What the???

[Off-Beat]

So a friend brought a qtx powered speaker in as it was cutting out... He had tried to open it up himself to have a look and bulled the speaker connections off the board so I soldered those back on.. I also noticed a small 470uF cap was very loose, because the legs had been shortened and from the position it was in it was difficult to solder it back in place so I replaced it with another 470uF 25v the exact same... Boxed it all back up after a quick service and switched it on, all working fine. I Thought it sounded a little distorted so I unplugged it, took the front grill off to check if the speaker coil was rubbing... It wasn't. Plugged it back in and now I have this nasty loud hum. I deff have the cap in the right way around and it was working until I unplugged it. Im a it confused with it now.

[nickb]

So a friend brought a qtx powered speaker in as it was cutting out... He had tried to open it up himself to have a look and bulled the speaker connections off the board so I soldered those back on.. I also noticed a small 470uF cap was very loose, because the legs had been shortened and from the position it was in it was difficult to solder it back in place so I replaced it with another 470uF 25v the exact same... Boxed it all back up after a quick service and switched it on, all working fine. I Thought it sounded a little distorted so I unplugged it, took the front grill off to check if the speaker coil was rubbing... It wasn't. Plugged it back in and now I have this nasty loud hum. I deff have the cap in the right way around and it was working until I unplugged it. Im a it confused with it now.

At the very least we're going to need the model number. Posting a schematic increases your chance of getting assistance immeasurably and is a nice thing to do anyway.

[Jazz P Bass]

I would approach the QTX as a new repair.

Forget everything else & start from scratch.

Troubleshoot it like it just came in the door.

[olddawg]

I've worked on a lot of powered speakers. Putting a PCB with large(ish) components causes ventilation and connection problems. The most common problem I see are the large rail caps (usually 4700uf) become domed, fail, and/or disconnect themselves. They are usually rated to close to the working voltage to be reliable over time. I usually replace them with higher voltage caps if there is room. Also be aware that the manufactures love to bury the components in hot glue or latex. And... Be aware that many times they are double sided boards where the component leads (especially those filter caps) provide the feed through connections and therefore must be soldered on both sides of the board. This requires the proper techniques, cleaning, fluxing, etc.

[Off-Beat]

Sorry guys I have been a little sick the last few days I will post more info on the speaker tomorrow and hopefully find. Schematic.