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

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    SWR Workingman's 15- Voltages- DC on Speaker

    Hi Everyone,
    I'm hoping someone can please take a look at these voltages on my SWR Workingman's 15". The unit is connected to a bulb current limiter. Negative of meter connected to chassis. No load at speaker jack and almost no light from bulb. With a 4 ohm speaker there is a full ON light bulb and hum from the speaker. Rotation of bias pot does change voltage but bulb stays full on and hum from speaker. I piggybacked a 470 ohm resistor to R27 bias resistor, no change. Semiconductors have been replaced or tested both in circuit and out. I did read that these could be fake/knockoffs. I hope I'm just missing something obvious and hope someone can provide somewhere else to check or what I'm missing. This is a great site. Thank you in advance for any help!
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    Lifetime Member Enzo's Avatar
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    With no load, DC on the output won't draw excess current. But put a load on it and voila, bright bulb. Bias shouldn't affect DC on output. So work with NO load for now.

    Q5 is the bias transistor, it sets the voltage DC difference between bases of drivers Q3, Q9. Roughly 2v, like yours. Whatever voltage is on Q5 will show up on the output. SO of Q5 ssits at +30, so will the output. This tends to mean the outputs are working.

    SO why is Q5 sitting at +30 instead of around zero DC?

    Once powered up +55v should come to Q5 by way of Q2 and R23,R24. It is getting there or we wouldn't have +30. The bottom end of Q5 is pulled down towards -55 by way of R38,R39,R39A and of course the voltage amp transistor Q8. And in my mind the problem centers there.

    Q8 is driven by Q7, with correction by Q7. So check Q8. What voltage is on its base? In other words is Q8 not able to conduct current? Or it it not being turned on enough by Q7? And the three Q8 resistors, are they OK? And are Q6,7 OK?

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    Hi Enzo,
    Thanks for diving in for me. Here is an updated voltage sheet.
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    I tried to rotate the picture. I hope this works.
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    Lifetime Member Enzo's Avatar
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    Pretty sure you have your B and C readings reversed on Q6,Q7. Clearly B of Q8 and C of Q7 MUST be the same, they are wired together.

    Check Q7,8.

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    Quote Originally Posted by Enzo View Post
    Pretty sure you have your B and C readings reversed on Q6,Q7. Clearly B of Q8 and C of Q7 MUST be the same, they are wired together.

    Check Q7,8.
    I think Q4 too. Also some of the transistor types are labeled wrong (NPN vs PNP), just don't want you to confuse yourself.

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    You were right, Enzo, thank you. Thanks glebert. Here is an update with voltages. Look at Q1 & 2 please.
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    Lifetime Member Enzo's Avatar
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    What about them, they seem to be working. Q8 is still a prime suspect.

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    Thank you for your help. Here is the corrected voltages for Q4. Thanks glebert, I loaded the wrong file last time. I diode tested Q8 in circuit. I got B-C .655, B-E .655, leads switched I got B-C open/infinite, B-E 1040. I switched to ohms reading and got the same 1040. It looks like R39A 220 Ohm and R37 820 Ohm= 1040 ohms in series with B-E. I do have another 2SC4382 to swap if you still feel necessary. I did plug the unit directly into a/c with the thought that maybe the voltage/current was just a little low. Q8 collector voltage went up to 28.8Vdc., still has DC on speaker jack. I'll put it back on the bulb limiter. Thank you for any suggestions.
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    Good morning,

    I removed and replaced Q8 with a new one. No change to the voltages. I rechecked the resistors around Q8 and they all tested good. Helped needed, please. Thank you.

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    Is the voltage at the collector of Q1 reading 0.014V? That seems pretty suspect to me. I don't know what Q1 and Q2 are supposed to be doing, but it looks like they are both shorted out. I don't know if Vce can be that low and just be in saturation region, or why someone would want this circuit to do that.

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    Lifetime Member Enzo's Avatar
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    It is a switch, it turns on Q2 to bring +55v into the circuit to enable it. It is a power up mute circuit. There is +55v on the collector of Q2, so that whole circuit is working.

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    Thinking out loud, without the clearest head:

    Going back to Q8, it is very strange that there is only a 0.1V drop across R39 (a 2 watt 180 ohm part), yet there is a 1.6V drop across R38a (220 ohm and presumably a 1/4 watt part), which would be 7mA. There is a 2.9V drop across the 470 ohm R38, which would be 6mA, so close enough. So current is going through R38 and R38a but not R39. Based on the wattage rating of R39 I would expect that there should be a much bigger voltage drop across it, or why put a 2W part there? But to have a bigger drop across R39 than R38a would mean more current would have to come from Q6. It almost seems like Q6 is sinking the current from Q8 instead of sourcing towards it.

    Now that I look are the voltages on Q6 shouldn't the base voltage be similar to Q7? Apparently there is current flowing into the base since there is a 20V drop across R35.

    edit: I'm wondering if that -54.1V beneath R39 is an error. The other voltage rails seem to be more like +/- 55.6V, which would give a more realistic drop across R39. Something seems wrong though as it looks to me like current is going into all three pins of Q6. There is 3mA going through R37 that has to be going somewhere too.

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    Last edited by glebert; 01-18-2020 at 03:46 PM.

