You can measure the 220uH inductor in circuit but it's best to do so with a DC current of 600mA, alternatively if you have a surface mount re-work hot air gun, carefully remove Q3 the shunt FDC655 N Channel MOSFET transistor used to produce the negative 4.5 volt rail and see if the +4.5 volt rail comes up. The capacitors can go either short circuit or open circuit and in my experience they usully go open circuit which is even more catastophic if they are used in switched mode power supplies to filter out the peaks to produce an average voltage. The bad electrolytic capacitors are usually identified by bulging tops or even rupture due to the build up of hydrogen gas but this is not always the case. It is very difficult to remove the 220uH inductor without first removing the electrolytic capacitors surrounding it because of the amount of heat required due to the large mounting pads of the inductor and there is a major risk of accidentally de-soldering all the components mounted underneath the inductor on the other side of the printed circuit board.įirst of all you need to establish that the inductor is your problem though as it is rare to have a shorted turn these days, in other forums several other people have had problems with the SB0300 power supply due to bad electrolytic capacitors made in Taiwan between 1998 - 2005 ( ). The 220uH inductor therefore needs to be able to handle this current without saturating and the surface mount ferrite core used has an air gap to prevent core saturation. The DC current required for the SB0300 model is about 600mA which exceeds the USB 1/2 current limit of 500mA hence the need for an external power pack/supply. Many others have had internal power supply problems with this particualr model but another common problem is faulty electrolytic filter/storage capacitors on the internal +4.8VDC rail.Īn exatract of the circuit involved is attached as is the -5VDC generator and some photos of the unit. The inductor was left in situ as it no longer matters if it has a shorted turn, the P Channel MOSFET will essentiall be ON all the time and given the 240VAC mains power pack has a nominal ouput of +5VDC the switching regulator is almost superfluous. My solution was to make a little daughter circuit board with a -5VDC generator circuit so that the shunt N channel MOSFET could be removed. Other more astute readers may be able to get the 220uH inductor off the board, open it up and re-wind it with new wire. I could not open up the special surface mount ferrite core used for the inductor as it was cemented together and not being able to obtain a suitable replacement core I made a rather drastic modification as a "work around" for the problem. The circuit is robust in that if an overload occurs the LTC1771 will limit the peak current and the ON time duration and this is what was happening with my unit. ![]() The specialty switched mode power supply IC is a LTC1771 device which is essentially designed for "current mode" of operation with a second voltage feedback loop. The internal power supply uses a series and shunt P channel and N Channel MOSFET transistors respectively, to obtain a stablised +4.8VDC and -4.3VDC power rails. Once heated up it would continue to work correctly until powered OFF and left to cool down whereupon it would fail to start up again. The fault turned out to be a shorted turn on the internal switched mode power supply series 220uH inductor that would go away once it was heated gently. ![]() The fault symptoms are: The internal RED SPDIF optical digital audio output LED is lit but the power, CMSS and audio mute LEDS are all OFF and the unit is not working. Faulty Creative Sound Labs SB0300 24bit external USB sound device.
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