Noise in one channel? Pioneer SX-400 - diyAudio
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Old 12th May 2011, 01:23 PM   #1
mfaughn is offline mfaughn  United States
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Default Noise in one channel? Pioneer SX-400

Hi all,

I'm sure there are about a zillion sources of how to troubleshoot this. I did a couple of forum searches and googled a bit but didn't quite come up with what I need.

I am working on a Pioneer SX-400 / Allied 935. I am getting distortion on one channel. I need to run sweep tones to make sure but at this point I would say that the louder you crank it the worse it is and is most noticeable in LF. I have replaced all the electrolytic caps except those on the heater supply and am working my way through the coupling caps. I have rolled tubes. I put a jumper across the tone controls in order to eliminate that as a source.

At this point my plan is to continue replacing caps until they are all replaced and then start on resistors in the signal path and then on to other resistors. If anyone has any advice on something else to do or where I might focus my efforts I'm all ears -- or a good url.

I am getting an oscilloscope next week. (It will be my first.) Might that be of help? (note: posting another thread about how to use it).
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Old 12th May 2011, 03:18 PM   #2
DF96 is offline DF96  England
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It would help if you identified where the noise is arising, rather than replacing the whole innards of the amp. A scope might help, although low levels of noise might not be visible. You could try temporarily shorting each grid to ground (apart from any not supposed to be at ground potential). If the noise stops then it arises in that stage grid circuit or an earlier point. If the noise continues then it comes from a later point, which may be in the same stage. In order to fix a fault, first find it!
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Old 12th May 2011, 10:37 PM   #3
mfaughn is offline mfaughn  United States
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Default Ooops. SA400 not SX

SX is solid state. This is a 6BM8 PP. It sounds quite good when it is working and I would really like to get this worked out.

Anyway, here's the schematic. I'm 99.9% sure that the problem is arising after it gets into the 6BM8 section. I've been measuring resistances between the various nodes in the vicinity, comparing the good channel to the distorted one and haven't found any discrepancies yet. As far as I can tell the noise/distortion is not separate from the audio signal but rather the signal is getting distorted. The level is also lower in the bad channel. So, it is both distorted and lower in volume. Any chance this is OPT? Can they fail in such a way as to produce results like this?
Attached Files
File Type: pdf SA-400.pdf (126.9 KB, 35 views)
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Old 13th May 2011, 10:22 AM   #4
DF96 is offline DF96  England
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A shorted turn in the OPT might cause this.

Do you have a means of measuring inductance? Some DMM can do this. Measure the primary inductance of each OPT, with no power applied. Either from anode to anode, or anode to HT. Compare the two channels. If the faulty channel has significantly lower inductance then this confirms a shorted turn.
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Old 13th May 2011, 11:31 AM   #5
mfaughn is offline mfaughn  United States
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I have an LCR. I'll give that a shot next.
Thanks.
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Old 14th May 2011, 03:10 AM   #6
mfaughn is offline mfaughn  United States
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Poop. That was it. OPT shorted. I happen to have two of these amps and the other one wasn't working either. swapped in an OPT from the donor and things are fine. Pulled OPT measures ~ 0.25H across the primaries -- the other three closer to 14H. Thanks for the help.
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Old 14th May 2011, 09:09 AM   #7
DF96 is offline DF96  England
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OK.
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Old 14th May 2011, 10:56 AM   #8
mfaughn is offline mfaughn  United States
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Thanks for the help DF96.
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