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My Velodyne CT120 subwoofer started making crackling sounds…

Customer Question
My Velodyne CT120 subwoofer started...
My Velodyne CT120 subwoofer started making crackling sounds at moderate volumes. When I play music through it, it almost sounds like the low end is kicking in partially, then cutting out and back in. Periodic almost, like a capacitor is charging and discharging every few seconds and you get bass then no bass. Any thoughts on what it could be?
Submitted: 2 years ago.Category: Home Theater-Stereo
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Customer reply replied 2 years ago
I am comfortable with a soldering iron and am looking for advice on how to figure out if a component is failing in the circuit boards and which one to replace. I do not think the driver itself is bad.
Answered in 4 hours by:
2/28/2016
Technician: Russell H., Service Tech replied 2 years ago
Russell H.
Russell H., Service Tech
Category: Home Theater-Stereo
Satisfied Customers: 12,615
Experience: 8+ years of professional experience
Verified

Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.

Test the driver (or sound-producing device that is going to the subwoofer) with some other speaker.

Once you have confirmed where the problem is, we can either troubleshoot the sub, or the amp/driver/etc.

The crackling sounds like a loose cable or slightly 'open' cable or connector.

But the intermittent bass sounds... that could be anything.

Make sure of the connections, wiring, and connectors (none dirty, all undamaged, connectors tight, wires not old/frayed/etc...) first.

Let me know what you think, and what you find out, please. Thanks.

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Customer reply replied 2 years ago
I took the amp out and resold weed a few of the connections that seemed to have a ring around the solder joint. I also reset the receiver to factory defaults (just in case). When I had taken the driver out, I pushed the speaker cone in and out and didn't hear or feel anything like coils rubbing, so I think the driver is ok. I was suspecting the built in amp perhaps crackling when the signal got too hot, so not sure how to test that. Btw, after I reassembled everything, the problem persisted. So are you saying to test this driver in another speaker cabinet or direct connect to a power amp and send low frequencies to it? I'll do that btw, and if it checks out ok, then what?
Customer reply replied 2 years ago
I took the amp out and resoldered a few of the connections that seemed to have a ring around the solder joint. I also reset the receiver to factory defaults (just in case). When I had taken the driver out, I pushed the speaker cone in and out and didn't hear or feel anything like coils rubbing, so I think the driver is ok. I was suspecting the built in amp perhaps crackling when the signal got too hot, so not sure how to test that. Btw, after I reassembled everything, the problem persisted. So are you saying to test this driver in another speaker cabinet or direct connect to a power amp and send low frequencies to it? I'll do that btw, and if it checks out ok, then what?
Technician: Russell H., Service Tech replied 2 years ago

Direct connect to a power amp and send low frequencies to it, yes... I was thinking of another sort of 'driver', I apologize.

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Technician: Russell H., Service Tech replied 2 years ago

I meant, try a different sound source, sound signal source, tuner/radio/player/whatever. To make sure the crackling, low-frequ-filtering, etc. is not (or, is) part of the subwoofer itself (and whatever built-in circuitry it has.)

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Customer reply replied 2 years ago
I tried the driver (speaker) by itself with a separate power amp and it performed fine. I swept from 35Hz to 200Hz and it was all good and loud. No crackling or popping. I them reassembled the subwoofer and plugged a signal generator directly into it (to eliminate my A/V receiver as being the problem) and sure enough, the cracking and popping came back. So as I suspected, the subwoofer's preamp or poweramp circuitry is having the problem and that is what I reached out to you for. I need help figuring out what components on the circuit board might have gone faulty. If this is not something you can help me with, let me know. Unfortunately I do not have an oscilloscope but I can hear that the bass sounds smooth at very low volumes and then starts to clip and compress (or cut out) as soon as you try to increase the volume even slightly. I sent an email to Velodyne support asking for schematics and will forward them along if I get them. In the meantime, I will attach something I found on the Internet that seems to be from my model. Let me know how we should proceed.
Technician: Russell H., Service Tech replied 2 years ago

I would guess a capacitor, possibly one with a bad solder joint.

Doubtfully also an inductor, but they're usually visibly broken if they're bad.

But I don't know what else would be the problem, or anything more specific.

The low-volume's-OK aspect... that might be from excessive amplification. But that's such an unusual problem I cannot recall what causes it, except perhaps for defective bias on the transistors in the amplifier.

If any transistors are biased with a capacitor on the base or the active side (emitter or collector, but not both, and depending upon the transistor being NPN vs. PNP), then the capacitor is paticularly suspect.

A schematic would help! if you get one.

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Technician: Russell H., Service Tech replied 2 years ago

Thanks for the schematics. I'll be looking them over for a little while. And post again with my thoughts.

