[SOLVED] Need help diagnosing a "no sound" issue from subwoofer

[Turrican3]

ISSUE FOUND:

Conductive paint!!! I was making who knows how many shorts!? Of course nothing was working as intended! And I guess I'll have to consider myself lucky if nothing actually blew up, as it's apparently the case.

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ORIGINAL POST

I'm having an issue with my home theatre system lately, that is driving me crazy... I'm a bit of a newbie though so please be patient with me even if I say something VERY stupid, which is quite likely I'm afraid.

I've recently had a sub fault (more on this later), it's a 15+ years old Infinity unit that according to service can't be repaired anymore due to lack of parts. So I decided to purchase a sub amplifier that felt like a nice drop-in replacement: a Monacor SAM-200D, connected to the RX-V671 via a single RCA cable --> sub out --> LFE in / left.

This brand new combo worked for like 15 minutes. Then, the sub apparently died. Whereas "died" means it stays in standby forever and nothing seems to be able to make it produce sound, i.e. power LED stays red, never turns green.

If I try to connect my smartphone to the L/R input channels of the Monacor, LED does turns green BUT volume is extremely low, nowhere near how it did for the 15 minutes it worked.

So... now I have no idea whether I have a faulty AV receiver, a faulty sub amplifier, or both!

Because you know, the 10+ years old Infinity subwoofer had the exact same issue: it turned on, but never left standby (LED stays red), the exact same issue I'm having with the Monacor. So this made me wonder: was the Infinity sub actually failed (by the way this would imply service didn't even check it as soon as they noticed it was too old, which I cannot say for sure), or was the Yamaha having an issue already before, that for some strange reason temporarily ceased to appear for the 15m the Monacor worked?!

Relevant settings on the AV: all speakers set to SMALL, crossover 80hz, subwoofer enabled, subwoofer level +4.0 dB (but tried up to the max, 10db, seeing no difference)
Relevant settings on the Monacor: crossover MAX, level/volume roughly at 50%

RCA sub cable checked for continuity, tried all sorts of playback stuff (Yamaha test tone, computer-generated 60hz sine, THX optimizer, blu ray discs, etc) that should have triggered the sub but... nada, nicht, zero.

I am able to do basic measurements/tests with a multimeter and/or an oscilloscope if needed (as a matter of fact I did the latter, and it feels like sub out is outputting... something! But I have no idea what I should be looking for in terms of waveform and/or voltage levels)

Any help would be greatly appreciated!

[m3vuv]

is the psu section linear or smps?

[tooki]

What kind of output do you get on the sub out RCA jack of the receiver, if you look at a test sine signal on a scope? In particular, look for a) what kind of peak-to-peak voltage you get, b) whether there is a DC offset, and c) whether the sine is symmetrical.

[tooki]

is the psu section linear or smps?

What difference would that make? And which device’s PSU? The receiver has its own power (almost certainly linear). The subwoofer is active, its power supply is built into the amplifier module.

[Turrican3]

Thanks @tooki, I'll see what I can do and will report back!

[Turrican3]

Here we go.

I generated a 60hz tone FLAC with Audacity, and tried to reproduce it via network (it's on a DLNA server)

Scope of the sub out signal from the Yamaha as per attachment... though I totally forgot to take note of the volume on the receiver, I can measure it again with any given volume if needed (guess it is, I apologize again but as I said being a newbie it's easy for me to make silly mistakes)

[tooki]

That’s very weird.

I would assume that at 60Hz, nothing comes out of the (non-subwoofer) speakers, right? (Such that it’s safe to turn the volume up high.) If so, try turning the volume up and seeing how the waveform changes. I would expect it to reach no higher than 2V RMS or so. (Turn on the Vrms measurement on the scope, it’ll make it easier.)

[tooki]

Also, see what happens if you play the sine wave on just one channel at a time. Either generate a new FLAC, or use your phone as a signal generator with an app.

[Alex Wolf]

It seems that you have faulty amplifier. Normally, it should amplify any line signal from any source, taking into account its frequency cutoff. First, I suggest to repeat the test with your smartphone connected directly to the subwoofer's built-in amplifier, but this time make sure that the volume is set to max level both on a smartphone and on the amplifier. Then, if the fault will be confirmed, disassemble the faulty unit, do a visual inspection and measure all its supply voltages. If there are difficulties with it, you can post a detailed photo of all the boards here. Although usually everything is clearly indicated on the silkscreen of the boards. Oh, and check the resistance of the speaker.

