bozallen Posted July 25, 2012 Share Posted July 25, 2012 http://s.ytimg.com/yt/img/pixel-vfl3z5WfW.gifAfternoon All, I hope someone can point me in the right direction with this one. I've got a problem with this Soundcraft Spirit Powerstation 600. The left channel when played through speakers is distorted. The left and right audio sounds clean using headphones. Every channel produces the same left amplified distortion. Keeping my input source mono, the right channel is amplified cleanly. I've made a sample video which is available on Youtube here.. http://www.youtube.com/watch?v=B-GmvOJh2JY The video was made using the same source / speaker / speaker cable / recorder / recording position / recording gain. I can solder and am happy to change components if required, but I don't know a lot about audio internals and wouldn't know where to start on this one. Any ideas?? Link to comment Share on other sites More sharing options...
boatman Posted July 25, 2012 Share Posted July 25, 2012 Unfortunately, unless you're very lucky, it's a job for someone who can use a meter and oscilloscope. Last time I fixed a Power Station 600 output amp fault it was the preset pot in the bias chain which was open circuit, but it wasn't easy to find. Getting to the power amps is quite an achievement, even before you try to find the fault. If you contact Soundcraft you can get the Power Station technical manual in pdf form. I think there's a nominal charge, but there's no way to fix it without the schematics. Link to comment Share on other sites More sharing options...
bozallen Posted July 26, 2012 Author Share Posted July 26, 2012 Thanks for your reply boatman. I've got a multimeter and my work has an oscilloscope and signal generator, so I may be in luck! I've contacted Soundcraft for the service manual, but no reply yet. If anyone has a copy please PM me http://www.blue-room.org.uk/public/style_emoticons/default/wink.gif I've opened it up, to see if anything obvious is wrong and spotted a couple of things on the left amp side. The diode circled, top left, seems to have some corrosion on it's leg. The resistor, bottom right, looks to have a bit of heat damage but still measures the same as it's counterpart on the right channel. http://i576.photobucket.com/albums/ss201/bozallen/26072012276.jpg Also this transistors metal heat transfer plate behind it seems to have slipped. Is the multimeter test method mentioned here http://www.kpsec.fre...ran.htm#testing suitable? http://i576.photobucket.com/albums/ss201/bozallen/26072012277.jpg Are there any more obvious things I should test before obtaining the service manual? Many thanks for any light people may be able to shine on this! Link to comment Share on other sites More sharing options...
boatman Posted July 26, 2012 Share Posted July 26, 2012 From its location, that looks like diode D1 (1N4148) which is in series with an identical diode D2. They provide the base bias for TR1 the constant current source for the input transistor pair. Taking great care (the power rails on this beast are +/-50VDC), measure across D1 & D2 when the amp is powered up and you should see approximately 1.3V if they are both good. Compare that with the voltage across D201 & D202 the same components in the right channel. Don't worry about the flaking coating on the power resistor, if it measures OK then it probably is, however if it has been getting hot then the solder joint on that component leg may be suspect. I would certainly be concerned about the slipped metal plate as that could short out the link which runs underneath it. Link to comment Share on other sites More sharing options...
bozallen Posted July 27, 2012 Author Share Posted July 27, 2012 Yes, it is D1. Measuring between D1 / D2 gives 1.3v as does D201 / D202. It turns out the plate behind that transistor isn't metal, but appears to be some kind of material. It was touching against LK96 anyhow so I've fished it out and re-seated it the same as the others. FR6 does measure fine (68ohms, same as FR206) even when measured between the collector of TR12 and R48 (I assume this rules out bad solder joints on the legs of FR6?) Can you suggest my next step? Should I drive the amp with a signal and trace it through with the oscilloscope, or work through testing the components as per the service manual values? Link to comment Share on other sites More sharing options...
