P38A Subwoofer 'popping'

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domino

domino

Well-Known Member
Posts
2,014
Location
Edinburgh
I've had a search for this and nothing has come up.

This problem only started happening last night on the way back from Manchester. Changing radio station, or between radio/cd/tape or between tracks (basically anything that stops and starts audio output) produces an audible pop from the subwoofer. I've also discovered that the woofer is producing it's own techno too. Rapid, muted popping like some kind of 160bpm hardcore. The latter comes and goes but there's no apparent pattern aside from it speeding up slightly when you're not idling.

Any suggestions as to what might be causing this and an appropriate fix? Haven't checked anything yet myself as it's ****ing down outside so any pointers first hand would be appreciated :)
 
I was thinking a long the lines of a capacitor myself..which suggests the amp itself is at fault. So, I need to either try and identify which capacitor it is and replace it or replace the amp.
 
Check the earths, grab a graphite core and wrap wrap this around the signal wire.

Failing external interference it's time for a deeper look into the amp.
 
Is the 'popping and banging' still apparent when the engine is not running?

Is the frequency of the 'pops and bangs' sympathetic with any of the vehicles systems, (engine rpm, wipers, heater, fuel pump, etc), in which case there may be some form of noise 'pick up'?

Have you, (or anybody else), undertaken any work just prior to the noise starting?

Are the bass driver units, (bottom of each door - if you have them), also popping and banging - this would suggest a common noise pick up in the 'head unit' itself rather than the sub woofer unit?

If you physically 'tap' the sub woofer can you get it to 'pop an bang'?

The 'dielectric' within most Electrolytic Capacitors will dry out as they age, (the lifespan of the P38 is sufficient time!), and the capacitors are then rendered 'open circuit', (occasionally they can break down and actually 'short circuit' - but this will usually have a more dramatic effect - especially in power supplies), and their 'smoothing' functionality will be lost!

Tracing audio noise can be a very frustrating process - unfortunately!
 
Wee update:
- The popping still exists when the engine isn't running (the popping from changing track/station/source/etc).
- When the engine isn't running, the techno-rave popping doesn't happen. For example, while it's there, if you switch off the engine, it stops immediately.
- Have discovered that this techno rave gradually slows down (e.g. 160bpm -> 80bpm) once you start moving.. and seems to stop completely after a few minutes.

- No work carried out on the car at all apart from an MOT last week.
- No issues with any other speaker in the car that's new. (drivers rear passenger has always buzzed; never gotten around to replacing)

This problem only started yesterday afternoon/evening returning from Manchester. The only 'special' circumstances would be the mental monsoon rain and winds experienced.. and there was a load in the boot which *may* have knocked on the siding in the boot but there are no obvious dents/marks/tears in the carpet to suggest that.
 
domino said:
Wee update:
- The popping still exists when the engine isn't running (the popping from changing track/station/source/etc).
- When the engine isn't running, the techno-rave popping doesn't happen. For example, while it's there, if you switch off the engine, it stops immediately.
- Have discovered that this techno rave gradually slows down (e.g. 160bpm -> 80bpm) once you start moving.. and seems to stop completely after a few minutes.

- No work carried out on the car at all apart from an MOT last week.
- No issues with any other speaker in the car that's new. (drivers rear passenger has always buzzed; never gotten around to replacing)

This problem only started yesterday afternoon/evening returning from Manchester. The only 'special' circumstances would be the mental monsoon rain and winds experienced.. and there was a load in the boot which *may* have knocked on the siding in the boot but there are no obvious dents/marks/tears in the carpet to suggest that.
Click to expand...

Feedback on the line somewhere ...
 
Alternator diode pack breaking down???

