SWR 2x10c distortion & low output

[Pryde]

Hey guys I have a SWR Working Pro 2x10c here with very low output and distortion. Isolated it to the power section. Preamp and speaker work fine.

No loose connection or solder joint that I can find probing around and would appreciate some thoughts on next steps to consider. Again the output is very low and the notes decay/fart out like bias issue perhaps but not sure.

Schematic is attached. Thank you.

[Enzo]

Module with a board on either side of an aluminum extrusion with a square fan on the end? I have a pair of those on my shelf.

Check the load itself, plug a different speaker cab into it, and use a different cord. And aside from that, the black and white wires from the power amp module over to the output jack board, disconnect them and use clip wires to connect a different speaker directly to the module. ANy difference?

If the problem is in the module, make sure the +/-97v rails are both up to voltage and clean.

Scope the output. With a sine wave input, what does it look like on the output. Do this with no load. Does it look OK with no load, but collapses under load? or does it look bad no matter what?

it may be stuck in mute, or halfway there. Power amp drawing lower left, Q15 under the diffy pair. it needs to turn on for sound. Those transistors left of there are the mute circuit/protect circuit. Explore that.

[Pryde]

Thanks Enzo:

Yes it is the same module you are describing I believe. So far:

-Checked load with different cable and speaker, problem still present
-disconnected white/black wires at module and jumped new leads over to another speaker, problem still present
-Per the schematic (WP400 model) I am reading +/- 70v at the rails so power is correct at the module

Admittedly not real proficient with my scope, it appears the sine wave is cleaner with the output unloaded. What can I conclude from this?

Thanks

[Enzo]

I am really looking for a obvious difference. Unloaded, the module just puts out a voltage, but without a load, no current is drawn from it. When you put on a load, not the load draws current when the voltage is present. That is Ohm's Law. SO if an amp makes clean waveform unloaded, but can't maintain that while loaded, it usually means the output stage cannot provide the current the load is demanding. That can mean an open emitter resistor, or an open output transistor, or a broken trace in the last stage, or similar things.

I like to use 100Hz as a test signal instead of 1kHz, but either will work. Without a load, how many volts of output can it generate? And then loaded at the same settings? When scoping load versus no load, leave the scope vertical on the same setting, so you can compare amplitudes.

[Pryde]

OK I included some shots of the scope to assist interpretation. Again I may have something set incorrect so eyeing the settings may be helpful.

The SG is at 1Khz with amplitude at 1/2 wat point

Scope is set at 5v/div and .1 time/div

The signal is going into the preamp input with gain at full and master volume turned up until clipping light comes on.

With this the unloaded signal appears to be a +/- 75v or so. With signal loaded the bottom clips off completely.at the -50v mark.

I appreciate the help with the scope dx.

[Enzo]

flip the AC/DC switch to ground for a moment and center your trace vertically. is your probe set to X10 please?


If you are losing the bottom half of the waveform, look at the negative side outputs. is the signal reaching the bases of those outputs?

[Pryde]

flip the AC/DC switch to ground for a moment and center your trace vertically. is your probe set to X10 please?


If you are losing the bottom half of the waveform, look at the negative side outputs. is the signal reaching the bases of those outputs?

OK the probe set to x10, centered line vertically with ground switch. Had to set 1 v/div now to view. The bottom waveform is cut off at the line.

[Pryde]

Bumping for more help. I am using a small signal tracer and testing the output transistor bases and emitters and getting signal on both -/+ sides. I also get signal on both sides of R34 and R43 (connected to C9). However on the other side of C9 (hooked to speaker (-) I get static/noise only. Is C9 my culprit as to why the bottom of the waveform is missing and cause for low output and distortion?

Don't want to rip apart the entire module and replace C9 unless sure.

Thanks

[Enzo]

C9? No. Look at the schematic. You have signal at the R34/43 end, but not the other end of C9? Well look at where that end connects - GROUND. I don't expect to see signal on ground.

Find R1 and L1. That is the path to the speaker. You have signal at R34/43, so the amp is working, we just are not getting it to the speaker. L1 is a coil of wire, so it will have a DC resistance of about zero ohms. Parallel R1 is 10 ohms, but with the coil in parallel, it SHOULD measure shorted. So power off, measure resistance of R1. If it measures open, then both it and the coil are open somehow. if you measure 10 ohms, the coil is open. if you measure zero ohms, all is OK with the parts. The coil is not likely really open, more likely one end broke free of its solder. If that happens, then all the output power tries to flow through R1, which then can burn out. So measure that.

