Bose Wave Radio/CD Hum after drop

[lcmtr]

I recently took over a Bose Wave Radio/CD AWRC2G that was knocked off of its stand by an overly curious kitten.
Since then, the unit shows the following behaviour:

Disclaimer*: Unfortunately I cannot say whether some of these traits are normal as I am a beginner in the field and have only had the unit in this state.

1) Constant Hum/AC noise(?) of constant volume on both speakers.
    - Audible irrespective of input used. Louder than actual audio only below volume 20/100.
    - Volume of hum is constant over entire volume level range.
2) Audio plays on volume level 0*.
    - This is not bothersome as it is only noticeable when glueing the ear to the speakers. However might be an indicator to the component at fault.
3) Coil whine from display unit main IC section*
    - Not bothersome. Normal?

Given that the noise is present on all inputs I am led to believe that either the amplifier or the power conversion circuit is at fault.
I checked the PCBs but did not see any disconnected or burnt up components.

Schematics are available if needed.

Thanks in advance,
Lucas

[mzacharias]

Possibly a cracked circuit board, or a main filter cap knocked loose.

I'm leaning toward a circuit board issue.

[lcmtr]

Cannot make out any cracks on the circuit board.

The filter caps (C6, C16, C202, C205) also seem rock-solid, no broken solder joints and no room for wiggling.
Although their size might hide problems below them.

I also checked the -15V voltage regulator (U200) as suggested in another forum post here.
Measuring -15.28V so this also does not look like the cause of the problem.

[lcmtr]

Edit: Attached circuit schematics.

I replaced the capacitors C202 and C205.
Unfortunately to no success, unit still hums.

Turns out I did miss something rather blatant on the circuit board:
There is a discoloration on the underside as if a component radiated too much heat.
Indeed, the 10V VRM (U201) for the audio circuitry (picture: top) heats up to a temperature that burns the skin.

The two smd resistors are the dissipating elements for the two regulators, respectively.

Help is greatly appreciated.
Lucas

[goaty]

Try to measure AC on the supply lines. If you got a scope or use AC mode on Multimeter to see if AC is above some mV. If so, the hum is from insufficient filtering, probably a damaged filter cap.

[andy2000]

Assuming it worked before it was dropped, there has to be some kind of physical damage.  Dropping it wouldn't cause a component to fail. 

Those regulators appear to have been running hot for a long time, so it's probably normal.  It's still worth checking their outputs though. 

Check carefully around all power supply and audio output components for damage (especially around the audio output IC, and large caps).  It's possible a pad was detached from the board leaving a hard to spot hairline crack in the copper. 

[Tom45]

Since the hum volume isn't affected by the volume control you know that the problem is downstream from the volume control.

And since there is more gain upstream from the volume control if it were power supply ripple, you would expect the earlier stages would be more affected by the power supply ripple. So it isn't a general power supply ripple issue.

The exception would be if one or more stages after the volume control have their own power supply. As both channels have the hum that does point to a power supply issue. Check the schematic to see if there is a separate power supply for the output stages.

[SpecialK]

I believe it's the main power supply cap.  The reason I say this is:

• I've looked at my Wave Music III which is newer but similar
• The hum isn't affected by the volume control
• This TO-220 regulator can't handle enough current for the audio IMO
• From what I can tell the main amplifier IC uses filtered but unregulated power from this rail.
• I think the preamp stages use the regulated 15V. I think there's a regulated 10V too.
• The transformer has two secondaries, the other creating a 5V rail for the microprocessor and stuff
• The cat incident could have caused the previous owner to try the unit after a long period of not being used.  Maybe the issue existed prior

I would solder a pair of wires across the 10000 uF main filter cap and run them outside the case.  Then I would measure AC voltage from the exposed ends with a low AC scale on my multimeter and see if it increases substantially once the unit was powered on.  I'm not sure what would be "bad"

Then if sufficient AC ripple exists, I would inject DC from a bench power supply that matches the unloaded DC voltage across those wires.

At any rate, I don't think I saw the right pages of the schematic.  Any chance we could get a download link?   

[Chris56000]

Hi!

Will these documents cover it?

For some reason they come out extremely faint – I assumed Bose used a high–speed draft mode when printing 'em into PDF from the EDA tool they used!

Since the unit was dropped/knocked off a shelf by an inquisitive pet, I would examine the PCB particularly round the mains transformer, C6, BR1 and the 15–pin Miniwatt TDA7374 power o/p chip U1 – note there are several different "earthy" points, denoted by letters A, D & P in triangles returned back to the negative pole  of the main reservoir electrolytic C6 – has C6 broken off the PCB and cracked the print somewhere?

Anyway you've got the full documentation here, download and print it off and then dismantle the PCB from the cabinet/chassis and check it very carefully for damage!

I can follow this up with more detailed fault–finding advice later if need be, but concentrate on the o/p stage  & PSU Sheet CD1 first!

Chris Williams

[lcmtr]

Hi,

First of all, thank you for the numerous replies!

Those regulators appear to have been running hot for a long time, so it's probably normal.  It's still worth checking their outputs though. 

The regulators output ~5.05V and ~10.08V, respectively.

Check the schematic to see if there is a separate power supply for the output stages.

Both preamp stages use the same Vcc supply, as pointed out by SpecialK.

I would solder a pair of wires across the 10000 uF main filter cap and run them outside the case.  Then I would measure AC voltage from the exposed ends with a low AC scale on my multimeter and see if it increases substantially once the unit was powered on.  I'm not sure what would be "bad"

I only have a multimeter with a 200V AC setting laying around.
When only plugged in, I measure 35.0V AC over C6, once audio playing this drops to ~33.4V AC, although the exact level is dependent on the volume.

I would examine the PCB particularly round the mains transformer, C6, BR1 and the 15–pin Miniwatt TDA7374 power o/p chip U1 – note there are several different "earthy" points, denoted by letters A, D & P in triangles returned back to the negative pole  of the main reservoir electrolytic C6 – has C6 broken off the PCB and cracked the print somewhere?

The solder points of C6 look good and the cap sits rock-solid flush on the pcb.
The bridge rectifier IC and U1 are same, the solder points look good and no leads are broken off.
C16 is a bit woobly, though.

Assuming it worked before it was dropped, there has to be some kind of physical damage.  Dropping it wouldn't cause a component to fail.

Is there the possibility that both speakers took a hit during the fall and that the PCB is fault-free?

Best,
Lucas

[wd5jfr]

Having fixed AC operated  radios for many years in the past that had hum at zero volume it was generally caused by weak power supply electrolytic filter capacitors.  The quickest way to tell is bridge the suspect unit or unit with one of similar size.  If the hum goes away you have identified the culprit. It could also be an invisible crack in a trace or a cold solder joint, so touch up the filter cap joint regardless of their appearance. If you have a scope you would see it on the DC line.  I"m not clear where you saw the varying 30 VAC but if it's on a DC line you have a bad filter. 
Good luck
Hank