My theory for this is thus (and is not scientiffical at all as I am not an electrician or Electrical Engineer):

You say the frequency of the techno rave gradually reduces then stops after a few minutes of engine running......??!!.....When you start the car, the battery takes a pounding so the Alternator has to top the battery up, this puts extra load on the Alternator and the diode pack mounted to it....until such time that the load is reduced as the battery gets topped back up again, once the load is shed the techno rave stops.....so could be electrical noise from the Alternator.....I wonder if you start the engine and time how long it takes for the Techno Rave to stop.....then turn off again....then start it up again and turn on the lights, heaters to fan speed full ahead flank, rear window heaters, and most other electrical loads, and see if the Techno Rave either a) lasts longer till it stops as the Alternator is taking more load thus charging the battery takes longer....or b) it continues indefinatly until you start turning the electrical loads back off again, which would reduce the 'stress' on the Alternator and then the techno rave stops.....

if any of that makes sense!
 
  1. Is this DSP system, or the older Sub with single amplifier ?
  2. Agree on checking ground point (E367 or E1351 if DSP Amp). This same ground point is used for LHS Tail lights.
  3. Also could be failed Capacitor in the Sub Amplifier.

Pete
 
Sounds perfectly logical to me - and certainly worth further investigation.

There is a possibility that an 'earth' has been lost, (close to, or inside, the sub woofer itself), is there a possibility that the 'load' has knocked the n/s trim, (that 'hides' the sub woofer), sufficiently to dislodge the molex connector slightly, (or to dislodge one of the ground leads within)?

upload_2016-2-8_15-4-57.png


This connector provides the audio feed as well as the 12 volts, (and ground return), to the sub woofer - mine is currently exposed as I have been repairing my boot floor - the connector to the CD Player is also part of the harness.

The buzzing from one of the doors may be a similar issue, (dislodged ground in the door amp connector or a faulty door amp itself - just replaced one of mine for £2.99 including postage - I couldn't have repaired it any cheaper)

My next step would be to use my Oscilloscope to monitor the input audio signal at the molex connector, (on the side of the sub woofer), if the 'noise' is abnormal I would expect to see the audio signal 'jumping' up and down every time there is a pop or a bang.

If there is no noise on the signal feed my next suspicion would be with the sub woofer itself - one of the internal noise suppressor capacitors/inductors could have failed, (as per my previous post), or it could be subject to a dry joint, (hence my suggestion to 'tap' the sub woofer to try and temporarily re-seat the dry joint.

Large coils, transformers, inductors, capacitors, power transistors, etc are all susceptible to dry joints as their 'mass' causes them to move/vibrate/resonate in sympathy with the vehicle movement, (or bass notes in this particular case - no pun intended), thus stressing the solder joint until it is no longer patent and the component is no longer in circuit, (wide temperature fluctuations can also result in dry joints), and vibration and wide temp variations are no strangers to vehicle interiors!!!!

Unfortunately I suspect that Domino will not have access to an Oscilloscope?
 
I think it's the 'older sub with single amplifier' model - I've never yet managed to figure out how to tell the difference without stripping all sorts..?

If you'd asked me 4-5 years ago, I had more oscilloscopes than most electronics labs ;)
As for the door buzz, it's not constant, that's more than likely a torn cone as it's only with certain frequencies and bass - been like that since I bought the car and just haven't gotten around to that particular to-do list item as yet!
 
Sounds like you will have no problem tracking it down then!

Just unfortunate that you can't, temporarily, replace the sub woofer with another, (known good one), as that would immediately narrow down the area where the fault resides.

I guess that, (if you have a small portable amplifier and speaker - such as a small guitar amp), you could disconnect the audio feed from the sub woofer and drive your spare amp, (under the same conditions as when the fault occurs), and if the noise pick up is in the vehicle wiring, (or as Saint suggests - the charging circuit/diode pack), you should still hear the noise.

If the sound is noise free, however, then the fault lies within your sub woofer amp - which should be a quick fix - although I have never taken mine apart to see what they use as the output stage?

Before I realised it was a sub woofer I thought it was an additional rapid heater system, (for the rear compartment), so I could be completely wrong!!
 