If it measures OK, then power back up and see if signal gets to the left (on the schematic) end of R1. There? Now how about the right end? Now how about right at P1 push-on spade? If you have it there, the amp is working.

Remember, we need a CIRCUIT for it to work. So the speaker return to P3 is just as important as the output at P1. So we want to make sure P3 is actually connected to ground.

One test for this would be to clip a speaker or load to P1 and P3. Now see if signal is at P1. if it is, see if you see signal at P3. If you do, then it is not grounded, but should be.


The signal is there, so trace it down to see where it goes away. A signal tracer is just a scope for your ears, we use them the same way.

[Pryde]

C9? No. Look at the schematic. You have signal at the R34/43 end, but not the other end of C9? Well look at where that end connects - GROUND. I don't expect to see signal on ground.

Find R1 and L1. That is the path to the speaker. You have signal at R34/43, so the amp is working, we just are not getting it to the speaker. L1 is a coil of wire, so it will have a DC resistance of about zero ohms. Parallel R1 is 10 ohms, but with the coil in parallel, it SHOULD measure shorted. So power off, measure resistance of R1. If it measures open, then both it and the coil are open somehow. if you measure 10 ohms, the coil is open. if you measure zero ohms, all is OK with the parts. The coil is not likely really open, more likely one end broke free of its solder. If that happens, then all the output power tries to flow through R1, which then can burn out. So measure that.

If it measures OK, then power back up and see if signal gets to the left (on the schematic) end of R1. There? Now how about the right end? Now how about right at P1 push-on spade? If you have it there, the amp is working.

Remember, we need a CIRCUIT for it to work. So the speaker return to P3 is just as important as the output at P1. So we want to make sure P3 is actually connected to ground.

One test for this would be to clip a speaker or load to P1 and P3. Now see if signal is at P1. if it is, see if you see signal at P3. If you do, then it is not grounded, but should be.


The signal is there, so trace it down to see where it goes away. A signal tracer is just a scope for your ears, we use them the same way.

OK coil and R1 check out ok at zero ohms.
But interestingly when I check P3 to chassis ground there is continuity however I am getting audible signal there as well with my tracer (which is the speaker (-) terminal).

So it is grounded...yet it isn't?

[Enzo]

What RESISTANCE to ground do you see at P3? Continuity can mean 1 ohm or 100 ohms, and those two tell very different stories.

Is the signal tracer signal at P3 the same loudness as at P1 - without changing the volume setting on it?

before going on, is your tracer grounded to the amp chassis? Or another way, if you just stick your tracer probe on the amp chassis, do you also hear signal?

[Pryde]

What RESISTANCE to ground do you see at P3? Continuity can mean 1 ohm or 100 ohms, and those two tell very different stories.

Is the signal tracer signal at P3 the same loudness as at P1 - without changing the volume setting on it?

before going on, is your tracer grounded to the amp chassis? Or another way, if you just stick your tracer probe on the amp chassis, do you also hear signal?

Sorry Enzo I think I am losing my mind here. For whatever reason I am NOT hearing signal on P3 now (just some static/noise). P1 has clear signal through the tracer.

Yes the tracer is grounded to the amp chassis properly.

P3 is showing ~10 Ohms resistance to chassis ground.

Sorry for the confusion and I appreciate you sticking with me.

[Enzo]

Well, I am now confused too. Do you have signal at both ends of R1/L1? With no load, do you have continuity (low actual resistance, not "continuity test") from R1 to P1?

[MarkusBass]

For whatever reason I am NOT hearing signal on P3 now (just some static/noise). P1 has clear signal through the tracer.
......
P3 is showing ~10 Ohms resistance to chassis ground.

I'm also confused. Are you talking about P3 test point on page #6 of the schematic? It should be connected to the ground so you cannot expect signal there. It should be also 0 Ohms to the chassis. Ground should be connected to the chassis of the amp on the Output PCB board (through J303 output jack). Is the output PCB board assembled in the amp?

Mark

[Pryde]

Well, I am now confused too. Do you have signal at both ends of R1/L1? With no load, do you have continuity (low actual resistance, not "continuity test") from R1 to P1?

Yes I have get signal at both ends of R1/L1.
Unloaded, I have 0.4 ohms resistance from R1 to P1

Again, the amp gets signal all the way to the speaker it is just low output and distorted/farty.