QuestionQUESTION: Hello,
I have an JVC MXGT90 bookshelf system that I am having some problems with. Whenever I play music (whether it be from cd, tape, radio, auxiliary input, etc.) I hear a popping sound through my speakers and then I hear no music and the sound level bars on the display stop moving, but the unit stays powered up. This problem happens about 90% of the time I use the system.
The only ways I've found to temporarily fix the problem is by either power cycling the unit, or switching to a different music source then going back to the original (I also sometimes have to adjust the volume knob to get it to start playing again when I switch sources). Even in doing those, the problem eventually reoccurs.
The unit has been in a closet for quite some time. I was thinking that maybe dust had collected on the heat sink (which is partially exposed on the back of the unit with a slotted plastic covering), causing the system to overheat and kick into some type of protection mode. I've looked through the manual and online and have yet to find a way to troubleshoot my specific problem. I figured I'd get your opinion before I start tearing it apart and possibly make matters worse. Thank you in advance.
Erik
ANSWER: ONE: Not good to keep in storage. Home electronic products should be fired up every couple of months at least; this will keep the capacitors and heat sensitive devices working good.
TWO: Dust is not that big of an enemy. From experience we learn that the cleaning causes more trouble than the dust. It is okey to clean it out but it must be done very, very carefully so as not to disturb something related with the operation of the units electronic circuitry.
THREE: Does the popping sound go away when the volume control is reduces to zero? Or does it stay on? That will help you chase down the problem.
FOUR: This problem could be related to bad/gassy capacitors in any stage of the amplification chain. Could also be tarnished or corroded switch contacts at the input selector switching system. Replacing suspect caps and cleaning the switch contacts will help to clear those up.
FIVE: Back to THREE: If the popping/clicking sound diminishes when volume is down it tells you the problem is electronically in the front half (ahead of) the volume control. If volume control does not affect the popping and clicking then it is related to the output stages such as the final power amplifier circuitry and/or the power supply (a gassy electrolytic in the ripple filter can shut down the DC line feeding the amplification system, for example.)
Hope this gets you to a solution.
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QUESTION: actually, theres only one or two single pops and then no sound. and if i (for example) am playing music through my auxiliary input and this occurs, i switch to cd then back to the auxiliary. most of the time i have to turn the volume up or down to get the music back, but this doesn't work unless i switch music sources or i can power cycle the unit.
this happens no matter source i use for music. cd, auxiliary, radio, tape, etc. hope this helps to narrow it down a bit and hopefully i can get some direction on what to do to fix the problem.
ANSWER: Yes, that helps.
It could be a blocking or coupling capacitor that has shorted out causing off bias on the following stage; when a signal comes along which sends a spike through the amplification chain the spike momentarily reverses bias and the amp opens up briefly, briefly enough to sound like a pop or click.
I suggest you check or replace all the ac coupling capacitors. Do you know how to do that?
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QUESTION: no, but my stepfather is good with electronics and a soldering gun. ill let him know and see if he can do something with it. just out of curiosity, is there a way to test the capacitors? and if i do indeed have to replace them, what kind would you recommend?
AnswerThey are best tested by replacing them! They are usually a few cents from Radio Shack and should be the closest equivalent to what is currently in the set. An electronics technician will know if one is polarized, mylar, or what type it maybe. In some of the more modern circuits they are surfaced mounted on a pc board.
But, checking the voltages on either side and comparing with what is supposed to be can tell if they are leaking dc through to the next stage and causing bias shift.
It may be difficult to tell if the set has a main pc board with all ICs. In which case it may require the purchase of the service manual (a good technician knows the way around these kinds of circuits and can usually guesstimate which caps would be signal carrying ones verses power supply or regulator types.
Experience is the key.
PS: There are capacitor testers but the cap must be pulled out of the circuit in order to properly test it; therefore, I think, you might as well just replace it since they are not terribly expensive.