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    Lifetime Member Enzo's Avatar
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    The bases should be similar, yes, but we have the 30v offset on the output sampled into Q6. Q6 normally tries to correct such differences.

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    Quote Originally Posted by Enzo View Post
    The bases should be similar, yes, but we have the 30v offset on the output sampled into Q6. Q6 normally tries to correct such differences.
    I'm trying to figure out how the BE junction can be reversed biased while current is flowing into the base.

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    Lifetime Member Enzo's Avatar
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    PNP. Base sits 10v more positive than emitter. Reverse biased then.

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    Thank you both for taking a look at the numbers again. I will double check those voltage again with my Fluke. All of these voltages were taken on the bulb limiter. I found an older schematic, not for this unit, and it showed rail voltages at -+ 59V. I will post new numbers shortly.

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    One question I have following along with this thread is about what is written on the schematic. For Q3 (2SC4382) is has PNP written above it, but that transistor is an NPN. So what transistor is in Q3 right now and is it a 4382? Probably nothing but misprinting pnp for an npn but just gotta make sure.

    https://www.digikey.com/product-deta...382-ND/3929372

    Edit: perhaps Glebert already pointed this out and it is not an issue. Just someone wrote pnp on the original reference schematic...
    Quote Originally Posted by glebert View Post
    I think Q4 too. Also some of the transistor types are labeled wrong (NPN vs PNP), just don't want you to confuse yourself.

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    Quote Originally Posted by Enzo View Post
    PNP. Base sits 10v more positive than emitter. Reverse biased then.
    There is current flowing through R35 (about 0.3mA) to cause that 20V drop, so it is either going into the base of Q6 or C16 is leaking it to ground. Usually for reversed biased transistor I expect zero current, but don't know if 0.3mA can happen due to some leakage.

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    Quote Originally Posted by DrGonz78 View Post
    One question I have following along with this thread is about what is written on the schematic. For Q3 (2SC4382) is has PNP written above it, but that transistor is an NPN. So what transistor is in Q3 right now and is it a 4382? Probably nothing but misprinting pnp for an npn but just gotta make sure.

    https://www.digikey.com/product-deta...382-ND/3929372

    Edit: perhaps Glebert already pointed this out and it is not an issue. Just someone wrote pnp on the original reference schematic...
    Yes, I had mentioned this up in #6.

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    If C16 is leaking then a drop would appear across R34. Is it?

    Or Q6 is defective.

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    All voltages with the Fluke moved just a little bit from where they were before. I appreciate any suggestions on what to check next.

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    I am so sorry for mislabeling the transistors. Here is the new sheet. No voltage drop across R34. Thanks again to everyone for helping me out.
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    Good morning,

    I really am sorry for not having the NPN and PNP written correctly on the sheet. I want this post to be helpful for anybody that encounters this same problem. It wasn't helpful that I made a mistake. I appreciate all the help I can get. I don't know what else to check. I even powered it up with a 4 ohm dummy load hoping I'd feel something getting warm/hot, nothing, the speaker jack was reading 6.65 volts dc. But I still had the bulb limiter in line. Any thoughts, please?

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    Don't forget the joker g1's Avatar
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    With limiter and load, power up without the load, then attach load without turning off the power.
    Some amps will not turn on properly on the bulb limiter while load attached, so you need to attach the load after power up.

    edit: this is assuming you have already got to the point of no DC on output with no load.
    If you are not at that point yet, you must first eliminate DC on output unloaded.

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    Looking at your voltages the first item that jumps out is the Base voltage of Q3 being so high.
    So either it is faulty or Q4 is (or both).

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    Hi, g1 & Jazz- thanks for your input. g1, I still have 31Vdc on speaker jack with no load. Jazz, I replaced Q3 (2SC4382) with a new unit. I don't have another Q4 (2SC3264) to swap. Diode test is the same for Q4 & Q10. No changes to voltages from last uploaded schematic. Any other thoughts, please?

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    Last edited by BStringThumper; 01-21-2020 at 12:03 AM.

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    Quote Originally Posted by BStringThumper View Post
    Any other thoughts, please?
    I think Q6 needs to be scrutinized, based on the fact that current appears to be flowing into its base (because of voltage drop across R35) despite it being reversed biased. See discussion in #13-15 and #19-21. Of course if R35 is actually not OK or there is a broken solder joint there that could also explain the voltages seen. Even if Q6 is bad though it still looks like current is flowing into all three pins (which is impossible).

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    Hi, Glebert- I will replace Q6 and inspect R35 and reflow the joints. I'll reread the mentioned posts. I'll be back shortly. Thanks!

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    My next step, seeing that Q3 has been replaced , would be Q2.
    Why is the Collector & Emitter voltage the same?
    A PNP transistor with a high voltage on the Base should have very low voltage on the Emitter.

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    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.

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    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.
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    Last edited by BStringThumper; 01-21-2020 at 01:43 AM.

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    Hey guys, maybe Iím off base here, but it seems like the circuit as drawn canít do the job. I suspect a schematic typo. Makes troubleshooting difficult, no? Hereís 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, OPís original results are not surprising. Gotta figure out whatís what before we can help the OP.
    Respectfully, Jcon.

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    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.

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    Last edited by glebert; 01-21-2020 at 03:07 PM.

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    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.
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    Last edited by BStringThumper; 01-21-2020 at 06:59 PM.

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