DCM TB1 Subwoofer (Amplifier) Repair. ESD Fail! - Page 1
Forum section: electronics / repair

author: MWisBest
author_rank: Contributor
timestamp: January 03, 2018, 11:53:20 pm
content: Well my new year is off to a terrible start. Killed this home theater subwoofer last night... it seemed like it was set too loud, so I hopped off the couch over to the back panel to adjust it, and ZZZAAAPPP! As soon as my hand got near the back panel it made the biggest ESD shock I've ever seen. It's not the fanciest subwoofer, but it was my dad's and holds great sentimental value with me so I'm going to fix it.(Note: click on images for hi-res versions)As soon as it's plugged in it makes an awful loud noise. Even if the power switch is off (the LED indicator works too), with nothing hooked up to the inputs, it makes the noise. At the very least the TDA7294 chip amp is fried... just going to start there for now.

author: wraper
author_rank: Unknown
timestamp: January 03, 2018, 11:58:44 pm
content: Quote from: MWisBest on January 03, 2018, 11:53:20 pmIf TDA7294 would fail, most likely fuses would blow up or something else burn, IC explode. IMO you suspect exactly what survived.

author: MWisBest
author_rank: Contributor
timestamp: January 04, 2018, 02:29:22 am
content: Quote from: wraper on January 03, 2018, 11:58:44 pmI don't think so. The fuse is a slow-blow 4A, and I unplugged it immediately. It was never left on more than a second. The TDA7294 has its tab tied to the negative supply rail, the tab was screwed to the heatsink, which is what I touched. According to the datasheet of the similar TDA7293, the ESD rating is 1.5kV. I have no doubt the shock exceeded that.The power LED appears to be tied to the mute or standby pin of the TDA7294, and that LED functions properly still. Even the auto input detection works. So if the signal to the mute or standby pin is correct as I suspect, there should be no way for the speaker to be making noise if the chip is functioning properly.It's certainly possible that everything else on the negative supply rail was taken out as well, but it's just a starting point. I can whack in a TDA7293 instead which is spec'd a bit better and has some features like clip detection I might be able to make use of somehow.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: January 04, 2018, 08:01:17 am
content: That "glue" they used is just horrible looks like baby sickGood luck with the sub-woofer repairBtw i would check all the resistors and diodes for damage

author: wraper
author_rank: Unknown
timestamp: January 04, 2018, 11:43:25 am
content: Quote from: MWisBest on January 04, 2018, 02:29:22 amSlow blow means it will maybe blow in say 100 milliseconds instead of 10 milliseconds with the same overcurrent. You certainly won't be able to unplug in time if it's doomed to blow.And why it should blow in the first place? The only part of TDA7294 that is exposed to outside is output which would be very hard to kill by ESD. Also it appears you powered it afterwards, if output stage is damaged, it's almost guaranteed that fuses will go out and/or IC will blow up. On the other hand there is a preamp which is much more vulnerable to ESD. And it's exposed to ESD via inputs and potenciometrs if their cases are not grounded. You could disconnect the input of TDA by removing capacitor on the input or by some other way and see if the noise still persists.

author: EHT
author_rank: Frequent Contributor
timestamp: January 04, 2018, 10:32:20 pm
content: Hi. Not sure what you mean by "ESD shock" here. You touched it while it was assembled and powered on and got a shock from it? I would think there would be a short from the line or a power rail to whatever you touched.Anyway, check the AC voltage from the PSU transformer and then the DC voltages going to the amp (if using DMM also check on AC range to see ripple). If possible you can disconnect the amp from PSU when doing this. Careful of the live side of the transformer.Data sheet says this should +/- 50V:http://www.st.com/content/ccc/resource/technical/document/datasheet/ab/d6/fa/99/84/36/49/ee/CD00000017.pdf/files/CD00000017.pdf/jcr:content/translations/en.CD00000017.pdf