I have a Focal Alpha 80 monitor that kept blowing fuses. It looks like the culprit was a TDA7294 on the LF channel that exploded spectacularly. Presuming this was just someone putting too much gain for the amp IC to handle, I replaced it and the fuse. On turning it on without any output, the power LED sputtered for a moment and there was a distinct pop. I turned it off before the fuse blew again and had a look inside. The second TDA7294 had exploded even more spectacularly leaving a dumpster fire's worth of smoke in the unit.
I removed it and looked for other issues but could not find any. The transformer is outputting 28V AC and 16V AC (slightly more than the rated 25V and 14V but I can't see that that is an issue). Pins 7/13 are measuring 39V and 8/15 are -39V. Cutting it fine with the rated voltage of 40V for a TDA7294 but again, should work fine. The equivalent pins on the HF channel are +/- 20V. There are no apparent visual problems but I may be missing something -- if anyone suggests measurements worth taking, I would be happy to try them.
Was the first blown amp due to too much gain, and did I just get unlucky with my replacement TDA7294? Admittedly I got a no-name part off AliExpress as virtually no money is coming out of this repair, but I'm wondering if that was a false economy. I have now ordered one from ST but that will take a little time to get here. Any pointers appreciated.
Oh, and I'm ashamed of the state of the pins where the 7294 goes, but two explosions had pretty much destroyed any chance I had of desoldering it cleanly 🙃
I removed it and looked for other issues but could not find any. The transformer is outputting 28V AC and 16V AC (slightly more than the rated 25V and 14V but I can't see that that is an issue). Pins 7/13 are measuring 39V and 8/15 are -39V. Cutting it fine with the rated voltage of 40V for a TDA7294 but again, should work fine. The equivalent pins on the HF channel are +/- 20V. There are no apparent visual problems but I may be missing something -- if anyone suggests measurements worth taking, I would be happy to try them.
Was the first blown amp due to too much gain, and did I just get unlucky with my replacement TDA7294? Admittedly I got a no-name part off AliExpress as virtually no money is coming out of this repair, but I'm wondering if that was a false economy. I have now ordered one from ST but that will take a little time to get here. Any pointers appreciated.
Oh, and I'm ashamed of the state of the pins where the 7294 goes, but two explosions had pretty much destroyed any chance I had of desoldering it cleanly 🙃
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For one thing, some of your solder pads on both sides are damaged and need repair. That was likely do to rough removal of the IC.
I'd check the small caps around the IC. The TDA7294 is VERY sensitive to overvoltage on pins 7 and 8 in relation to pins 13 and 15 (see example circuit diagram below). Even if operating under the max voltage, the chip can't have more voltage on 7 and 8 at any given time than on pins 13 and 15.
It only takes a split second transient when the smaller filter caps around the IC are charging or discharging (or one of them is bad). Under no circumstances should pins 7 and 8 have more voltage than pins 13 and 15, even for a fraction of a millisecond ie. when the power is turned on/off and the caps are charging or dis-charging unevenly in relation to each other.
So my advice would be to remove and test all the small electrolytic caps around the IC. The solder pads need repairing as well. If those aren't reliable, you'll be wasting alot of ICs and further damage the PCB from excessive component swapping. You may want to invest in a cheap desildering tool that sucks the solder off the pads instead of desoldering wick. I always re-flow the old lead free solder first with 60/40 lead/tin solder before removing the IC. This helps with removal of the old solder.
There's also the obvious issue of counterfeit parts. Be sure to buy your ICs from a reputable source.
I'd check the small caps around the IC. The TDA7294 is VERY sensitive to overvoltage on pins 7 and 8 in relation to pins 13 and 15 (see example circuit diagram below). Even if operating under the max voltage, the chip can't have more voltage on 7 and 8 at any given time than on pins 13 and 15.
It only takes a split second transient when the smaller filter caps around the IC are charging or discharging (or one of them is bad). Under no circumstances should pins 7 and 8 have more voltage than pins 13 and 15, even for a fraction of a millisecond ie. when the power is turned on/off and the caps are charging or dis-charging unevenly in relation to each other.
So my advice would be to remove and test all the small electrolytic caps around the IC. The solder pads need repairing as well. If those aren't reliable, you'll be wasting alot of ICs and further damage the PCB from excessive component swapping. You may want to invest in a cheap desildering tool that sucks the solder off the pads instead of desoldering wick. I always re-flow the old lead free solder first with 60/40 lead/tin solder before removing the IC. This helps with removal of the old solder.
There's also the obvious issue of counterfeit parts. Be sure to buy your ICs from a reputable source.
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