Also do the usual check for corrosion on C207 behind LHS passenger footwear kick-panel. This feeds both Left Doors, and Sub Amplifier.
 
if you have the single harmon kardon amp in the rear then its the start of a slippery down ward slope to eventual amp failure mine still hisses from the rears traced it down to component level in the amp not repairable so for now iv turned the balance to front only still get a slight hiss but some music is better than none for now may replace the whole system with a new set up in the summer
 
well the faulty components are not avalible they are coverd in hard protective plastic to secure them to the circuit board and protect them iv been trying to fix mine for a while now iv a faulty amp whos power supply has gone dead still drawing my own cicuit diag for it in the hope i can fix it and get my sound system back to normal trust me the amp is not fixable unless its the main amp o/p ics which can be replaced but they aint causing the fault
 
Here is the amp iv traced my fault to an I/c under the black circle which has been hot melted at the factory the heat sink is under the PCB bit of a bad design which I suspect is the cause of the failure. Using thermal shock I can cure and recreate my fault.
 

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Dave,

Very interesting - a mixture of both discrete and surface mount components.

I note what appears to be a two channel system with 4 x Power Amp chips, (two for each channel - possibly 'bridged'), so I wonder how many bass units are there in the sub woofer assembly itself, (possibly two but definitely not four)?

I note that the primary incoming power rail 'smoothing' is accomplished by the large electrolytic and coil in the bottom left of your picture and Domino should take a closer look at these components, (or at least their respective solder connections), and I would re-solder them both regardless of whether the joints look suspect or not.

The 'isolated' circuit in the upper left part of the board, (surrounded by a foil border), is somewhat intriguing and without reference to a diagram I can only assume that it is either a pre-amp or a muting or power switching circuit, (I note that muting and power control is present in the door amplifier modules).

The possibility of a repair will be subject to where, exactly, the fault lies but if it is due to one of the surface mount chips it will be difficult, (at the very least), and impossible if it is one of the epoxy covered components or one of the others if you are unable to source a replacement.

I did manage to replace a surface mounted logic chip on a Pioneer CD Changer Unit, (it was located in a lake under the rear seat of a Jaguar that had a serious leak!), and several legs, (on the chip), had corroded away.

I managed to remove it, (approximately 20 to 30 pins if my memory serves me correctly), without the use of a hot air solder station by winding a piece of piano wire to my soldering iron tip and I then gently prised each leg away from the board in turn and then, finally, levered the body of the chip away from the pcb, (they are usually bonded to the board during manufacture).

Installation of the new chip was the exact reverse of removal - taking care to minimise the amount of heat delivered to each leg.

We have gained a tremendous amount of benefit from VLSI, (very large scale integration), technology - utilised in today's electronic products, (lots of functionality, cheap manufacturing costs, much smaller circuit 'footprints', less heat dissipation, reduced current consumption, etc), but the downside is that we now live in a 'modular' society where most repairs are now only possible down to a 'module' or board level, (and not down to component level), unfortunately.
 
biketeacherdave said:
well the faulty components are not avalible they are coverd in hard protective plastic to secure them to the circuit board and protect them iv been trying to fix mine for a while now iv a faulty amp whos power supply has gone dead still drawing my own cicuit diag for it in the hope i can fix it and get my sound system back to normal trust me the amp is not fixable unless its the main amp o/p ics which can be replaced but they aint causing the fault
Click to expand...

biketeacherdave said:
Here is the amp iv traced my fault to an I/c under the black circle which has been hot melted at the factory the heat sink is under the PCB bit of a bad design which I suspect is the cause of the failure. Using thermal shock I can cure and recreate my fault.
Click to expand...

Martian said:
Dave,

Very interesting - a mixture of both discrete and surface mount components.

I note what appears to be a two channel system with 4 x Power Amp chips, (two for each channel - possibly 'bridged'), so I wonder how many bass units are there in the sub woofer assembly itself, (possibly two but definitely not four)?

I note that the primary incoming power rail 'smoothing' is accomplished by the large electrolytic and coil in the bottom left of your picture and Domino should take a closer look at these components, (or at least their respective solder connections), and I would re-solder them both regardless of whether the joints look suspect or not.

