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Subwoofer has no musical sound just a constant pulsing

R

rudkinst

Active Member
Hello all. Trying to learn electronics and I have a Sherwood SP-165W, 8 inch subwoofer that I thought I would try to fix. I know the very basics and I have an ESR and a DMM to work with.

Sub has power, but no inputted sound will come out. It just makes a constant, even, pulsing sound (like a motor boat). The sound does not change with volume, it stays exactly the same. I checked the woofer, 7.6 ohms. No visibly damaged components. It is pretty simple, so I am just going through and checking every component. I am measuring capacitors in circuit with the ESR (is that OK?) and I have found 3 so far that are obviously bad and replaced them. No effect.

I will just keep checking every component, it is not a large board, but I thought I'd see if anyone has any input. I've seen this on some Polk 111 subs and it was capacitors.

What do you think? Thanks.
 
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Chances are high that electrolytic capacitors have lost their capacitance and the amplifier has become unstable, leading to the motorboating. A recapping project might solve the problem.
 
egellings said:
Chances are high that electrolytic capacitors have lost their capacitance and the amplifier has become unstable, leading to the motorboating. A recapping project might solve the problem.
Click to expand...

Thanks. That’s what I figured. I’ve got a ton of caps and I’ve started. I’ve seen mixed information. Is an ESR accurate when the cap is in circuit? My readings seem correct. Also do I have to zero it with every cap or just once then do a bunch?

Thanks for any feedback.
 
Let us know if that solved the problem. ESR of a cap is usually a tiny fraction of an ohm, while circuit resistances paralleling the cap would be many times larger, so they would only slightly influence (reduce) the ESR reading. For complete confidence in the readings, though, I'd disconnect one end of the cap from its circuit and then do the reading. You could do an experiment, and try the reading both ways and compare the results, too.
 
rudkinst said:
Thanks. That’s what I figured. I’ve got a ton of caps and I’ve started. I’ve seen mixed information. Is an ESR accurate when the cap is in circuit? My readings seem correct. Also do I have to zero it with every cap or just once then do a bunch?

Thanks for any feedback.
Click to expand...

Most of the times yes, I have an esr meter and I'd say that 80% of times the reading is accurate in circuit, when not is due to a circuit configuration related to the measured capacitor and not a fail in the esr meter. The fail of that sub sounds more related to a bad capacitor or a transistor failure in the power stage
 
jfzea said:
Most of the times yes, I have an esr meter and I'd say that 80% of times the reading is accurate in circuit, when not is due to a circuit configuration related to the measured capacitor and not a fail in the esr meter. The fail of that sub sounds more related to a bad capacitor or a transistor failure in the power stage
Click to expand...

I’ve found four caps that measured poorly in circuit and out so I switched them. I’ll finish with the cap testing tomorrow then go through transistors and maybe resistors if that doesn’t solve it. I am a rookie so I can’t recognize the power stage even of a board this straight forward. But I’m getting there.
 
OP, I admire your willingness to learn and try new things. I am also glad that a place like this exists where you can get support and advice.
 
If you have found some bad electrolytic capacitors, I'd just replace them all. No point in checking each and every one of very inexpensive parts, and it's very hard to test the function of caps in circuit. Smaller ones are more likely to be bad that larger one, but a voltage spike can kill the big ones too.

From there, you need to get a schematic with voltages listed and check each point. "motorboating" is an ocsillation at low frequency, very often caused by bad capacitors but can also be due to a bad power supply voltage regulating transistor (the big ones near the main (big) power supply caps, often with heat sinks). One of those bouncing the supply rail up and down will do it.

A cheap "toy" component tester is handy. They are not particularly accurate, but will easily find bad transistors or serious mis-maches with specs and will give you a quick capacitance and ESR reading for caps. Again, not particularly accurate, but better than a DMM for transistors. I have access to an in-circuit transistor tester too, but it's not always good at finding faults, some circuits will show the transistor bad when it's not. Works fine out of circuit, but it's ancient and not easily found used.
 
Well did every electrolytic cap and the pulsing is gone except for a slight pop at start up and shut down.

But no sound! I’ve tried the speaker level and line level inputs. Nothing. Ready light on back switched from red to green when the bass kicked in but no sound. Again the speaker tests at 7.8 ohms.

I’ll test the output and regulator transistors. There are only 3. I know how to test out of circuit using diode setting but there is no schematics online.

If anyone wants to recommend another test method feel free and thanks again for the input.
 
Hmm, out of my league I'm afraid. I would check to make sure that all the solder connections are good though, the vibration in a sub-woofer could easily lead to cracks. I just "repaired" a KT-400 tuner that was inert by re-soldering the pins on the transformer.

Transformer is glued to the board and directly soldered on, and all three pins on the input side had lifted traces. Works like a charm now!

If the inputs have cracked solder joints the amp could be prefect, but since it's not getting any signal, will be silent!
 
rudkinst said:
Well did every electrolytic cap and the pulsing is gone except for a slight pop at start up and shut down.

But no sound! I’ve tried the speaker level and line level inputs. Nothing. Ready light on back switched from red to green when the bass kicked in but no sound. Again the speaker tests at 7.8 ohms.

I’ll test the output and regulator transistors. There are only 3. I know how to test out of circuit using diode setting but there is no schematics online.

If anyone wants to recommend another test method feel free and thanks again for the input.
Click to expand...

It is possible to check transistors in circuit, first check that there are not short circuits between its terminals, this is valid for bjt and mosfet types. Second if the transistors are bjt with the multimeter settled in diodes, see the datasheet of the terminals of the transistor and check Vbe should be aprox 0.7v. and Vbc less than Vbe. Sometimes is better to desold the part.
 
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