JBL monitor speaker is crackling

[VMP_09]

Hello this is my first post,

Suddenly, my left JBL LSR 308 monitor speaker makes a crackling sound out of nowhere.
See the video for an example of the sound:


So far, I've tried the following:

• Replaced the power cable.
• Disconnected all inputs.
• Tested on a different power outlet.
• The other speaker is still working fine.

What could be the issue?

I've inspected the circuit board and don't see any visible damages,
except possibly in photo 3, where R10, R11, and L901 (?) diode (?) appear blackened due to heat.
(Update: I have just checked the pcb board of the other (good working) speaker, including the EEVblog #1322 video below have same similar black colored resistors R10, R11 and inductor L901, so that's probably not causing the issue)

If it's something simple as a faulty capacitor, I can replace it.

For reference, here's the YouTube video of EEVblog #1322 repairing the same speaker:

Please see the attached photos of the circuit board.

Thank you in advance for your responses.

[DavidAlfa]

Those parts are fine (L=inductor).
It could be the power supply making large spikes, the audio buffer, or power amplifier itself.
This is one of these repairs thar require some tech equipment, really hard to tell without hooking a scope to it.
Nothing seems bad at glance.

[VMP_09]

Thank you for your reply.

I have two oscilloscopes, so far I haven't measured any electronic components with them yet (so please let me know how to do so).

These are the two scopes I have:

Velleman PCSU200 -
USB PC oscilloscope and signal generator.
https://www.velleman.eu/products/view/?id=407512

Frequency range: 0 .. 120Hz to 12MHz

The second one is a Chinese kit:
JYE Tech DSO138 DIY Digital Oscilloscope Kit 13804Kg

Max frequency range: 200kHz

https://www.banggood.com/nl/DSO138-DIY-Digital-Oscilloscope-Kit-13804K-Version-With-Transparent-Acrylic-Housing-p-1051617.html?rmmds=search&cur_warehouse=CN

[Audiorepair]

That doesn't sound to me like any kind of power supply issue, or any analogue issue.

That is some kind of digital processing problem.

[VMP_09]

That doesn't sound to me like any kind of power supply issue, or any analogue issue.

That is some kind of digital processing problem.

Thank you for your answer, have you encountered something like this before?

[Audiorepair]

That doesn't sound to me like any kind of power supply issue, or any analogue issue.

That is some kind of digital processing problem.

Thank you for your answer, have you encountered something like this before?

Yes, this is my job.


It is difficult to diagnose from a video, for starters I don't know how accurate the video sound is. 
It has a lot of high frequency content but is clearly broadband noise of the type not normally associated with analogue circuitry. 
SMPS power supplies aren't generally noisy like this, they usually just stop working instead.

But it sounds to me like a digital problem where the crackling contains anything from zero to full scale deflection randomly.

Maybe you could put one of your scopes on it and post a picture here, it might be interesting to actually see what that noise looks like zoomed in.

[VMP_09]

An update: I brought the speaker to an electronics repairman today. He says that the power amplifier is working fine, but the preamplifier part isn't functioning properly. (Similar to what you said Audiorepair)

However, there's one issue: he can't find the schematic diagram or the service manual for the JBL LSR 308.

Now, I have a question for you: can you find the schematic diagram? Or do you know a way to find it?

[dietert1]

We have four of those and they are marvels when working. One of those came from ebay as non-working and i happened to solve the problem by visual inspection, reconnecting plugs and the like.
It will be difficult to get the schematic, unless some customer made one. But once you identify the parts, you can look into their datasheet or find an application note to understand the circuit.
As these speakers don't have a digital feed, the preamplifier is an audio ADC. That doesn't work equally well with all audio sources. Once i connected it to a Sony laptop speaker output and it was noisy.
Maybe the ADC part is bad (ESD?). As far as i remember the ADC is integrated with an audio DSP to implement the crossover filters.

Regards, Dieter

[VMP_09]

Thank you for the feedback Dieter.

I hope somehow we can come across the schematics.

Best regards,
VMP

[andy3055]

That sounds very much like semiconductor noise. It could be a bad transistor or a diode failing under power.  It could also be a really bad solder joint. If your repair guy has confirmed that the power amp is ok, check the solder side of the preamp board. Since you have the other speaker working ok, compare it with this. It could be a bad cap also! Many possibilities.

[dietert1]

One may think about the reliability of SMD and connector technology in the presence of strong vibration. The circuit board has like six mounting points and the input connector provides some damping, too. But the upper end extends into free air. Possible damage will also depend on the average sound level of the monitor and duration of usage. I mean when using it as a party speaker at elevated levels for many hours, some SMD part can fail.

Regards, Dieter

[David_AVD]

If the noise is only there with the volume turned up, it may be the input protection diodes.

[VMP_09]

Thank you all for the feedback.

Yes it is only when the volume is turned up. It gets worse the louder the voume goes up.
Without any input connected.

VMP

[David_AVD]

A lot of pro audio gear have diodes on each balanced input line going to V- and V+ to help prevent damage to the front end.

Unfortunately the diodes sometimes go noisy. They are usually SMD versions of the 1N4148 or similar.

I could also be the front end IC that accepts the balanced input. They also go bad and can cause static type noise.

[VMP_09]

Update: unfortunately the electronics repairman cannot find the schematic so says he cannot repair it.

What I could do is see if I can replace the front stage parts one by one myself, to see if it works.

I'll try to post a picture of the scope readout aswel as user 'audiorepair' asked.

[Audiorepair]

Thank you all for the feedback.

Yes it is only when the volume is turned up. It gets worse the louder the voume goes up.
Without any input connected.

VMP

If this is true, then the noise source is before the volume control, and there is not a lot of circuitry before that.
This would normally be established in the first few minutes of a repair.

Usually that would either be the first op-amp, the protection diodes as David suggested, the ubiquitous electrolytic caps or all 3.
Being as these get damaged by stuff coming in from the outside.

You can remove the protection diodes and the unit should still work, there are normally 4 connected in a bridge rectifier type arrangement.

[andy3055]

Get hold of a good soldering iron and heat up/reflow all the solder points of components up to the volume pot. Especially all the semiconductor devices. If there is any bad/ dry joint tthere, this will take care of it. If the fault does not clear, it is time to check the individual components. It will be easier if you have a scope. I am surprised the repair guy needs a schematic to find this. Does he have a scope?

[VMP_09]

Hello andy and Audiorepair,

Please see the attached image, it is the close up of the input part on the board, which would be the diodes that cause the problem?

The blue arrow I drew are the pins to the back connector.


Best,
VMP

[andy3055]

There seems to be a number of diodes on that board. Without a diagram it is hard to identify just with a photo. I also see some residue near your blue arrow. Better to check where that came from. Is it some cap leaked?

[dietert1]

The image shows the main board witht the arrow pointing towards the socket of the input board. The socket itself and the input board are hidden behind/under the main board. Please post an image of the input board itself (the one with the audio input connectors).

Regards, Dieter

[Audiorepair]

Dieter is correct, that is not the input board.

The protection diodes are going to be close to the input XLR.
I have found that if one or more are damaged/noisy, they will measure a different Vf in circuit to the others using a meter on diode mode.

This is pretty much a definitive test for replacement.

[Koray]

I guess, Dieter is mentioning this board:

[Audiorepair]

Yes, I don't know this particular speaker, but the op-amp in the photo is also a likely suspect.

Perhaps there are more SMD components on the other side which are not visible, the protection diodes should be there if it has any.