[Alex Wolf]

By the way, if you want to check the amplifier separately, then disconnect it from the speaker, connect a 16...32 Ohm powerful resistor instead, apply a signal to the input (for example, a 100 Hz sine – there are a lot of apps to generate sound frequency signals with your smartphone's headphones output), connect the first channel of the oscilloscope to the input signal, and the second channel to the load resistor. The signal at the input should repeat the signal at the output, only the amplitude of the output signal should be significantly greater and changes as you change the volume.

[Turrican3]

@tooki

Again, I will perform this test and report back. Just one clarification, one channel at a time for the sine playback you mean I should create a stereo track (well, actuall two) that only sports the wave on left and right respectively?


@Alex Wolf

I assume you mean the subwoofer amplifier, right? Unfortunately I don't think I own a powerful enough resistor, just standard 1/4 or 1/2 W while I believe you're suggesting far higher ratings.

I wonder about the auto standby/poweroff though, would that allow me to correctly check voltages? I will try anyway!

Last but not least, the sub speaker should be fine since it works when connected as a regular speaker (i.e. not to the sub amplifier), EDIT multimeter says roughly 4.5 ohms.


Will report back as soon as possible.

[Alex Wolf]

I assume you mean the subwoofer amplifier, right? Unfortunately I don't think I own a powerful enough resistor, just standard 1/4 or 1/2 W while I believe you're suggesting far higher ratings.

I wonder about the auto standby/poweroff though, would that allow me to correctly check voltages? I will try anyway!

Last but not least, the sub speaker should be fine since it works when connected as a regular speaker (i.e. not to the sub amplifier), EDIT multimeter says roughly 4.5 ohms.

Yep, subwoofer's built-in amplifier. You need at least one 20 Ohm 10 W resistor, or twenty 470 Ohm 1/2 W resistors. 😁 But since the speaker resistance looks normal/typical (and I guess it matches the markings on it), then it can be used for tests instead of a load resistor.

I haven't seen the power supply's schematic of the amplifier and I'm not sure if it actually cuts off the main power and goes into standby. But if in doubt, use your smartphone as an input signal generator when measuring supply voltages. In fact, it is even useful to compare voltages with and without the input signal.

[tooki]

@tooki

Again, I will perform this test and report back. Just one clarification, one channel at a time for the sine playback you mean I should create a stereo track (well, actuall two) that only sports the wave on left and right respectively?

Exactly, so that the input has the sine on one side, silence on the other, and we see what the output of the system is, since at some point it has to mix the two channels down to one.

The reason I want to see this is because only every second wave appears to be distorted. So I wonder if they’re somehow getting mixed weirdly.

[Turrican3]

Guys... it's working!?

This doesn't even remotely make sense to me, but it's working.

Something weird going on with the Yamaha sub out or what?! I literally did nothing except leaving everything turned off (totally disconnected from mains) since my first update 3 hours ago.

What the what?!??!??

[Alex Wolf]

The reason I want to see this is because only every second wave appears to be distorted. So I wonder if they’re somehow getting mixed weirdly.

This could be caused by software generation or DAC decoding inaccuracies (depending on the used bitstream compression format). This definitely cannot lead to a "no sound" problem. But I noticed it too, looks a bit suspicious.

[Alex Wolf]

I literally did nothing except leaving everything turned off (totally disconnected from mains) since my first update 3 hours ago.

What the what?!??!??

It's hard to tell, floating faults are the hardest to diagnose. Generally, a complete blackout helps when the overload protection trips. But this is way far from the whole story. 😁

Now I suggest to heavily load the whole sound system, and see what happens. To do this, it is enough to turn the volume up and put some bass and dynamic track for playback on a loop for about 15...30 minutes. Of course, if it's possible so as not to bother anyone nearby.

[Turrican3]

Ok let's try this too!

I'll report back... I mean, as long as no neighbor slams my door and destroys everything with a huge hammer.

[Turrican3]

More than 30 minutes at decent volume, nothing strange happened, sub still going strong. And no angry neighbors in sight!

Perhaps I'm crazy but the next thing I'd do now is bring back the (allegedly?) failed Infinity original sub amplifier. It wouldn't surprise me if it worked.

[Turrican3]

Nope, Infinity sub amplifier seems to be actually failed. What a pity, but maybe someday I'll be able to fix that too when/if I'm knowledgeable enough.

[Turrican3]

Another update, perhaps the most significant?

I was able to make the Monacor fail again (i.e. not turning on when sub signal is being sent by the AV receiver), just by reinstalling it on the sub chassis.

I was like "uh, not again..."

Then I removed the Monacor from the chassis, and it worked again.

It was a matter of mere seconds... wait, what if... but... naaah, it can't be...

Meter in continuity mode. Let's have a look at this plastic box that used to hold original amplifier... BEEP.

