DrV Posted July 4, 2017 Share Posted July 4, 2017 You should check that the local decoupling capacitors are still there and securely soldered. These would be a couple of ceramic caps soldered very near the supply pins of the LM3886. If those are missing/faulty you may well get oscillation at ultrasonic or RF which you won't know about until something gets hot or goes pop. Ideally you should stick a scope on the output with a dummy load connected before exposing your new tweeters to the experience of becoming a medium wave transmitting aerial! Dave Link to comment Share on other sites More sharing options...
timsabre Posted July 4, 2017 Share Posted July 4, 2017 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? Could be. A bit of poking with a voltmeter once you have the chip removed would tell you that. I think you'd really notice the missing HF if you ran without it... everything would sound like an AM radio. Link to comment Share on other sites More sharing options...
i_hate_fisicks Posted July 7, 2017 Share Posted July 7, 2017 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? Yes, it is important. As you lose your top end hearing it's all the more important for the clarity to be there in the first place. The crossover point will be fairly low, so will sound very bad without the HFs. Link to comment Share on other sites More sharing options...
Gerry Posted July 10, 2017 Author Share Posted July 10, 2017 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?Had time today to do a bit of dismantling/ testing.With the chip on got 30V at the input, pins 9 and 10 :angry: So now wonder got 30V at the output pins to the HF speaker.Removed the chip and just Mv at the input pins :D :D So looks like a new chip might cure the problem.Should recieve new chips, by post, in a day or two.CheersGerry Link to comment Share on other sites More sharing options...
Gerry Posted July 15, 2017 Author Share Posted July 15, 2017 Replaced the LM3886 and the decoupling capacitors.Just a few Mv at the output now.Connected a speaker from an old PC monitor set and can now hear a 10kHz signal PK so the fix has worked.Took the HF drivers apart, a strong smell of burnt varnish. Needed to prise the diaphragm out as the burnt varnish was holding it in place.The gap, into which the diaphragm coil fits, needs a good clean out. Used folded over masking tape but will that shift any burnt on varnish?CheersGerry Link to comment Share on other sites More sharing options...
timsabre Posted July 15, 2017 Share Posted July 15, 2017 Maybe a strip of emery paper, possibly soaked in white spirit if necessary Link to comment Share on other sites More sharing options...
Gerry Posted July 15, 2017 Author Share Posted July 15, 2017 Maybe a strip of emery paper, possibly soaked in white spirit if necessaryI wondered about that but I thought it was important not to scratch the metal surface?CheersGerry Link to comment Share on other sites More sharing options...
Gerry Posted July 17, 2017 Author Share Posted July 17, 2017 New diaphragms fitted and all working OK. :D :D Thanks for your help/ advice.CheersGerry Link to comment Share on other sites More sharing options...
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