SWR Workingman's 15- Voltages- DC on Speaker

[BStringThumper]

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!

[Enzo]

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?

[BStringThumper]

Hi Enzo,
Thanks for diving in for me. Here is an updated voltage sheet.

[BStringThumper]

I tried to rotate the picture. I hope this works.

[Enzo]

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.

[glebert]

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.

[BStringThumper]

You were right, Enzo, thank you. Thanks glebert. Here is an update with voltages. Look at Q1 & 2 please.

[Enzo]

What about them, they seem to be working. Q8 is still a prime suspect.

[BStringThumper]

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.

[BStringThumper]

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.

[glebert]

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.

[Enzo]

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.

[glebert]

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.

[Enzo]

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.

[glebert]

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.