Onkyo Integrated Amp With Incredibly Low Volume

Discussion in 'Electronics Repair' started by KilgoreCemetery, Aug 12, 2017.

  1. KilgoreCemetery

    KilgoreCemetery

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    I have an Onkyo A-5 integrated home stereo amp that I was using up until about a week ago when the volume dropped to almost nothing. I feel I need to stress the "almost" part. With the volume knob maxed out you can hear music playing, but it's whisper quiet and seems to affect both channels equally. I've included a couple pictures and the schematic. Hope it helps. Here's what I've tried so far:

    • Different speakers
    • Different outputs (Speaker set A, B, A+B)
    • Different input types (like Phono vs Tape)
    • Turning the "Muting" button on and off
    • Turning the "Mode" button on and off (Stereo vs Mono)
    • Turning the "Subsonic" button on and off
    • Turning the "Loudness" button on and off
    • Turning the Bass and Treble buttons on and off
    • Adjusting the Balance knob seems to work as normal despite the low volume
    • Checked speaker fuses
    • Checked main fuse
    • Jumpers between Pre-Out and Main-In are solid metal and only fit one way. Tried a good RCA cable just in case though
    • I haven't found any sign of bad caps or burnt components
    • Transformer is outputting +40VDC and -40VDC
    • Diode check on main power transistors (Q001 - Q004) was good
    • A bunch of other things that may or may not be important

    I'm still learning how to read schematics and no more than a novice when it comes to circuit board repair. However, I have found a resistor (R702) that gets super hot within a minute or so of being powered on. It's in the "Transient Killer" portion of the schematic, but I think it has more to do with handling power fluctuations than murdering drifters. I removed and tested the resistor. It's a 1.2k ohm res that measures 1.186k ohms. As for the capacitors around it, unfortunately my multimeter isn't able verify that high of capacitance. C706 is 330uF, 50V. C705 and C707 are both 220uF, 50V. This also holds true for the main caps C002 and C003, which are 13000uF, 50V.

    At this point I'm probably going to order replacement capacitors regardless, but for the moment I'm not going to let this set back stop me. I may not be able to tell the capacitance, but I do know that the voltage is still (relatively) where it should be throughout the circuit board.

    For instance, the Tone OpAmps (Q301, Q302) are reading similar to what is on the schematic, but I don't think the notations on the schematic are 100% accurate for this part. It looks like they were hand written and I have no idea if it was done at the factory or just someone in their basement. The OpAmps used on this board are TA7136AP's (datasheet included).

    For what it's worth:
    Q301
    1) 15.8V
    2) -111mV
    3) -113mV
    4) -17.4V
    5) -15.8V
    6) 118mV
    7) 16.7V

    Q302
    1) 15.8V
    2) -73mV
    3) -73mV
    4) -17.4V
    5) -15.9V
    6) 81.7mV
    7) 16.7V


    I've also found a pair of JBT's with no output voltage close to the Tone OpAmps, but pulling them and doing a diode test showed they are fine. They are NPN transistors (Q303, Q304), 2SC1636, They both show -6.2V on the Base, but zero volts on the Collector and Emitter. These connect to the "Subsonic" switch so I don't know if this is how they're supposed to function or not.

    Everything else that I've tested has seemed completely normal to the best of my knowledge. I lifted one leg of the diodes D701 - D708 in the "Transient Killer" section and they were all in the 0.5V range one way and 0.L the other. The close-by transistor, Q701, measured voltages similar to the schematic of 33.28V (33.1V), -5.79 (-4.9), -6.38 (-5.5). I feel like I've basically checked every component in this part of the circuit board and I'm running out of ideas.

    Help me, electronicspoint. You're my only hope!
     

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    KilgoreCemetery, Aug 12, 2017
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  2. KilgoreCemetery

    kellys_eye

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    Have you checked the source signal is ok? No short on the input?
     
    kellys_eye, Aug 12, 2017
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  3. KilgoreCemetery

    dorke

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    "Help me, electronicspoint. You're my only hope!"
    No need to be so dramatic.;)

    This is were a scope will be very helpful 'but you probably don't have one.
    How about building a simple audio probe?
    Like this guy does.
     
    dorke, Aug 12, 2017
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    KilgoreCemetery likes this.
  4. KilgoreCemetery

    KilgoreCemetery

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    I've tried running the source signal (in this case, my laptop) through a different amp, but didn't have any way to test it throughout the Onkyo.

    Just watched the videos, dorke. Gonna see if I have a capacitor that would do the trick.
     
    KilgoreCemetery, Aug 12, 2017
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  5. KilgoreCemetery

    KilgoreCemetery

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    Thanks again, dorke (I intend that to be way nicer than it sounds). I went ahead and build a gadget for inspecting and started tracing the input signal. Seems it doesn't make it past the selector switch. I'm about to do a little disassembly to determine why
     
    KilgoreCemetery, Aug 13, 2017
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  6. KilgoreCemetery

    KilgoreCemetery

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    Turns out the selector pin in the back of the switch wiggled loose. Turning the knob wasn't actually switching between anything despite the nice clicks of the detents.
     

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    KilgoreCemetery, Aug 13, 2017
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