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Old 28th January 2012, 12:03 AM   #1
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Default Cause of weak Preamp channel

What typically is the cause of a weak channel in a preamplifier? This is on a Rotel and it doesn't have a balance control. I have schematics.
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Old 28th January 2012, 05:24 AM   #2
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Quote:
Originally Posted by orbanator7577 View Post
What typically is the cause of a weak channel in a preamplifier? This is on a Rotel and it doesn't have a balance control. I have schematics.
Dirty contacts whether inside a pot or selector switches or the panel connectors. Track it down with a scope in minutes. Possible but less likely are components changing values. Resistor changes would usually maintain flat response (unless in a EQ network). Capacitor (usually electrolytic) often roll off low frequency response. The scope and square waves will pinpoint it pretty quickly.

G
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Old 28th January 2012, 10:03 PM   #3
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Hm..., what if the less lout channel is OK and the other increased its gain? E
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Old 28th January 2012, 11:57 PM   #4
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I posted this in the wrong area, but here is is.

This was my first time hooking up to an oscilloscope, but this is a screenshot of the output @ 50hz. What do I need to look at to determine the issue at hand? The Right channel definitely has more volume.
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File Type: jpg 50hz.JPG (91.5 KB, 75 views)
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Old 29th January 2012, 07:55 AM   #5
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Quote:
Originally Posted by orbanator7577 View Post
I posted this in the wrong area, but here is is.

This was my first time hooking up to an oscilloscope, but this is a screenshot of the output @ 50hz. What do I need to look at to determine the issue at hand? The Right channel definitely has more volume.
Yep. At 50 Hz one channel is definitely louder. Is the same true at 1 kHz? 10 kHz? Try your test again with a 400Hz square wave. If the shapes are the same between channels the odds are VERY high that the response is the same between channels and the gain is simply different. Then you'll want to see if there is a level tracking problem on the volume control or a faulty balance pot.

G
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Old 31st January 2012, 05:50 PM   #6
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I'm leaning towards the volume or tone control. Ill know for sure then I replace the rest of the caps. Unless one of you know better? I have the schematics if you want to take a look.

The pics are 400hz, 1khz and 10khz
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File Type: jpg 400hz square.JPG (73.5 KB, 62 views)
File Type: jpg square 1khz.JPG (54.3 KB, 58 views)
File Type: jpg square 10000khz.JPG (92.7 KB, 57 views)
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Old 1st February 2012, 07:53 AM   #7
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Quote:
Originally Posted by orbanator7577 View Post
I'm leaning towards the volume or tone control. Ill know for sure then I replace the rest of the caps. Unless one of you know better? I have the schematics if you want to take a look.

The pics are 400hz, 1khz and 10khz
I think you're on the right path. That leading edge ringing is very odd though. Show the prints if you can.

G
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Old 1st February 2012, 01:06 PM   #8
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here ya go. hope this helps.
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File Type: jpg 2.JPG (226.8 KB, 25 views)
File Type: jpg 3.JPG (198.8 KB, 25 views)
File Type: jpg 4.JPG (168.6 KB, 22 views)
File Type: jpg 5.JPG (170.9 KB, 9 views)
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Old 4th February 2012, 11:39 AM   #9
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To be honest I feel rotel took three steps back when they changed from using AD827 opamps to 2604 or 2114, no wonder that step response look that bad. Cutting costs has its negative effects. I think its half way to oscillation and probably because of the 2114. From what I can remember theres a partial fix for this on the 2114 datasheet.
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Old 5th February 2012, 07:25 AM   #10
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Originally Posted by homemodder View Post
To be honest I feel rotel took three steps back when they changed from using AD827 opamps to 2604 or 2114, no wonder that step response look that bad. Cutting costs has its negative effects. I think its half way to oscillation and probably because of the 2114. From what I can remember theres a partial fix for this on the 2114 datasheet.
I don't believe the step change has anything to do with the specific opamp. You're talking 400 Hz and even an awful 741 can do that well.

The schematic does not 'read' well in that it shows the ICs as 'pictorials' so I have to look up the pinouts and translate back to standard form. It doesn't help that the text is unreadable - at least what I see.

Out of curiosity what is the signal source and 'scope' ?

G
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