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Kenwood 104 AR Stereo Receiver problem

 
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smith544



Joined: 25 Oct 2005
Posts: 2

                    
PostPosted: Tue Oct 25, 2005 4:46 pm    Post subject: Kenwood 104 AR Stereo Receiver problem Reply with quote

I have a kenwood receiver, 104 AR, that is no longer working. It was reported to zap my sister, so I brought it home to isolate the short. When I plugged it in, and turned it on, I found something quite different. The on/off switch is a relay, and somehow, a 240 hz (Or possibly 120 Hz, I counted 20 clicks in 5 seconds, I think that each click is a quick on off, but one click could be on, and one off) oscillation is occuring in the relay control circuit. I assume that it's a leaky cap or IC, but I'm more of a tube radio hobbyist and all these ICs and circuit boards are a little overwhelming. Does anybody have any ideas? Thanks
Scott
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smith544



Joined: 25 Oct 2005
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PostPosted: Fri Oct 28, 2005 9:48 am    Post subject: Reply with quote

I guess I figured this out, but I haven't bought the cap yet. So, with a 6o Hz wave ac input, rectifyed, that makes 120 positive peaks per minute. The filter caps, the really big 6800 uF ones, are charged when the voltage peaks, and discharge when the voltage drops off, so, if one of the caps is leaky, it wont maintain a constant power supply, and the relay will turn on, and then off, at every peak, and make 240 clicks per minute. If that cap had zero dc resistance when my sister first turned it on, then it would have shorted to ground, and shocked her, but the current supply going through it could have repaired the leak somewhat by reforming the oxidation, without restoring the capacitance up to the needed value. When I took the main board off, and discharged the two filter caps, only one of them had any electrons in it to speak of. The only problem with this is, on a fairly new system in normal use, how did the cap develop a leak. I hope the answer isn't reverse polarity, because I don't know why this would have happened. Does anybody have any ideas as to whether or not this sounds plausible? thanks
Scott
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underquark
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Joined: 20 Jun 2005
Posts: 874
Location: UK

                    
PostPosted: Fri Oct 28, 2005 10:36 am    Post subject: Reply with quote

Last time I checked, Hertz referred to cycles per second so a 60Hz AC supply (whether 120 or 240V) would give a low hum and not a series of countable clicks (unless you have grasshopper's ears). A full rectify would give 120/s or a slightly higher-pitched hum (well, one octave higher to be pedantic). Still sounds relay/cap-related, though, just think the Hertz thing is a red herring. I'm not sure that these big cap's can actually discharge as fast as 120Hz and are there to smooth things out.
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zaphod7501



Joined: 02 Aug 2004
Posts: 533
Location: Peoria Illinois

                    
PostPosted: Fri Oct 28, 2005 7:11 pm    Post subject: Reply with quote

First thing to do is disconnect the speaker wires from the back of the receiver and try it again briefly. (This may be irrelevant since you may have never connected any for your testing.) Do not reconnect them for additional testing. If a speaker or wiring shorts out the receiver can do different things, depending on the design.

Start with checking things that don't involve changing parts.

First of all, the large filter caps are probably not the problem. The reason one had little voltage could be for a couple of different reasons. First of all one is 35-100 positive volts while the other will be negative (meaning the neg. terminal will be the one with voltage) and you might have measured the wrong terminal and measured the voltage on ground. The positive voltage usually feeds additional circuits and they will draw the voltage down much much quicker than the other. Check for open fuses. Sometimes these are on the circuit boards, sometimes they are on the transformer terminal board -- on the bottom side. These fuses usually fail after an output shorts. Remember a fuse follows a failure it can not predict a failure. If an output device is shorted and you replace an open fuse without checking for shorted parts, you will do additional damage, sometimes catastrophic.


The rate of clicking almost sounds too slow for a filter failure however.

Normally, for a rapidly clicking power relay, I would say the culprit was the standby power supply filter. When it opens up the raw voltage sits on the relay causing it to "chatter". These are not usually high quality parts in critical circuits so failure is fairly common. You could trace the connection from the relay coil back to the filter and the rectifier diode (not the one accross the relay coil). Combined with a 'zap" however I would change the rectifier also since if it were to short it would have the same effect but replacing the cap would last about 1 minute and it could literally "blow". This circuit can be very simple or it can have several regulators in it.

Other scenarios:
There are no relays on the output IC's or transistors and the micro actually shuts the unit down rather than going into "protect". with the speakers disconnected you would need to measure the various speaker terminals with a DC meter. You are looking for a large DC voltage present for a fraction of a secong until it shuts off (5 volts or more, up to 100). If there are relays on the outputs, then you would need to measure the voltage on the input side of the speaker relay since it is directly connected to the output circuit.

It is possible that it didn't zap your sister, she zapped it (static electricity). Unfortunately this could kill the microprocessor and you are either going to be heading for major (possibly factory level) repair or replacement of the receiver..

In summary; first check for open (possibly hidden) fuses, then for DC voltage on the outputs. It's hard to guess further until some of these things are checked.
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Just call me Zaphod (or Steve) --- I never should have started using numbers in a screen name but I just can't stop now.
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