BEEP. BEEP everywhere.

Conductive paint!!! I was making who knows how many shorts!? Of course nothing was working as intended! And I guess I'll have to consider myself lucky if nothing actually blew up, as it's apparently the case.

Geeze, conductive paint... it seemed like harmless plastic!
I suppose it was added to shield the electronics, or what else?

[m3vuv]

leave it for a rainy day when you see things from another angle .

[Alex Wolf]

Another update, perhaps the most significant?

I was able to make the Monacor fail again (i.e. not turning on when sub signal is being sent by the AV receiver), just by reinstalling it on the sub chassis.

I was like "uh, not again..."

Then I removed the Monacor from the chassis, and it worked again.

It was a matter of mere seconds... wait, what if... but... naaah, it can't be...

Meter in continuity mode. Let's have a look at this plastic box that used to hold original amplifier... BEEP.

BEEP. BEEP everywhere.

Conductive paint!!! I was making who knows how many shorts!? Of course nothing was working as intended! And I guess I'll have to consider myself lucky if nothing actually blew up, as it's apparently the case.

Geeze, conductive paint... it seemed like harmless plastic!
I suppose it was added to shield the electronics, or what else?

Congratulations! 👍🍾 You found the fault, despite the fact that you acted not quite methodically. I must point out that if you did the tests described above, you would also establish the fact that both the amplifier and the speaker are working properly. You just went a more complicated and less reliable way, and it's just a matter of luck that the problem was found so quickly. Of course, it's up to you, but I advise you to act more methodically next time, checking and excluding faults module by module.

And, please, if it doesn't make it too difficult, next time post photos of the faulty area. More experienced engineers and hobbyists could see much more than you can see at first glance. For example, in this case it's pretty interesting which parts of the board were shorted with conductive paint (and you didn't analyze or describe it). What is its average resistance per cm by the way?

P.S.: Shielding amplifiers and active speakers in which amplifiers are built in is a common practice to protect against external interference.

[Turrican3]

leave it for a rainy day when you see things from another angle .

I assume you're talking about me repairing the Infinity amp sometime in the future?

Congratulations! 👍🍾 You found the fault, despite the fact that you acted not quite methodically. I must point out that if you did the tests described above, you would also establish the fact that both the amplifier and the speaker are working properly.

Oh well, definitely NOT methodically! But I guess it's somewhat understandable being a newbie, right?

By the way, the test you're referring to is the resistor one? (I already knew about the speaker working properly; perhaps I should have made a *written* list of all the tests I had made during the last week though, I just went by memory here)

And I definitely apologize for the lack of photos, I would have gladly provided them if anybody had asked for. Also please understand that as far as I understand nothing blew up (unless you're talking about the Infinity amp, that one unfortunately does seem to be broken indeed, but perhaps I should open another thread if I want to investigate or even possibly repair it?)

Thanks again everyone for your precious support!

[Turrican3]

Wait I think I saved one shot so perhaps it may help everybody see what I was dealing with.

What's shown in the attached photo is the back of the subwoofer with the amplifier in a working position, you can see the inner part of the plastic box with conductive paint.

Now, the amp was initially mounted in a way upper position. And it was VERY tight. So my conclusion is that the solder side of the electronics that can be seen here were making contact in at least one point, making a short or something like that.

As a matter of fact, as soon as I removed the amp from the chassis (or simply shifted the installation in a lower position, as per photo) it started working again.

[Alex Wolf]

I assume you're talking about me repairing the Infinity amp sometime in the future?

By the way, I think that you can already cope with this task. It's not the gods who burn our pots. You always have to start somewhere, and some non-working "junk", for which a replacement has already been bought, is a great start. It's not scary if it doesn't work out or if you make it even worse. You will gain experience, which is priceless.

about the Infinity amp, that one unfortunately does seem to be broken indeed, but perhaps I should open another thread if I want to investigate or even possibly repair it?

It would be nice if you have the right mood and desire to tinker with it. 😃

Oh well, definitely NOT methodically! But I guess it's somewhat understandable being a newbie, right?

This is absolutely normal even for professionals, we are not robots after all. 🙂 Even Dave acts a little chaotic at times, which sometimes leads to letting the magic smoke to escape. 😁 This is not some kind of hard-and-fast rule that distinguishes newbies from mega-pros. It just makes life a little easier.

By the way, the test you're referring to is the resistor one?

In short: yes, "disassemble the amplifier from the case and test it as a separate module" test. Since the speaker was already tested and proved to be working, it could be used in place of the load resistor.

Thanks again everyone for your precious support!

I'm glad to help! 😊