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Thread: SWR Workingman's 15- Voltages- DC on Speaker

  1. #71
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    If you can I would do direct measurements of Vbe values on Q8 and Q5, since the consensus is that there isn't enough current flowing there. Especially if the voltages are varying I think it is useful to take that directly rather than subtracting the emitter and base voltages that might have been taken with some time separation.

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  2. #72
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    New voltage readings- plugged directly to mains. I'm going back to read and read all the posts. Thank you again.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  3. #73
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    I know you said you replaced Q6 but now it looks like current is flowing in reverse (out of the emitter) back towards Q7. This may explain where the current going into the base is going, but seems broken to me.

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    When you back-bias a base-emitter junction, it breaks down at about 6V and conducts reverse current. Look at the data sheet for Vebo. If not limited, it will break the junction, BTW. It's doing what you'd expect.
    Enzo, I think Q8 is fine. Working like a current mirror. 2.5mA in, ~7mA out (depending on Vbe which, as OP noted, changes as it warms up) is about right for the Q8/R37/R39A configuration. Hope I'm not missing anything.
    Enzo is correct, Q5 is working correctly as a Vbe multiplier. Adjusts for crossover current.
    Jcon.

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    My mistake: Q8 current mirror: 3mA in, 8mA out. (I originally forgot to consider the reverse current from back-biased Q6)
    Jcon.

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  6. #76
    Lifetime Member Enzo's Avatar
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    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?

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    Thatís 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 donít doubt that the amps work. I just think we arenít seeing everything in the schematic. I get a sense youíre seeing my point. Am I right? Are you seeing anything that Iím missing?
    Jcon

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  8. #78
    Lifetime Member Enzo's Avatar
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    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.

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    Respectfully, Enzo, can we really say that the schematic doesnít have an error? 1.) it doesnít seem like the circuit can do what itís 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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  10. #80
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    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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    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 itís overstressed). Why does OPís schematic have 220 ohm to limit q8 current? Why is is called R39A? Whatís the A for? Is it adjustable in some way?
    Jcon.

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  12. #82
    Lifetime Member Enzo's Avatar
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    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.

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  13. #83
    Lifetime Member Enzo's Avatar
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    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.

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    Quote 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.

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    Last edited by glebert; 01-25-2020 at 07:52 AM.

  15. #85
    Lifetime Member Enzo's Avatar
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    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.

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    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

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    Last edited by log1982; 01-25-2020 at 05:00 PM.

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    Quote 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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  18. #88
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    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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  19. #89
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    Someone should really notify Fender about those schematic errors. Maybe they care enough to edit the schematics they have on their servers?

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  20. #90
    Lifetime Member Enzo's Avatar
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    Ah, so more like the LA15 schematic then?
    Attached Thumbnails Attached Thumbnails LA12 and LA15 Power Amp Rev A.pdf  

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    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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    Lifetime Member Enzo's Avatar
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    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.

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  23. #93
    Don't forget the joker g1's Avatar
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    Quote 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!

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

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    Quote 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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  26. #96
    Lifetime Member Enzo's Avatar
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    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 Attached Files

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    Lifetime Member Enzo's Avatar
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    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?

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  28. #98
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    Quote 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.

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    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!

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    Bstring, I should remind you that we think R23 as a 1watt resistor is overstressed. SWR seems to think so too. In Enzoís corrected schematic, itís listed as 2w. Suggest you double check to see if itís 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, itís 1.22, and at 200, itís 1.5. I donít even want to think about what it looks like in saturation.
    Jcon

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    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...ÖÖÖÖ.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!

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  32. #102
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    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

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  33. #103
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    If running on limiter lamp, do not power down to connect load.

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    Quote 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 Thumbnails Attached Thumbnails Click image for larger version. 

Name:	SWR Bias Procedure----WRONG!.jpg 
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    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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