Brian Posted July 27, 2012 Share Posted July 27, 2012 Should I drive the amp with a signal and trace it through with the oscilloscope,...Yes but I'd start with a DC voltmeter. Since you have a good channel to play with do a side-by-side comparison of DC values with your meter. You can then move on to doing so with a scope and the same signal fed into both channels but there will come a point where the presence of negative feedback will start to blur the picture. If the output is distorted then you'll get a distorted signal fed back to the input via the NFB path which in turn will distort the signal being fed forwards. Link to comment Share on other sites More sharing options...
bozallen Posted July 27, 2012 Author Share Posted July 27, 2012 Thanks Brian. I'll get on that this evening. This is going to sound very much like a beginner (sorry) Should I use a common ground such as the leg of R1 and then measure the DC through the amp chain as Brian says, comparing the good amp chain with the bad? I don't want to be the cause of additional component damage! Also, I've noticed the following difference between the left and right circuits. The left channel has this physically smaller resistor for FR2 which measures 68ohms. (Just right of the words 'Stereo Power') http://i576.photobucket.com/albums/ss201/bozallen/27072012281.jpg However the right channel has a much larger resistor, also measuring 68ohms. http://i576.photobucket.com/albums/ss201/bozallen/27072012279.jpg Could this cause a problem? I assume the resistor on the right channel is larger in size for a reason. Link to comment Share on other sites More sharing options...
Brian Posted July 27, 2012 Share Posted July 27, 2012 Any resistor labelled FR is a Fusible Resistor ie one designed to burn out in the event of a fault. The one on the right channel looks like a previous repair using the wrong component because that isn't a fusible resistor. FR2 (and FR202 on the right) is at the top of the bias chain with FR3 at the bottom. It might be worth checking the other FRs with the amp off. FR1 should be 300RFR3 47RFR4 10RFR5 68RFR6 68R Link to comment Share on other sites More sharing options...
bozallen Posted July 27, 2012 Author Share Posted July 27, 2012 Mine measure this...FR1 297FR2 68FR3 46FR4 10FR5 67FR6 68 FR201 290FR202 67FR203 47FR204 10FR205 68FR206 67 Which looks fine to me! Link to comment Share on other sites More sharing options...
Brian Posted July 27, 2012 Share Posted July 27, 2012 Looks good so far. Once you hook it up to a scope a useful indication of where the problem might lie will be what the output looks like. ie is the output clipped on the +ve part, the -ve part or both. Link to comment Share on other sites More sharing options...
bozallen Posted July 27, 2012 Author Share Posted July 27, 2012 I've got a pretty interesting update! Hope you guys know what to make of it... So I thought I'd start with the real basic oscilloscope tests. I put a 600Hz sine wave through the amps and got it on the scope from the leg of R122. Which gave me this on both amp channels. So a nice clean signal coming in... http://i576.photobucket.com/albums/ss201/bozallen/LInp600Hz.jpg Then I put the probe on the speaker output of the left channel which gave this. Oh dear. The +ve bit of the wave looks chopped off! http://i576.photobucket.com/albums/ss201/bozallen/LSp600Hz.jpg And the right speaker output, perfect! http://i576.photobucket.com/albums/ss201/bozallen/RSp600Hz.jpg Finally I touched the probe between D1 and D2. The left speaker instantly became clear! Putting the probe back on the left speaker output now gives me this... http://i576.photobucket.com/albums/ss201/bozallen/LSpFinal600Hz.jpg What should I make of it? Dodgy diode?? Link to comment Share on other sites More sharing options...
Brian Posted July 27, 2012 Share Posted July 27, 2012 Could be as simple as a dry joint. Link to comment Share on other sites More sharing options...
timsabre Posted July 27, 2012 Share Posted July 27, 2012 Poking component legs with a bic pen or something non-conducting is a good way to find solder joint faults. Link to comment Share on other sites More sharing options...
bozallen Posted July 27, 2012 Author Share Posted July 27, 2012 Great! I'll remove the suspect diode, clean it up and re-attach it tomorrow. Are the components just surface mounted on the top or do they go all the way through and need de-soldering from the rear too? Link to comment Share on other sites More sharing options...
boatman Posted July 28, 2012 Share Posted July 28, 2012 Great! I'll remove the suspect diode, clean it up and re-attach it tomorrow. Are the components just surface mounted on the top or do they go all the way through and need de-soldering from the rear too? They are all through-hole components on that board. To make a proper job of it, you'll have to remove the board and de-solder from the track side. If the diode is suspect just replace it with a new one don't waste your time trying to re-use a component which costs less than 10P. Link to comment Share on other sites More sharing options...
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