Gerry Posted June 28, 2017 Share Posted June 28, 2017 Just bought 2 The Box PA108a active monitors off a well known auction site. Described as:-"For parts or not working, Both HF drivers don't work" Description correct as the HF drivers didn't work. Bought 2 new ones from Thomann for €14.80 each plus €20 postage. Put them in and powered up the speakers, no input, bass and treble controls at zero and volume set low. Audible hiss from HF drivers for a few seconds then a pop and silence. Multimeter confirmed that both HF drivers had blown. Any ideas? CheersGerry Link to comment Share on other sites More sharing options...
Jivemaster Posted June 28, 2017 Share Posted June 28, 2017 Crossover failure? Wrong drivers fitted before and refitted by you? Possible internal HF feedback causing ultrasonic extreme overdrive. Link to comment Share on other sites More sharing options...
Gerry Posted June 28, 2017 Author Share Posted June 28, 2017 Crossover failure? Wrong drivers fitted before and refitted by you? Possible internal HF feedback causing ultrasonic extreme overdrive. The correct drivers where fitted by me.How can I check the crossover?CheersGerry Link to comment Share on other sites More sharing options...
Jivemaster Posted June 28, 2017 Share Posted June 28, 2017 If it's simply a crossover then replace it, If it's an active crossover on the main board ???? Link to comment Share on other sites More sharing options...
timsabre Posted June 28, 2017 Share Posted June 28, 2017 Sounds more like there could be dc coming out of the amp module. Or hf oscillation. Almost certainly a problem with the amp module in either case. Link to comment Share on other sites More sharing options...
KevinE Posted June 28, 2017 Share Posted June 28, 2017 HF probably handled by an LM3886 or similar all-in-one. Sometimes a little soot is visible at the legs. No DC protection on powered speakers at this price point! Link to comment Share on other sites More sharing options...
Gerry Posted June 29, 2017 Author Share Posted June 29, 2017 Just measured the voltage at the speaker terminals.39V DC :angry: CheersGerry Link to comment Share on other sites More sharing options...
timsabre Posted June 29, 2017 Share Posted June 29, 2017 Do the LF drivers work? it might not be a difficult fix if so. As Kevin says the HF drivers are probably powered by their own little power amp chip. If you can identify this chip then it may not be too hard to swap it. Link to comment Share on other sites More sharing options...
Gerry Posted June 29, 2017 Author Share Posted June 29, 2017 According to the Thomann blurb the PA108a is an "Active 2-Way Bi-Amped Fullrange Box"so, presumably, it should have a separate amplifier for the HF part.The LF works OK.I need to open it up to see what is inside the cabinet.CheersGerry Link to comment Share on other sites More sharing options...
timsabre Posted June 29, 2017 Share Posted June 29, 2017 Take a pic and post it here and we can do some amp IC spotting Link to comment Share on other sites More sharing options...
Gerry Posted June 30, 2017 Author Share Posted June 30, 2017 Yes there are two LM3886 chips inside.The LF chip is OK but the HF chip is giving 40v at the output so not surprising the HF driver is blowing.The chips have 40- 0 40 as their power supply.I know I can just replace the chip, looks like it's already been replaced but the different colour to the solder, but what is causing the LM3886 chip to blow?CheersGerry Link to comment Share on other sites More sharing options...
Gerry Posted July 3, 2017 Author Share Posted July 3, 2017 Sorry to be a nuisance but has anybody any idea what might have caused the LM3886 chip to be giving 30V at the output?CheersGerry Link to comment Share on other sites More sharing options...
timsabre Posted July 3, 2017 Share Posted July 3, 2017 Basically one of the transistors in the output stage of the chip has gone short circuit. Common fault with power amps (integrated chips or otherwise), many amps have dc protection circuits to detect this fault and disconnect the speakers. Does it look damaged, or like it's been hot? Without history of the unit it's hard to say, someone may have blown the hf drivers then botched the repair, or blowing the drivers may have damaged the chip too. Link to comment Share on other sites More sharing options...
Gerry Posted July 4, 2017 Author Share Posted July 4, 2017 I've ordered 2 new LM3886 chips. Will install them and check the voltage at the output before buying 2 new HF diaphragms and installing them.Most of the actors and audience, in the am dram group I am with here in Spain, are retired people.I know I can't hear anything above 10Khz so will it make a big difference if the HF side of the powered speakers doesn't work?CheersGerry Link to comment Share on other sites More sharing options...
alistermorton Posted July 4, 2017 Share Posted July 4, 2017 Basically one of the transistors in the output stage of the chip has gone short circuit. Common fault with power amps (integrated chips or otherwise), many amps have dc protection circuits to detect this fault and disconnect the speakers. Does it look damaged, or like it's been hot? Without history of the unit it's hard to say, someone may have blown the hf drivers then botched the repair, or blowing the drivers may have damaged the chip too. Maybe something on the input side of the LM3886 has failed and is pulling the input hard to the rail, thus pulling the output hard to the rail? Link to comment Share on other sites More sharing options...
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