zeron Posted January 16, 2017 Share Posted January 16, 2017 (edited) Hello, I've had an SW-12 since 1993/1994. It had been working very well until about two weeks ago. About two weeks ago, it started making two kinds of noises after it gets warmed up for a while, like 6 - 35 minutes. One kind of noise is static-like and the other kind is what I'd call crackling kind (could be the sound of clipping). These noises come on whether any source is connected or not. Until these noises come up, the subwoofer appears to work as good as before. Once these noises start, none of the controls (LEVEL, PHASE, and LOW PASS) has any aural effects on the noises as far as I can tell. I've plugged the power cord of the unit to many different wall outlets in my 2-story house and also with or without my computers, cable modem, router, etc. ON, and the problem shows up anywhere. So, I'm leaning towards ruling out things like ground loop or interference from Wii-fi, etc. as the cause of the problem. Even after the noises show up, the source material keeps getting reproduced as usual. I suspect the problem has something to do with overheating of the amplifier because they don't show up immediately after I turn the unit on. Also, I found one area of the top plate of the plastic cover that covers the amplifier has a sign of getting soft before, but I have no idea when it happened. I have connected the speaker leads inside to a regular speaker and the noises came on, so I'm ruling out the problem lies with the subwoofer's speaker unit. I have cleaned the pots for the controls fairly well, too, so they're probably not causing this problem either. I'm now concentrating on the amplifier. As a starter, I downloaded a service manual of SW-12 and SW-15. It's here: http://www.audiolabga.com/pdf/SW12-15 I.pdf Pages 15-17 are the schematic of SW-12. The top portion of Page 16 (Sheet 2 of the schematic) is the part I'm concentrating on [labeled DRIVER CKT filter driver bd.] now because the location of the cluster of 4 transistors (Q7, 8, 9, and 10) coincides with the previously softened area on the amplifier's plastic cover and also the area on the circuit board these four transistors lie on is not burned badly but dark brown. (Each of these transistors has its own little heatsink, so they are probably known to get hot, but maybe with my unit, they may be getting too hot to affect the performance of other components in the amplifier.) Now, I'm completely clueless when it comes to electronics. I'm hoping that someone here can help me with questions I have about the schematic and electronics in general. Tonight, I removed the amplifier and measured about a dozen resistors' values with the unit turned off without any source signal being fed to the amplifer. Since my multimeter can't test capacitors, I'm not going to mess with capacitors now. Obviously, I don't want to remove any components from the circuit board to take their measurements if I can avoid. As far as I could tell, there were no apparently damaged components or suspicious solder joints. Four resistors caught my attention, though: One is R45 1M. It's first resistor on the DRIVER CKT (on the left on the schematic). When I take a reading, the value is only about 109K. Here is my first set of questions: 1. Do I need to remove at least one end of R45 to get an accurate reading? 2. If it's faulty and I install a good one, would it make any noticeable difference in the circuit? Two other resistors which give different values than the specs' are R74 and R73. Their values should be 2.7K ohms according to the schematic and the parts list in the service manual. My meter reads about 1.42K ohms. Here's my second set of questions: 3. Do I need to remove at least one end of R74 and R73 to get their accurate readings? 4. If they're faulty and I install good ones, would they make any noticeable difference in the circuit? And here's my final question for tonight: 5. If you were tackling this problem, where would you start? Thank you in advance for reading this post and possibly offer some answers/advice/suggestions. Happy Listening! ------------------------------------------------------------- Additional comment: The main heatsink on the back of the amplifier never gets hot. It remain just mildly warm. On the other hand, the small heatsinks on the four transistors on DRIVER CKT get too hot to touch. Edited January 17, 2017 by zeron corrected errors due to dropped/extra words, etc.; also added "Additional comment" at the bottom. Quote Link to comment Share on other sites More sharing options...
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