Audiovox AV-350

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JdOwNj
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Location: Garland, Maine
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Audiovox AV-350

Post by JdOwNj »

This Is a really fun one... I am helping a friend fix his audiovox car
radio/cassette. The syptoms of the prob were that it would come on
normally, front panel would light up, would tune, display proper time,
but absolutely nothing at the speaker outputs. Under normal
circumstances I would have expected a blown transistor from somebody
fooling with speakers, etc, but this system was professionally
installed, and when replced with an almost identical unit, the
speakers work normally, so a short in the speaker system seems to be
ruled out. Opening the cover I found a smoked resistor, R440, seems
to be a 1/2 watt green(wire wound????) resistor. It is impossible to
read color codes from it. I dont have a schematic available, so I was
wondering if anybody knew where to get one, or at least the specs of
that resistor. Also something caused the resistor to fail... it
failed nice. It discolored the board right through, small crator in
the componant side. Any input on what might have caused that to fail?
Thanks very much in advance!
analogee
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Location: Aurora, OR
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Re: Audiovox AV-350

Post by analogee »

This is not necessarily a very likely scenario, but since I've experienced something like this myself, I'll tell you.<p>Many power amplifiers have what's called a snubber network on the output. It is a capacitor (something like 0.01 uF) in series with a power resistor, from the output of the amp to ground.<p>I've seen a power amplifier oscillate (generally, in my opinion, due to poor design) with such amplitude, and high enough frequency (the capacitor pretty much blocks any significant current at audio frequencies), so that the power resistor burns out. In the case I'm familiar with, the oscillation was caused by one side of the speaker being disconnected, but all the wiring to the speaker still being connected to the other output wire. This load was non-standard enough (I'm guessing capacitive) to cause at least one amplifier oscillate so badly that it burned out the snubber resistor.<p>Anyway, the solution was to replace the resistor, and make sure the speakers were connected properly before powering up the amplifier. By the way, if you can verify that the resistor is a snubber (trace it out on the PCB to see if it is connected as I described above), the value is typically 1 Ohm or 1.5 Ohm, and shouldn't be ultra critical. I'd just try a 1.5 Ohm 1/2 W resistor in case you don't know the value of the original. Or, if you can find the part number of the power amplifier (in case it is an IC), look up the manufacturer's recommended snubber network in the datasheet.<p>Regards,
Todd
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JdOwNj
Posts: 95
Joined: Thu Dec 06, 2001 1:01 am
Location: Garland, Maine
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Re: Audiovox AV-350

Post by JdOwNj »

Thanks for the reply... is a carbon film resistor fair game at least as a temp replacement?
analogee
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Location: Aurora, OR
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Re: Audiovox AV-350

Post by analogee »

I don't think carbon film would hurt, circuit-wise, if you want to give it a try temporarily. Past experience probably leads you to flame-proof as a long term solution!<p>Regards,
Todd
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Wir werden wissen.
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