Help in identifying SMD component K4A on Bose Wave System II

[rfengg]

Hi All,

An SMD component marked K4A on my Bose Wave system II heats up like mad.
Could anyone help me identifying this component marked as "K4A" as per the attached pic?
Also if anyone has a link to the service manual / schematic of these Bose Wave system II manufactured around 2003, that would be most helpful

thanks.

[eb4fbz]

DMMT3904W

[fzabkar]

BOSE MODEL CD2000 FE2000 ACOUSTIC WAVE MUSIC SYSTEM II Service Manual:

https://elektrotanya.com/bose_model_cd2000_fe2000_acoustic_wave_music_system_ii.pdf/download.html

K4A in smd markings database:

http://www.smdmark.com/en-US/search/code?id=k4a

[rfengg]

My apologies fzabkar, the model that I have looks more like the attachment I have included below......sorry for the confusion......I was led to believe that its called a Bose Wave System II.
Any chance I can get hold of a schematic or SM  for this?

[fzabkar]

That was the only similar item I was able to find at Elektrotanya.

[tooki]

Hi All

Could you change the image to something that’s not obnoxious? You’ve literally got a blurry thumbnail in a sea of whitespace.

[rfengg]

Hope its better now.....

[Jeff eelcr]

Bose has a sticker on the back or bottom of the unit with a model number/ type on it.
A picture of that  would would be a better way to ID this unit.
Jeff

[rfengg]

Over the weekend, I managed to change all the SMD caps on the radio (all except the ones in the shielded can that houses the uP)  to leaded ones from Panasonic and Nichicon and relieved that the radio now powers up normally and plays audio.

However, I have 2 strange issues.......

a) The headphone jack gives no audio out.....when plugging headphones, the speakers mutes as they should , but I hear zilch on my headphones.
Do these radios have a separate headphone amp or are they driven by the TDA7376 itself?

b) On AM and the AUX input the audio is perfectly fine, however on FM, there is a consistent buzz from the radio.....this is confusing me no end, as the AM/FM chip is common (LA1837) and I assume that the finals should be fine as AM/AUX are perfect......any pointers on where I should start looking?

For the life of me, I cant find a schematic for this version of the Bose Wave...........

thanks

[tekky]

I realize it has been a couple of years but wondered if you had any success?

I have an AWRCC1 Wave System II which also uses the LA1837 and TDA7376B you mention so maybe it is same model? I did a recap on mine and fixed a few things (bad IR sensor, no power, no audio) . The 6pin K4A is likely a dual NPN. To fix a power issue I successfully replaced one of these on mine using a DMMT3904W: https://www.diodes.com/assets/Datasheets/DMMT3904W.pdf
As I recall, there was a bit of gunk around and under that part, maybe cap juice.
 
After replacing all but the two largest caps, I am now having the same symptom you described - background hum only on FM and no headphone output. I think we can both rule out the speakers, the TDA7376B audio amp, and possibly power supplies. The fact that it is FM specific points to something upstream of the audio amp, maybe around the FM tuner and preamp. That LA1837 does not seem to be available anymore so hopefully it is not that.

I have not been able to find schematics and the pinout of that preamp is completely different from the LA1836 and LA1851 used in other models so the best I can do is probe pins looking for noise sources. I'd be interested if you were able to fix the problem or learn anything.

[rfengg]

I realize it has been a couple of years but wondered if you had any success?

I have an AWRCC1 Wave System II which also uses the LA1837 and TDA7376B you mention so maybe it is same model? I did a recap on mine and fixed a few things (bad IR sensor, no power, no audio) . The 6pin K4A is likely a dual NPN. To fix a power issue I successfully replaced one of these on mine using a DMMT3904W: https://www.diodes.com/assets/Datasheets/DMMT3904W.pdf
As I recall, there was a bit of gunk around and under that part, maybe cap juice.
 
After replacing all but the two largest caps, I am now having the same symptom you described - background hum only on FM and no headphone output. I think we can both rule out the speakers, the TDA7376B audio amp, and possibly power supplies. The fact that it is FM specific points to something upstream of the audio amp, maybe around the FM tuner and preamp. That LA1837 does not seem to be available anymore so hopefully it is not that.

I have not been able to find schematics and the pinout of that preamp is completely different from the LA1836 and LA1851 used in other models so the best I can do is probe pins looking for noise sources. I'd be interested if you were able to fix the problem or learn anything.

@tekky : Unfortunately no.....you are right in that the issue may be  upstream......I just did not  have time to look into it and as we moved to NY at that time and so I donated it to charity.
What I missed out on replacing,  were the caps inside that shield that houses the uP......but I think its a long shot......please let us know if you find something....atleast for academic purposes.

[tekky]

@tekky : Unfortunately no.....you are right in that the issue may be  upstream......I just did not  have time to look into it and as we moved to NY at that time and so I donated it to charity.
What I missed out on replacing,  were the caps inside that shield that houses the uP......but I think its a long shot......please let us know if you find something....atleast for academic purposes.

I did replace the caps under the shield but I agree they probably wouldn't explain the symptoms. I found an additional quirk with mine. If I switch back and forth between AM and FM the audio is normally muted briefly during the switch and takes a second or two to retune? and unmute.  On mine the audio sometimes does not come back no matter how long I wait, and I have to power cycle via cord or remote or keep switching bands to recover. When it happens it could be either band and it does not seem to matter if the unit is warm or not. The display gives every indication that things are normal.

If I have audio working in either band, and instead I switch between presets of that band, the audio does not fail so it is something specific to switching between bands. I don't know if any muting is used here. From the datasheets I see that both the LA1837 and TDA7376B have a mute signal on one pin so I can at least probe those to see when or if they are being utilized.

Update - the mute pin on the TDA7376B seems to be used only when the unit it turned off. The mute pin on the LA1837 is used when switching bands and maybe selecting presets and seems to work as expected even when I have the no audio issue, but there is another internal mute control from the 'IF buffer' so maybe that is closer to the root cause of the no audio problem. Maybe some failure of this circuit to tune properly keeps the chip muted.

Another mystery is the mute on the remote does not utilize either of the mute pins (or standby pin). I have not been able to find any obvious control signal that changes with the remote mute.
Without schematics it will take too much time and I may have to give up. At least the CD player works.