Noob Question - Will a Bad Cap Fry a Resistor? - diyAudio
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Old 6th January 2012, 03:46 PM   #1
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Default Noob Question - Will a Bad Cap Fry a Resistor?

I have a little experience in electronics repair and want to improve my skill and knowledge. I recently acquired at no cost a JVC RX-715VTN receiver that I would like to put it into service. I found a service manual online and have learned the basics about the unit.

When I first powered it up, one of the larger resistors on the amplifier board started smoking. And there are four other large resistors that are burned, two of which are near the smoking resistor. There are two large 10000MF capacitors on the amp board, one of which has a slightly convex top, and it appears some crusty stuff seeped out of it onto the board. The transistors appear ok - no obvious burns or meltdowns - but I have not checked them with a multimeter.

Hence my first noob question. Will a bad capacitor cause resistors to fry, or should I be looking for other failed components along with the capacitors?

Second noob question (that perhaps should have been my first question). Does anyone know of a good basic online amplifier troubleshooting primer for a consumer level receiver such as this? My primary interest is solid state consumer, audiophile , and pro sound reinforcement amps. I've got an old Marantz receiver with a bad channel that I want to tackle next.
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Old 6th January 2012, 03:58 PM   #2
yajnaS is offline yajnaS  India
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A capacitor that failed short-circuit can smoke anything in series with it. Electrolytic capacitors do not usually fail short.

You should probably put the repairs off for a later date and build some small circuits instead so you get a hang of troubleshooting while you're dealing with simple circuits that you understand.
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Old 6th January 2012, 04:46 PM   #3
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Start by checking the large power transistors. Those resistors are emitter resistors (the emitter is one of the three legs of a transistor). A very common failure is for the transistor to develop a short between the emitter and collector which causes big current to flow through the emitter resistor and it then cooks.

Pull the transistor and use an ohmmeter. Check for resistance between the emitter and collector. Since this is a diode junction (the current flows one way only), you should make two measurements between the emitter and collector, reversing the leads from the ohmmeter. A healthy transistor will have a low reading one way and a high reading the other. A shorted transistor will have a low reading both ways.
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Old 6th January 2012, 05:04 PM   #4
DF96 is offline DF96  England
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The large capacitor with the bulge and ooze will need replacing. Whether it is the culprit or an innocent victim remains to be seen. One possibility is an output stage fault, which fried the resistors and also caused excessive ripple current so overheating the capacitor.
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