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

  1. #36
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    To clarify....the resistor swap matches the schematic.

    Update: C12 tested good but was replaced while I was there. No change to original condition.
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    Last edited by BStringThumper; 01-21-2020 at 10:58 PM.

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

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    Hi Glebert, yes, C12 was fine.

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    Update: Q5 is not a 2N6474, it is a substitute part MJE15028G recommended by Newark.
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    Have you checked R28? I don't see that marked OK on the drawings, but maybe I missed one.

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    Lifetime Member Enzo's Avatar
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    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.

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    Yep, R28 is ok Fluke reading 99.4 ohms.

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    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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    Quote 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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    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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  11. #46
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    If this amp was on my bench :
    I would desolder Q4 and Q10. The drivers Q3 and Q9 have DC-paths on their own and should be able to stabilize the output (No-load) to zero volts.
    If that doesn't happen, well maybe some new voltages would learn me something.

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    Noticed the schematic calls out "power ground" over by the zobel network. I don't know what is being used for reference for these measurements but it seems like that is where they should be referenced to.

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    Quote Originally Posted by glebert View Post
    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.
    Hi Glebert, I will investigate the POWER GND reference, but I think my voltage rails are reading correctly. I'll try anything. I also changed the resistors on R30A & R30B to exact 220 ohm (matching). The numbers don't make sense to me either.

    Update. I upload the new voltages. I only updated the numbers in the circle because everything is reading higher today, so please ignore the outputs and bias string.
    The POWER GND is going to the speaker jack negative side. It has good continuity to ground...…….. the jacks had star washers on them.... but maybe they should be isolated? That wouldn't be much of a ground. I'll unground them next try. The Q6 & 7 numbers changed.... again. Maybe the matched 220 resistors?

    Thank you everyone for helping.

    Update: ungrounding speaker jack from chassis ground made no difference to the 31Vdc on output.
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  14. #49
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    Do you still have this powered up with a light bulb limiter? I ran out of ideas a long time ago, now grasping at straws.

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  15. #50
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    I know that you replaced Q5 with a good substitute. I was looking at the picture that you had posted of the main board and was looking at the screw missing connecting the Q5 to the heat sink, at least that's what it looked liked. Very tough to tell, as I only could see there was thermal past but could not tell if there was mica insulator in place. It does look like other screws are missing and I would want to confirm there are no ground connection missing connected to the power amp board.

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    As I wrote before, and as glebert alluded to, the circuit is doing exactly what schematic says it should do. Need more pulldown current than what the schematic is showing.
    Wanna test the theory? Change R23 from 2.7k to 27k.

    Jcon.

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    Quote Originally Posted by DrGonz78 View Post
    I know that you replaced Q5 with a good substitute. I was looking at the picture that you had posted of the main board and was looking at the screw missing connecting the Q5 to the heat sink, at least that's what it looked liked. Very tough to tell, as I only could see there was thermal past but could not tell if there was mica insulator in place. It does look like other screws are missing and I would want to confirm there are no ground connection missing connected to the power amp board.
    Hi DrGonz78, Q5 is not bolted down. There are mica insulator there for Q4, Q5, Q10, so no grounds there. There are also no grounds underneath the circuit board hold down screws. Great thought. Please keep them coming.

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    Quote Originally Posted by log1982 View Post
    As I wrote before, and as glebert alluded to, the circuit is doing exactly what schematic says it should do. Need more pulldown current than what the schematic is showing.
    Wanna test the theory? Change R23 from 2.7k to 27k.

    Jcon.
    Hi log1982, thanks for the suggestion. 27k seems a bit extreme, should I try something in-between 2.7k & 27k? I'll give it a try.

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  19. #54
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    Yes, sir, still on the limiter. I'm grasping at straws and pulling what's left of my hair out.

    I'll look at it again tomorrow. Good night.

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    Quote Originally Posted by BStringThumper View Post
    Yes, sir, still on the limiter.
    Is the bulb glowing at all? If not you should be able to take it off the limiter, might make the voltages stabilize better.

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  21. #56
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    Quote Originally Posted by BStringThumper View Post
    Hi log1982, thanks for the suggestion. 27k seems a bit extreme, should I try something in-between 2.7k & 27k? I'll give it a try.
    What value do you find R23 in your amp to be?
    If log1982 is correct, then there is a typo and someone has changed that resistor in your amp to match the typo. Obviously the amp worked at some point so it's not possible this is some design flaw.
    What is the history of the amp? Something someone tried to fix or was discarded? Or something that was working one day and not the next, as seen by the owner?

    edit: also please post the values found installed for the other resistors log1982 mentioned:
    R23, R24, R30, R39a, or R37.

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    Lifetime Member Enzo's Avatar
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    I thought log was suggesting the different resistor only to prove a point that it would change the current through the VAS. I didn't get the impression he thought that was what was supposed to be there.

    I could be wrong too.

    One thing to remember is this amp used to work. Unless we believe this thing always put 30v on the speaker right from the factory. SO redesigning the circuit wouldn't seem to be the real way to fix it. Like if my closet door won't close right because a coat hanger is in the way, I don't get out a plane and shave the door down to "fix" it.

