This powered speaker is externally very similar to Thump TH-15A, but circuitry is completely different. Like the other repair I explained in a recent thread, this one has no sound at all, but I can hear some hiss that tells me that power amplifier is probably good. OL (overload) LED is always on.
Visual inspection OK. I detect +15V but not -15V. It comes directly from a ON Semiconductor linear regulator MC7915CT [U2]. +15V are provided by MC7815CT [U3]. Input correct (±20V). No shorts in consumer side. I replace it with a scrapped Fairchild KA7915. Now -15V OK, OL led is off but... still no sound!
I don't have schematics, but I detect 4 SMD diodes (1N4148W "A2") near input XLR/jack that protect input. Two of them (D6 and D7) aren't conducting in either direction. I replace them with axial classics, still no change in sound.
First OpAmp that has contact with these diodes is U16. Check input with scope, OK, but no output. Replace this Mackie custom M-80 DMP8 packed OpAmp with a scrapped NJM2068MD and now sounds OK!
Something was badly connected to input that blew protection diodes and first OpAmp. BTW, what is this M-80 clearly rebranded OpAmp? Mackie says in promo stuff that it "performs exceptionally well, offering low noise, high drive and class-leading channel separation", but provides no datasheet at all. Hope that nobody trying to repair any equipment that carries these "classy" OpAmps wastes his/her time finding exact M-80 replacements, instead of any low-noise NJR / JRC OpAmp in DMP8 packaging.
Visual inspection OK. I detect +15V but not -15V. It comes directly from a ON Semiconductor linear regulator MC7915CT [U2]. +15V are provided by MC7815CT [U3]. Input correct (±20V). No shorts in consumer side. I replace it with a scrapped Fairchild KA7915. Now -15V OK, OL led is off but... still no sound!
I don't have schematics, but I detect 4 SMD diodes (1N4148W "A2") near input XLR/jack that protect input. Two of them (D6 and D7) aren't conducting in either direction. I replace them with axial classics, still no change in sound.
First OpAmp that has contact with these diodes is U16. Check input with scope, OK, but no output. Replace this Mackie custom M-80 DMP8 packed OpAmp with a scrapped NJM2068MD and now sounds OK!
Something was badly connected to input that blew protection diodes and first OpAmp. BTW, what is this M-80 clearly rebranded OpAmp? Mackie says in promo stuff that it "performs exceptionally well, offering low noise, high drive and class-leading channel separation", but provides no datasheet at all. Hope that nobody trying to repair any equipment that carries these "classy" OpAmps wastes his/her time finding exact M-80 replacements, instead of any low-noise NJR / JRC OpAmp in DMP8 packaging.
Attached Files
what did we say about hoarding too much stuff and depriving others of them?! *cough* socialism *cough* lol! 
