Cause of weak Preamp channel

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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.

 
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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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.

 
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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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.
 
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' ?

 
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Here's what I would do... I take it you haven't a "real" scope and generator.

Get yourself a CD with some test tones on it preferably lower than 0db and in the 100 to 400hz range.

Play the tone and turn the volume up until you read around 1 vac across the line outputs terminals. Do that for both channels and post the results here. eg ch1 1.02 volts and ch2 0.89 volts. Bass and treble centred.

Reason for 100 to 400hz... any meter will measure that frequency range accurately. Reason for tracks lower than ODB... you would be turning the volume only a few degrees to get full output.

Your squarewave shots show bass a little accentuated (bass boost).

There are some tracks here for you to use, post #2
http://www.diyaudio.com/forums/mult...-power-do-your-speakers-need.html#post2869380

Use the 220hz one.

Lets see what that shows.
 
Found the issue. The volume pot. I had no idea it could be adjusted. I have it centered now for the most part. Is there such thing as a volume pot that is balanced? I did replace all the caps today with higher grade.
 

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Pleased you have fixed although I don't quite understand what you mean, "The volume pot. I had no idea it could be adjusted"

It looks like an ALPS motorised pot... very good quality.

For a scope to begin with you want an analogue type (that means with a traditional CRT display) of at least 20MHz bandwidth. Dual trace can useful too as you get more experienced.
 
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