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Technician: Russell H., Service Tech replied 2 years ago

In the second schematic, the one with the op-amps and transistors, I would guess you should look at

C501

and

C723

near the op-amps. Also

C701

on the output of the upper op-amp.

Also C606 near the speaker at the right.

I would look at or replace one or all of those. But I could be wrong, I'm sorry to have to say, in all honesty. My recommendation is surely good simply as 'where to inspect components closely' though.

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Customer reply replied 2 years ago
Hi Russell, sorry I was away on business travel and did not get a chance to respond back to you. I had reached out to Velodyne support and they sent me their copy of the schematic for my subwoofer (which I will upload). The font is strange and hard to read but that is all they had for this old unit. Would you mind taking a look and letting me know if you think it is still the same capacitors I should look at? If you want to circle the schematic and re-attach it, that would help. I also borrowed an oscilloscope from a friend and will do some poking around to see where the signal deteriorates. Any tips are much appreciated!Thank you,
Damien
Technician: Russell H., Service Tech replied 2 years ago

I would not really know (though I have used an o-scope) how a 'crackly' waveform looked. Doubtless rather erratic and transient.

Show me the schematic by all means.

But I would guess crackly sound would imply erratic connection/disconnection. 'Sparks', only in little, and probably at some non-soldered or poorly soldered and deteriorating connection.

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Customer reply replied 2 years ago
I had uploaded it before but I guess it didn't come through. I am attaching it again. Let me know if you don't see it.Damien
Customer reply replied 2 years ago
Hi Russell, have you looked at the revised schematic I got from the manufacturer (uploaded Mar 19)? If so, any thoughts on what to look at?
Technician: Russell H., Service Tech replied 2 years ago

The triangles are amplifiers called op-amps.

The big triangle? I'm not sure. It might be an amplifier chip?

The transistors are of course circles.

Measuring with the o-scope, the signal path would be through the triangles and to some extent through the transistors also.

But crackling is most likely to be owing to a loose connection, bad solder joint, etc. Finding them by eye and re-soldering them, heating them a little, would be the most likely procedure. (Be sure to prevent components from overheating by 'clipping' their lead on the other side of the board from the soldering, with a clip-type soldering heat-sink.)

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Customer reply replied 2 years ago
Hi Russell,I think I found the problem. It wasn't any of your suggestions but I was able to identify it with the scope. This may not be the entire problem, but at least I found a faulty component. After I replace it, I will dig further. Turns out, one of the large capacitors (4700uF 63V) right after the diode bridge (right after the transformer secondary), wasn't doing its job. On the oscilloscope, the bottom capacitor was giving a fairly smooth negative DC, but the one on top had too much of a ripple (10+ V peak to peak on top of a DC offset). I took both of them out to measure their capacitance. The bottom one was measuring about 4600uF but the top one was dead. I'm not worried so much about the 4600 measurement of the good one, because it was just a quick arduino circuit I put together so the accuracy is questionable. But the fact that it measured one successfully but the other one just appeared as an open circuit tells me that the second one is definitely faulty. So I am going to replace both with fresh capacitors today.When listening to the subwoofer I noticed that it didn't crackle so much on the upper frequencies (120Hz to 200Hz) until you turned it up. But on the lower frequencies (35Hz - 100Hz) it crackled with a lot less volume. I used an audio generator app on an iPhone and sent a sine wave to the sub and would sweep the frequency. I had connected the oscilloscope on the input and output (2 probes) and noticed that the crackle coincided with the top peaks of the output sine wave changing shape. Instead of a smooth sine it would look more like someone took a bite out of it (like the Apple logo?). Interestingly, this was only on the positive half of the waveform. The bottom half was still smooth. This all seems to point to the top capacitor on the power supply circuit being faulty because it could no longer provide a smooth positive DC voltage to drive the rest of the components. After I replace both of these capacitors, I will of course see what else might be broken. Either something else went bad and caused the capacitor to fail, or this capacitor malfunction could have fried something else down the line in the circuit that wasn't expecting such a variation in the voltage. So I am not out of the woods yet. Anyway, that is where I am with this. To be fair, this is the kind of insight I was looking for from JustAnswer.com but unfortunately none of your suggestions pointed me to this part of the circuit. I understand it is difficult to diagnose something like this via email, but considering all of the above, I do not think I should be charged for this repair by JustAnswer.com. So what do we do?
Technician: Russell H., Service Tech replied 2 years ago

Pricing, charges, amounts, etc. are beyond my scope as a JustAnswer.com Expert.

For all issues of billing, payment, subscription terms and status, etc., I must refer you to

Customer Support at JustAnswer.com, which may be contacted through:

http://ww2.justanswer.com/contact-us

They will no doubt address your concern, if you contact them, with reasonable promptness.

(I think I provided information and followed through on the case. That may count for something.)

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