Receiver output testing

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  #1  
Old 04-27-19, 06:14 AM
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Receiver output testing

Any audio repair technicians in the audience? I just purchased a factory refurbished Marantz SR5012 receiver and am getting some discrepancies when I measure the output voltage and DC resistance of the left, right, and center channels. What led me down this path was a speaker that wasn't playing as loudly (although I believe the actual speaker is at fault). I took some measurements using a digital multi-meter (I realize this isn't the appropriate tool for this type of thing, but I was taking some simple comparative measurements) and found there to be over 100mV difference between left and right channels with music playing. However, with test tones playing, the voltages were the same. I then took resistance measurements of each channel (receiver on but no music) and got 28 ohms for left, 6 ohms for center, and negative 2 ohms for right. I realize that I'm not going to get accurate resistance readings with the power on, but I at least expected the values to be the same. Is there some other test I can do? I have an old analog oscilloscope and could measure the amplitude of some test tones across the spectrum and see if they match.
 

Last edited by mossman; 04-27-19 at 06:37 AM.
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Old 04-27-19, 08:39 AM
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There should be very near 0vDC on all outputs. DC will fry a speaker. You can measure the audio output on the AC scale. You'd see the value change wildly with music. A few different tones would be needed for a conclusive test. You can't measure resistance with the amp on and even with the amp off it wouldn't tell you much.
 
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Old 04-27-19, 10:12 AM
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There are setup disks and I'm sure you can download them as well. You can then pipe tones or white noise through your system. With some consistent tones or white noise you can probably detect differing speaker output easier than by listening to the wild variations in music.

I don't know your receiver. Does it have a calibration feature? If so, since it was refurbished, I would do a factory reset or manually zero out any corrections that might be in memory from the previous owner. Then you could run the auto calibrate (if the receiver has it) and see if it can balance out the audio level for you.

Before getting my current receiver that has a built in calibration feature I cheated and used the calibration feature of my subwoofer. I'd play the test tones through each speaker and read the audio value from the sub's calibration screen and manually adjust each output from the receiver.
 
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Old 04-27-19, 10:30 AM
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Like Pete said resistance measurements are not very meanigful. AC measurements can be useful but you should check the spec sheet of the meter for the maximum frequency capability. Many less expensive models are limited to 1 KHz or less for accurate measurements. Unless you have a single tone frequency, a true RMS meter is needed for accurate measurements. Because of sampling effects DC measurements may not be accurate with an AC signal present especially if it's random like music. Also, some amplifiers may have a nonpolarized capacitor in series at the output so you might be some observing DC voltage stored on such a capacitor.

An oscilloscope would provide good comparative measurements of test tones, as long as you're not expecting 0.1 dB accuracy. For audio levels 1 dB accuracy is more than adequate.

Have you exchanged speakers between the different channels to see if the lower output stays on the same channel or the same speaker?
 

Last edited by engr3000; 04-27-19 at 11:24 AM.
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Old 04-27-19, 10:40 PM
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I realize a multimeter isn't the appropriate tool to measure audio equipment, but I found it odd that I got a stable 28.5 ohms on the left channel, a steady 6 ohms on the center, and a fluctuating negative 2 ohms on the right. Accurate or not, shouldn't I at least be getting similar measurements between the left and right channels?

I did switch the speakers around and the speaker that wasnt playing as loudly didn't play any louder on the other channel, which tells me the speaker is an issue. However, I'm still trying to determine why the output voltage and resistance are different.

I already did a factory reset, and have the receiver set to multi-channel stereo.

I had my multi-meter set to Vac and used the Max feature to measure the maximum amplitude, both while playing the same soundtrack and when playing the test signals (pink noise). Curiously, the music resulted in the 100mV+ delta between left/right channels, but I got the same exact voltage when playing the pink noise test tones.

I have a test tone disc that I used to tune my car stereo, so I could play that while observing the signals on the oscilloscope. If I don't see any discrepancies between the left/right channels, I guess I'll just have to ignore the odd difference in resistance readings.

By the way, I measured the DC resistance of the left, center, and right channels of my old receiver and they were nearly the same.
 
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Old 04-28-19, 09:01 AM
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It's kind of hard to explain but an ohmmeter runs a small DC current thru the test subject and measures the voltage drop. If you try to check for resistance on a circuit that has DC voltage on it..... the measurement will be incorrect or skewed.
 
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Old 04-28-19, 12:25 PM
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Right, I realize that, which is why I said I don't expect the results to he accurate, but did expect them to be consistent and they aren't even close.
 
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Old 04-28-19, 12:45 PM
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It could be something as simple as the bias set in the output stages.The bias basically sets the match between the PNP and NPN output transistors. Just being off by a few miilivolts will change your ohms reading. A slightly leaky cap can do the same thing.
 
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Old 04-28-19, 06:48 PM
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Yeah, makes sense. I guess I won't worry about it as long as the signal amplitude is good.
 
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Old 04-29-19, 05:45 AM
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Do not use the ohms (or resistance or continuity) function of a meter on a circuit or system that is powered on..
 
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Old 04-29-19, 09:18 AM
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Do not use the ohms (or resistance or continuity) function of a meter on a circuit or system that is powered on..
Yeah, I know you're not supposed to do that, but if I don't, the output relays will not be energized and I will measure open circuit. No audio was playing during the measurements, so I figured it would be okay.

I downloaded the service manual and as expected, the amplifier circuitry for the left, right, and center channels is identical, so I'm not sure why I am getting such different readings.

Here's a screenshot of the final output stage passive components for L/R/C channels. Not sure how I'm getting a 28.5 ohm or even a 6 ohm reading. The negative reading is more understandable because I must be charging that capacitor.

https://www.mediafire.com/view/50o5l...krOut.png/file
 
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