The 'isolated' circuit in the upper left part of the board, (surrounded by a foil border), is somewhat intriguing and without reference to a diagram I can only assume that it is either a pre-amp or a muting or power switching circuit, (I note that muting and power control is present in the door amplifier modules).

The possibility of a repair will be subject to where, exactly, the fault lies but if it is due to one of the surface mount chips it will be difficult, (at the very least), and impossible if it is one of the epoxy covered components or one of the others if you are unable to source a replacement.

I did manage to replace a surface mounted logic chip on a Pioneer CD Changer Unit, (it was located in a lake under the rear seat of a Jaguar that had a serious leak!), and several legs, (on the chip), had corroded away.

I managed to remove it, (approximately 20 to 30 pins if my memory serves me correctly), without the use of a hot air solder station by winding a piece of piano wire to my soldering iron tip and I then gently prised each leg away from the board in turn and then, finally, levered the body of the chip away from the pcb, (they are usually bonded to the board during manufacture).

Installation of the new chip was the exact reverse of removal - taking care to minimise the amount of heat delivered to each leg.

We have gained a tremendous amount of benefit from VLSI, (very large scale integration), technology - utilised in today's electronic products, (lots of functionality, cheap manufacturing costs, much smaller circuit 'footprints', less heat dissipation, reduced current consumption, etc), but the downside is that we now live in a 'modular' society where most repairs are now only possible down to a 'module' or board level, (and not down to component level), unfortunately.
Click to expand...

252849242_373d779ccd.jpg
 
Martian said:
Dave,

Very interesting - a mixture of both discrete and surface mount components.

I note what appears to be a two channel system with 4 x Power Amp chips, (two for each channel - possibly 'bridged'), so I wonder how many bass units are there in the sub woofer assembly itself, (possibly two but definitely not four)?

I note that the primary incoming power rail 'smoothing' is accomplished by the large electrolytic and coil in the bottom left of your picture and Domino should take a closer look at these components, (or at least their respective solder connections), and I would re-solder them both regardless of whether the joints look suspect or not.

The 'isolated' circuit in the upper left part of the board, (surrounded by a foil border), is somewhat intriguing and without reference to a diagram I can only assume that it is either a pre-amp or a muting or power switching circuit, (I note that muting and power control is present in the door amplifier modules).

The possibility of a repair will be subject to where, exactly, the fault lies but if it is due to one of the surface mount chips it will be difficult, (at the very least), and impossible if it is one of the epoxy covered components or one of the others if you are unable to source a replacement.

I did manage to replace a surface mounted logic chip on a Pioneer CD Changer Unit, (it was located in a lake under the rear seat of a Jaguar that had a serious leak!), and several legs, (on the chip), had corroded away.

I managed to remove it, (approximately 20 to 30 pins if my memory serves me correctly), without the use of a hot air solder station by winding a piece of piano wire to my soldering iron tip and I then gently prised each leg away from the board in turn and then, finally, levered the body of the chip away from the pcb, (they are usually bonded to the board during manufacture).

Installation of the new chip was the exact reverse of removal - taking care to minimise the amount of heat delivered to each leg.

We have gained a tremendous amount of benefit from VLSI, (very large scale integration), technology - utilised in today's electronic products, (lots of functionality, cheap manufacturing costs, much smaller circuit 'footprints', less heat dissipation, reduced current consumption, etc), but the downside is that we now live in a 'modular' society where most repairs are now only possible down to a 'module' or board level, (and not down to component level), unfortunately.
Click to expand...

Yes exactly my faulty chip is under the black epoxy thermal shock tracked it down to this chip. The gold ringed part is the main PS to the board im currently working out a diagram and measuring voltages on my working board to compare to a faulty one I have which has a complete power failure. I to do not like to give up on things electrical as a time served TV/video/audio engineer I'm use to fixing thing but when it comes to something like this designed and built not to be fixed I'm up against a brick wall.
Still I'm hoping to see if I can get the dead amp going I'm thinking a surface mounted resistor has failed but as it involves sitting in the boot for hours on end tracing voltages it will have to wait until better weather
 
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