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Computer speakers power issue

KDLrZ9S
Hello. I have these old computer speakers. The sound of the left speaker comes and goes depending on how you wiggle the cord. By opening the main speaker (right) I found that the underside of the left power output seems scorched (?). These are harmon/Kardon speakers. According to the power adapter the model number seems to be A481511OT.
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KDLrZ9S
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https://bbcode0.com/full/2021/1/4/5180e7aae28640d1ba9b3ace56741313-full.jpg.html
 
Last edited:

davenn

Moderator
Sorry. I am having trouble uploading images. I think the code I inserted should have the images.

yeah, they didnt show, you may have tried some other format ?

Use .jpg, around 100kb max, ~ 900 x 600 max

try again :)
 

Harald Kapp

Moderator
Moderator
The pcb looks badly corroded. Try cleaning the pcb (use pure alcohol and light pressure, if need be use a light abrasive household cleaner). After cleaning, remove any residues of the cleaning agent with a liberal dose of distilled or purified water and a soft brush.
Re-solder the affected joints, replace the electrolytic capacitors. After all, the speaker seems to work basically but has obviously a loose contact or cold solder joint(s).
 
It looks like it had water soluble flux residue remaining on the PCB and then got wet, causing metal salt conductive bridges.

You may need something more aggressive to clean that off. I usually use (because I happen to have some) brasso metal polish and a toothbrush. It's the mild abrasive particles in it that help more than the oxalic acid. After scrubbing with the polish, then I'd scrub with hot detergent solution and a final water rinse. If it had fine pitch SMD chips I might do a final rinse with alcohol but for that, I wouldn't bother.

If after scrubbing all that corrosion off, there is bare copper left behind, you might want to paint it to seal off from oxygen, whether it be with a lacquer, conformal coating, nail polish, or anything really is better than nothing, except some coatings could interfere with any later repair efforts and anything opaque, also interfere with ease in tracing the circuit tracks.

However, I wonder if this is coincidental, that the real issue is one speaker connects to the other with a 3.5mm or similar jack and the spring tension contacts in the socket have gotten loose or tarnished a bit so the manipulation of the cord, even if the power cord instead of the link to the other speaker, is moving the plug in the socket just enough to restore the connection.

If that's what's happening, you could of course source a replacement socket with same form factor, or do without the connector and solder the wires directly on, or since there is so much empty available space on a speaker housing, just put a hole in the housing for a panel mount jack, air-wired to the PCB contacts.

Otherwise I don't see how wiggling the cord would be doing anything about the corrosion on the PCB. If the power input socket were intermittent that should affect both channels.
 
A common failure mode with these sort of things is that one of the conductors in the lead fractures close to the connector. This often happens after many years of handling. Try replacing the either just the cable or cable and connectors.
 
A common failure mode with these sort of things is that one of the conductors in the lead fractures close to the connector. This often happens after many years of handling. Try replacing the either just the cable or cable and connectors.
I think I might try to replace the cable first before anything else. I have done some soldering in the past but not a lot. Where would I get these cables/connectors? How etc? Also I never remember getting the speakers wet.
 

Harald Kapp

Moderator
Moderator
You don't have to replace the full cable. Usually these break wehre they are constantly moved and bent, typically at the entry to the case, If you can spare a few centimeters, cut the existing cables at a distance of ~ 1 cm from the case, then strip the insulation (use the remaining cable ends as an example). Then solder the nnew cable ends in the same spot where the former cable ends were.
 
Where would you get cables? The internet! Kidding aside, depends on where you're located. A fully made cable might be cheapest on ebay or Amazon while individual connectors, I'd check Digikey, Mouser, etc, electronics suppliers.
 

bertus

Moderator
Hello,

I have taken three pictures from the openings post and rescaled them.
THX1138_A.jpeg This looks like a deposit of the corrosion of the PCB.

THX1138_B.jpeg
Not much to see on this picture.

THX1138_X.jpeg
Massive corrosion around the diodes.

Bertus
 
The crud in your pictures looks a bit like battery leakage. Have there ever been batteries in your speakers? Or perhaps a capacitor has leaked.
 
As for the alcohol. Try some undiluted screen wash additive for the car. It is usually Ethanol or IPA or sometimes a combination of the two. It's cheap and it works. Another source in an aerosol is de-icer which is very similar to screen wash.
 
Just from a quick look the capacitors haven't leaked and don't look bulged. I added some pictures with a better view of the board. Could the moderator please rescale these?
20210105_195925_480x853_240x426.jpg20210105_195810_640x360_320x180.jpg 20210105_200205-1_240x292.jpg
 
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