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From: Kevin McMurtrie
Subject: Re: Cracks and pops on audio Op-Amp
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Sun, 08 Dec 2002 09:14:22 GMT
NNTP-Posting-Date: Sun, 08 Dec 2002 01:14:22 PST
In article ,
firstname.lastname@example.org (Adrian Black) wrote:
>My friend recently gave me a 4-channel Kenwood car amplifier with one
>channel not working. It uses a decent design with a switching power
>supply, two NEC uPC4574C quad bipolar op amps, a uPC4570C dual op amp
>and 4 LM391's to run the output transistors. After looking at the PDF
>of the quad op amp, it seem to be similar to the LM837N. Low noise,
>high slew, etc.
>Testing with my scope show three channels work well but one channels
>is filled with noise. Not humming, but crackling and poping.
>After poking around with my scope sending a 1khz signal through it, I
>find that the noise appears at the output of the first op-amp the
>channel hits. The signal then goes to "Line-Out" connectors on the
>unit and back into the same IC to a second op-amp.
>But I'm at a loss to what could be causing the noise. I desoldered the
>connection to the "line-out" thinking it was somethng downstream
>causing the noise, but that had no effect.
>Could the chip itself be bad? Or would it more likely be one of the
>resitors, diodes or capacitors? All of those visually seem ok. I
>resoldered all of their connections to make sure no cold joints were
>I thought the power supply might be noisy (+ and - 15v) but all three
>other channels including the other "half" of this particular IC work
>perfectly. I desoldered the +15v feed into the chip and when looking
>at the power, it was totally clean and steady.
>When I removed power from the chip, the noise went away. Somehow audio
>was going all the way through the amplifier and conncting a speaker
>confirmed a nice steady tone. Coneection the power again brought the
>gain up and all the noise right back. The noise happens wether a
>signal is present or not.
>Looking at the input into the inverting input on the op-amps shows an
>oscillator...? (it's what is looks like on thte scope and all the
>other stages look the same.) The invering input is connected to the
>output through a resistor as well as the non inverting input through a
>cap and ground through a resistor and cap. It also looks like the
>other channel going through the amp has the same capacitors, diode and
>resistor as the bad channel.
>Anyway, if you guys have any ideas for what I should try, I'de love to
>hear them. I was thinking of ordering a LM837N and installing it as I
>can't seem to find the NEC part. I just wonder if it's possible the
>noise could be coming from the chip itself ...?
It sounds like moisture creeped into the op-amp. Sometimes such a chip
will magically work again for a few days if you hold a soldering iron to
it for a few seconds. Then it goes all random again.
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