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Post by gjaky on Jul 21, 2014 8:22:27 GMT -5
Have this subwoofer on service, the thing works generally, but there is some faint crakle coming from the speaker, regardless of input connection or volume setting. I've narrowed down the problem to the preamp/crossover board, I can see the noise on the output of this board with oscilloscope, without a schematic it is hard to find out the problem. I've redone quite much solder joints but without success. The construction looks like some generic chinese stuff, here is a picture off the board in question. The IC '62429' is unheard of me, and can't find any reference.  |
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Post by mastertech on Jul 21, 2014 9:50:26 GMT -5
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Post by gjaky on Jul 21, 2014 12:10:21 GMT -5
Thanks, I did not find it...
This is an active subwoofer with line and high level pass through, but only powers itself.
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Post by mastertech on Jul 21, 2014 13:13:24 GMT -5
I meant on that preamp board. I can not see if those connector pins are marked or not.
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Post by gjaky on Jul 21, 2014 23:25:51 GMT -5
There are four connectors on that board, only one is marked pin wise.
-three pins for AC input
-three pins for signal out (ground, signal, relay switch signal)
-six pins from the input terminal board (says: st, in, gnd, in+, in-, out)
-seven pins for the frontal control interface (IR reciever, 2*7 segment LED display, rotary volume control, switch for the crossing frequency selector), no idea on pinout.
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Post by mastertech on Jul 22, 2014 10:05:53 GMT -5
Best I can tell from that is only 1 channel going to the sub amp. Got a pic of the amp board?
Are you sure what you are seeing on the scope is the same crackle you hear from the speaker?
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Post by gjaky on Jul 22, 2014 11:19:02 GMT -5
Unfortunately no picture off the amp board, it is a bridged construction.
I am pretty sure I see the noise, unplugging the preamp board 8from the amp board) instantly makes the noise go away.
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Post by mastertech on Jul 22, 2014 12:17:33 GMT -5
Usually, disconnecting the preamp signal the amp stops switching and would not be a good indicator. But without seeing specific diagrams I could not say for sure.
If it were me I would be testing that preamp output with another receiver/speaker/probe setup, if you have read any of my posts on this in the past. Probing along until you hear the noise eliminates false signals.
Have you tried running the sub in a different location and different source input to make sure nothing in the room or on the power line is causing the crackle?
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Post by gjaky on Jul 23, 2014 11:48:11 GMT -5
Today I had a little time to look at this stuff again. 100% sure the problem is coming from the preamp section, the amp board with open input is dead silent, but taping my fingers makes it hum, so it's not muted. Also the scope "noise signal" is corresponding to the noise I hear, when no noise the signal is also clean.
I also sniffed around the volume control ic, thanks to the datasheet I could see that only one channel of the IC is used, the other side's pins are completely left open. And guess what, this noise is present both on the open input and output of this IC, while with the used channel, the input is kept on constant level, only the output is infected with this crackle, so I guess this must be the IC itself.
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Post by gjaky on Aug 13, 2014 2:23:25 GMT -5
So here is an update: the cause of the crackle wasn't the IC itself, but it's supply (a simple zenner shunt regulator made 5V from the 12V) replaced both the diode and the serial resistor, the crackle went away. Now I'm faced with an other problem which is also a weird one. If the input signal is larger than 250mV there is some odd noise modulating the signal's amplitude, sounds exactly like an unsmooth idle of an engine (if the input is sine). On a scope screen there is no sign of signal clipping, but can see the problem: again comes from the preamp board.
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