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Audio Amp Problem - Distortion Left Channel

 
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Gemo



Joined: 08 Aug 2007
Posts: 3

PostPosted: Wed Aug 08, 2007 3:49 pm    Post subject: Audio Amp Problem - Distortion Left Channel Reply with quote

Have an old Pioneer SX-424 stereo and the left audio channel has a problem that has me stumped!

When the unit first turns on the audio is fine - but after about 10-15 secs the left channel audio begins to distort badly (ie. very clipped, scratchy and much reduced volume). The right channel is working perfectly - as does the rest of the radio (AM, FM...)

If you turn the stereo off for 20-30 secs and then back on, the left audio works OK for about 3-5 secs and then distorts. The distortion isn't gradual - it's like something gets to a certain voltage then the distortion 'switches' in...

At first I thought it must be heat related but I've tried chilling all the left amp components with cool spray which has no effect/improvement in the distortion - and the symptoms (ie how the distortion 'cuts' in) don't totally sound like a heat problem.

Since the right audio continues to work fine it has to be something related to only the left side. The power supply voltage (40VDC) is OK and DC voltages on the left channel output transistors seem OK and comparable to the rt audio.

Hope someone can provide some devine guidance.

P.S. Wish I had a schematic... but I understand quasi-complimentary amps fairly well...
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Thu Aug 09, 2007 10:58 am    Post subject: Reply with quote

Try supplying a 1 kHz sine signal from a tone generator to an AUX input and look at the output signal by oscilloscope. See what kind of distortion it is (i.e crossover dist, peak clipping, HF oscillations etc). Turn the volume and tone controls back and forth to see if the distortion changes shape, this can tell whether the problem is in the power amp or in any stage before the controls.

It might also be interesting to check out the waveforms at other places, i.e at the volume control, at the power amp input etc.

Also, check the stabilized supply voltages for the preamplifier stages.
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Gemo



Joined: 08 Aug 2007
Posts: 3

PostPosted: Thu Aug 09, 2007 1:49 pm    Post subject: Reply with quote

Thanks for the very good suggestions torbjorn.

Wish I could scope out some waveforms but my old scope has given up the ghost so I'm trying to troubleshoot this unit the very "old fashioned" way!

Good thought on checking the voltages for the preamps - which I'll do tonight. Would have guessed that any preamp voltage supply w/b common to both channels, but on second thought maybe each channel has a sepearte regulated supply to minimize noise/hum, ground loops etc.

I did notice that when measuring the audio AC voltage (on an analog VOM) that the AC voltage swings on the bad channel (measured at the amp side of the ouput capacitor) are very weak compared to the same point on the good channel - suggestiung I trust, that the distortion may be coming from earlier in the circuit (eg preamp...?)

I'm also going to have a listen to the preamp output audio (ie. from the "tape" jack) and see what the audio quality is at that point.

Tomorrow I'm going to get a schematic for this unit - so I'll have a better idea.

Thanks again for your commets - welcome any other ideas you might have...

Regards
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Gemo



Joined: 08 Aug 2007
Posts: 3

PostPosted: Tue Aug 14, 2007 6:53 pm    Post subject: Reply with quote

Sorry for the delay - have been traveling out of town for a week.

Think I've fixed my problem. Determined this weekend that the distortion in the left channel was in fact caused by a faulty "Tape Monitor" switch.

I guess the left channel contacts in this switch were just flakey enough to severely distort the audio. Wiggling the switch a bit would make the problem come and go - so cleaning it has solved the problem.
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