohms for hundreds of usecs. I'd avoid that if I were you.
                          Jcon

                          Comment


                          • If running on limiter lamp, do not power down to connect load.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • Originally posted by log1982 View Post
                              Very likely that Q4 is dead and Q3 died trying to carry his load.
                              2 Ohm vs 8? Large difference. For example, with a 20V p-p signal, Q4 on average dissipates 20 Watts @ 8 ohms, and 80 Watts at @ ohms. With a 40v p-p signal, Q4 dissipates 35 watts @ 8 and 140 @ 2 ohms. I suspect you damaged Q4 (and Q10 for that matter) with a 2 ohm load, and Q3 died soon thereafter. In case you're wondering, peak dissipation is several hundred watts @ 2 Ohms for hundreds of usecs. I'd avoid that if I were you.
                              Jcon
                              Hi log1982,

                              Thank you for the wattage analysis. I agree, 2 ohm is a large difference from 8 ohm. Your calculations just proved why it blew up... twice. Here is the attachment for the test procedure I followed. I did retest the components after the first failure, it was working until I tried to do the bias procedure again with the 2 ohm, then it shorted the outputs and drivers on the second failure.

                              With the help of your calculations I think I'm going to order the parts and try it again. This time with an 8 ohm load and just look for the crossover distortion to just disappear and leave it. Thank you!!!
                              Attached Files

                              Comment


                              • Hi g1, thanks. After R39A was replaced with the correct 22 ohm (not 220 ohm) the amp would power up on the limiter even with a load. I did read that some amps will not be able to do this because of the soft start not stabilizing first. This has been quite the learning curve.

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