Recapped 840av with intermittent sound

[d3jsp]

Have a 840av that is already recapped with red polymer caps. Sound from the headphones jack work just fine but onboard speaker is intermittent. Sometimes it works and is loud, but most of the times it's extremely faint or nothing at all.

I would assume this is a bad c48 from bomarc's schematic's right? But it just seems so unlikely that a polymer cap is already going bad.
Anyone have similar experiences with the 840av and can this be anything else on the board?

 

[croissantking]

It might be dirty contacts inside the headphone jack. When you remove the headphones, it closes a contact that routes the audio to the onboard speaker, and these often get dirty or corroded with age. Then you get issues like you’re having.

- try spraying contact cleaner into the jack
- try inserting and removing a headphone connector multiple times

 

[cheesestraws]

Yes, likely to be the contacts in the jack. But check the whole signal path always, don't just blindly assume it's capacitors, there are plenty of other things to go wrong in those machines.

 

[croissantking]

Yes, likely to be the contacts in the jack. But check the whole signal path always, don't just blindly assume it's capacitors, there are plenty of other things to go wrong in those machines.

Yes - it’s exceedingly unlikely to be a brand new capacitor that’s causing the issue unless they installed it backwards

I’m working on an 840AV board right now that has no video and no sound (except for the startup chime). The original caps ravage these boards so it’s generally corroded traces that are the issue.

 

[cheesestraws]

Yes - it’s exceedingly unlikely to be a brand new capacitor that’s causing the issue unless they installed it backwards

Given those symptoms, though, might also be a dodgy solder joint on one of those new capacitors... being fully open or high resistance much of the time but then either warming up or physically flexing to make contact sometimes.

 

[d3jsp]

The headphone jack itself is fine. Traces around speaker looks fine too. My guess is that it could be the TDA7052A amplifier chip? Since it doesn't seem like headphone sound goes through this

 

[d3jsp]

Swapped for another DA7052A from aliexpress but still no sound. The new chip is getting extremely hot even though the input measurements look correct from specsheets

pin1 VCC ~8V
pin 4 mute is ~0.9V. This goes to pin 40 on the 2300 instead of 37 like what is in bombarc's schematics, strange!
pin 5 and 8 goes to speaker. Gives around 5-6V all the time, not sure if this is right
pin 3 and 6 are ground
pin 2 signal seems correct

 

[d3jsp]

with the original chip, I am getting
pin1 VCC at ~ 12V
and pin 4 at 5v
and pin 5 and 8 at ~5.8V

Even though it doesn't work and there is no sound, the chip does not get hot at all. This leads me to suspect if these chips have different specs or did I get fake Chinese chips?

 

[d3jsp]

Got them here by the way, as they do look kind of sketchy: https://www.aliexpress.us/item/3256806577569338.html

 

[d3jsp]

Yup, turns out to be the 7052a. Looking around these seem to fail pretty frequently. Also, beware of fake chips from China, glad it didn't fry my board.

 

[croissantking]

Got them here by the way, as they do look kind of sketchy: https://www.aliexpress.us/item/3256806577569338.html

I need to get the same chip for my LC475. Just to confirm, it’s safe to buy from this link?

 

[d3jsp]

no, do not buy from there, they sell fake chips

 

[croissantking]

no, do not buy from there, they sell fake chips

Ah, Ok. Do you have a link for legit chips?

 

[d3jsp]

look on ebay and buy the ones where they have pictures with chips made by NXP or Philips