    I hada no idea we were still on the bulb limiter. The limiter is only for saving fuse blows and not stressing an amp with serious problems. But this amp is not blowing fuses, so get rid of the bulb, it does skew all voltage readings.

    All that follows could be wrong:
    I think the circuit wants Q8 turned on harder. You now seem to have 10ma through R39 and 7ma through R39A. The more positive the base of Q6 the more it turns off (remember PNP and "upside down") That tends to drop current through R39, making the voltage atop it more negative. That lowers the voltage on Q8E, relative to the base. That turns it on harder. Likewise reducing current through Q6 means the drop across R30/30B drops, making it more positive. Over on Q7, the base is tied to zero volts where the signal enters. If R30 goes more positive, that means the emitter of Q7 goes more positive relative to the base. The base thus more negative. Again PNP, so that should turn on Q7 harder, when pulls Q7C more positive. it is wired to the base of Q8 so that should turn on harder, which is what we want.

    Looking at some other SWR circuits I see the same 2.7k 470 470 and bias transistor in the collector circuit of the VAS.

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    Don't know if it helps much, but this thread (https://music-electronics-forum.com/...ad.php?t=34764) has some voltages called out for this model amp that is a lot closer to working than the OPs. See post #9.

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  24. #59
    Lifetime Member Enzo's Avatar
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    His numbers show that our amp here is trying to correct the DC offset, and can't.

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    Rather than saying Q8 isn't pulling enough current could it be that Q5 is not allowing it to have enough? Since Q5 is replaced by a substitute part and there is a note about changing the value of R27 based on the Q5 beta I wonder if that factors in. Original part had a large hfe range (15-150), replacement part specd at 40.

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    Lifetime Member Enzo's Avatar
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    Q5 has 2v across it, that is its job, to maintain a couple volts space between bases of Q3, Q9. I'd have to say it works.

    If we turn on Q8 harder it will pull that whole mess down, but Q5 will continue to make that 2v space. That is how the amp works. Q5 establishes that 2v space to prevent crossover distortion. The Q8 yanks it up and down with the music. The outputs follow whatever Q5 is doing.

    In fact you can even short Q5 E to C. That will increase crossover distortion, but the amp will run coolest possible. And I think you would still find 30v offset.

    I could be wrong.

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    Quote Originally Posted by glebert View Post
    Don't know if it helps much, but this thread (https://music-electronics-forum.com/...ad.php?t=34764) has some voltages called out for this model amp that is a lot closer to working than the OPs. See post #9.
    Yeah I had pulled that thread up a few days ago and drew all the voltages out on a schematic. I wrote Q4 (61.3v) instead of 66v as the thread said. I kept wondering was why there is 500mv at the base of Q7 and 10v at the base of Q8. When they should be about 100-200mv each.

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  28. #63
    Lifetime Member Enzo's Avatar
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    Doc, I am confused. Your drawing shows 10v on Q8 collector, not base. 176mv on base of Q7.

    The main rails were 61v in one case and 66v in another? NO big. Units vary, and mains volts vary.

    One major difference is the other thread only had a 400mv offset, whereas we have a 30v offset here.

    The 60v on collector of Q5 looks wrong to me.

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    Quote Originally Posted by Enzo View Post
    Doc, I am confused. Your drawing shows 10v on Q8 collector, not base.
    I'm guessing he meant Q6 base of the OPs amp, not Q8.

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  30. #65
    Lifetime Member Enzo's Avatar
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    OK, I'll buy that.

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    Yeah I meant 10v on base of Q6. Still I think Enzo is right that Q5 is doing its job and that Q8 is behaving strange.

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    Could the pins of Q8 be scrambled ? Could the Base and Emitter be reversed ? What is that note in the lower left corner of the schematic ? Shouldn't the pinout be B C E if you are looking at the top surface of the part with the leads pointing down ?

    Edit: Q8 looks OK in the pic at the top of this page, but the B-E Voltage looks too high. Something has to be wrong if Q8 can't pull the output down below ground.

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    Last edited by loudthud; 01-24-2020 at 12:03 PM.
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    Well, actually, yes, I did, sorta. Enzo, G1, OP, thanks for considering my assertions.

    27K is about the right value to bring the output to zero when difference amp q6/q7 is in equilibrium.
    At rest, (no signal) we want the output to be zero volts. That means we have to drop 50 something volts across R23 and R24.
    To maximize linear dynamic range in the feedback circuit, Q6/7 should share current equally when the output is zero.
    *If the schematic is correct*, then the kooky current mirror Q8 will sink about 2mA.
    We need something like 25k in R23 to drop 50 volts with 2 ma
    ** Remember, I said if the schematic was correct.
    R37, R39A, R30, and R23/24 each play a role.
    The most logical resistor to suspect is R23 because it is listed as a 1W. If it were 2.7k, it would be over-stressed in this application.
    Jcon

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    Good morning,
    Thank you everyone for the suggestions and detailed explanation of the circuit.
    Yes, I had it on the limiter for these tests. I will take new readings. I did try it direct to mains and then tried plugging in the speaker load and it still had high Vdc on speaker. I'll post new reading asap. Thank you again for all the help!

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  35. #70
    Lifetime Member Enzo's Avatar
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    ALWAYS check for DC before connecting a load. That 30v offset without one will not go away with one.

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