SA-7100 Ripple Current measurements-effects on protection board?

Discussion in 'Pioneer Audio' started by Gobstopper, Nov 3, 2018.

  1. sregor

    sregor AK Subscriber Subscriber

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    Thinking out side the box - turn off any LED or fluorescent lamps in the room - there has been a few cases where the problem was external. Just a thought....
     

     

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  2. Gobstopper

    Gobstopper Active Member

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    210
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    Back from break. Shorting plugs inserted into connections above above had no effect. I also checked the resistance between chassis and power plug pins. Both pins registered as infinite on an autoranging DMM. I did notice that the quality of the buzz changes when I engage the Tone Defeat switch, and is also slightly reduced when I engaged the -20db muting switch. Finally turning off overhead CFL had no effect.
     
  3. Gobstopper

    Gobstopper Active Member

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    210
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    Oakland, CA
    I didn't expect it to, but plugging the shorting plugs into the main amp inputs didn't help.
     
  4. Gobstopper

    Gobstopper Active Member

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    210
    Location:
    Oakland, CA
    Had some free time this evening, so thought I would double check the pre-outs and main-ins using another unit (Unit B). SA-7100 Pre-out into B's Main-in produced a buzz in left channel, I switched the L and R connections into B to double check, and buzz followed the left channel-out from 7100. The other way around(Unit B Pre-outs into 7100 Main-in), I could hear the signal from Unit B (AM radio in this case), but there was a 120 hz humm/buzz underneath it. The buzz from the control amp is much harsher sounding.
    I then tried to go back to my poor-man's oscilloscope or audio signal trace. I connected the grounds of both units, and then put a DC blocking cap in the probe path. I had done this before, but for some reason tonight, regardless of where I put the probe (chassis, or any pins ), I kept getting a buzz, not sure why. In one sense this might be a validation of my "2 problem theory", in that the main amp has a 120hz issue, and the control amp, or upstream, has something going on with the left channel (and possibly something related to the 120hz issue on main amp). I did try turning off lights to double check that they weren't causing the problem.
    Any suggestions out there?
     
  5. Gobstopper

    Gobstopper Active Member

    Messages:
    210
    Location:
    Oakland, CA
    Realized that shorting plugs were responsible for finding buzzing everywhere when using audio probe last time. Today removed all shorting plugs and no buzz when touched chassis or grounds. I poked around the control amp. The gate on the FETs (Q1 and Q2) was quiet, but both sink and drain had the buzzing. Disconnecting the jumpers, I tested inputs to main amp: quiet, but output also had buzz. Perhaps I'll see if any particular set of shorting plugs sends buzz to chassis.
     
  6. Gobstopper

    Gobstopper Active Member

    Messages:
    210
    Location:
    Oakland, CA
    And the testing continues:
    Findings, (inputs and outputs refer to audio signal)
    Main Amp
    1)with pre/main jumpers connected, there is no buzz at inputs to power amp, despite buzzing at outputs of control amp.
    2)If I enable Tone Defeat, I get some buzzing on left channel input, but not as strong as control output. No effect on right channel input
    3) buzzing at +B and -B pins

    Control Amp
    1) no buzzing at +B and -B pins
    2) buzzing at output pins, and all those related to the tone controls

    Equalizer Amp
    1)Buzzing on outputs, not on inputs
    2) Base of Q1 and Q2 had an odd, almost heartbeat sound. Maybe 2 sets per second, with each set having two beats close together.
    3) very faint buzzing at +B pin

    Finally, not able to recreate chassis buzz. I had all 6 shorting plugs in when it happened, but using one plug and trying the different inputs didn't do anything
     

     

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  7. Gobstopper

    Gobstopper Active Member

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    210
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    Happy 2019 everyone! Hopefully this will be the year I fix this unit! I had a quick question. Using an audio probe, would one expect a buzzing on the outputs of the full-wave bridge rectifier? MTF made a comment earlier on about the possibility one of the snubbing caps being shot, but I don't know if that would explain the buzz I hear at the pins ( 6 and 7 ) that correspond to the output of the bridge rectifier (these go to main amp board). The pins from the other rectifier are different: 12 and 13 are quiet, but 11 has a faint buzz that is lower pitched than that at 6 and 7 (more of 60hz hum I would guess). Is there a way I can check that the transformer isn't the cause?
     
  8. Gobstopper

    Gobstopper Active Member

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    210
    Location:
    Oakland, CA
    Woah! I moved the grounding wire that connects to pin 10 on the main amp away from some neighboring wires (see attached pic), and noticed that it greatly reduced the hum/buzz on the power amp when I disconnected the jumpers. Left it in place, that is to say away from other wires, screwed the bottom back on and tried the unit out. Plugged my phone into the Aux 2 input, and with the audio probe started testing inputs and outputs of the different amps. Bad news: with Tone Defeat Turned off, no signal reaches the main amp. Great news: with Tone Defeat engaged, I get perfectly clear output!? At first just tried output pins of main board, but plugged my headphones in and Voila! Buzz is gone. Not sure why, or if it's indicative of a more serious problems, but I'll take it. Now to figure out what is up with the Tone Defeat Switch. Still can't believe it......
     

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  9. Gobstopper

    Gobstopper Active Member

    Messages:
    210
    Location:
    Oakland, CA
    Alas, my joy was short lived.. Last night I tested it again after writing, buzz was there, but then went away on its own. Today I fixed up the tone defeat switch, all kinds of gunk on the inner part was not letting the signal go through. Reassembled, and buzz is back, but at least signal is making it all the way through this time. Much louder when tone-defeat is not engaged, and also dampened with the muting switch, so somewhere after flat amp I would guess, will have to bust out the audio probe again. Odd thing is when touch the outer parts of the pre-main jumped jacks, or the on-off switch with my hands, buzz gets a bit softer, also if touch the power switch. Moving the back grounding wire also changes the intensity. So at least I've had a few windows of no buzz!
     
  10. Gobstopper

    Gobstopper Active Member

    Messages:
    210
    Location:
    Oakland, CA
    SO CLOSE! PLEASE, PLEASE! I hope someone can help me on this last bit. It turns out that if I touch the exposed metal end of capacitor C11 on the power supply board and the chassis with same hand, the buzzing goes away completely! Capacitors C2, C4 and C5 had a similar effect but wouldn't kill the buzz completely, just reduced it. None of other capacitors on that board displayed a similar behavior. I tried touching the side of C11 instead of exposed metal end, or pushing on the exposed metal end with a plastic tool instead of my finger, and neither had an effect. I think this would argue against a bad solder joint, but not convinced of my deductive skills anymore. I lifted the board off the plastic mounts to look at solder joints, and in this configuration, touching the exposed metal ends of either C2, C4, C5 or C11 reduced buzz, but didn't eliminate it completely. Same with touching Pin 14 (ground for C2,4,5, and 11). This most likely indicates a grounding problem, but not sure what else to do. I truly hope someone can point me in the right direction, I feel like I almost there!

    edit: added a jumper between Pin 14 and Chassis. Buzz reduced even with board not on plastic mounts, but now "just" touching the exposed metal end of the capacitors listed above gets rid of buzz, don't have to touch hand to chassis as well
     
    Last edited: Jan 14, 2019 at 12:52 AM
  11. Gobstopper

    Gobstopper Active Member

    Messages:
    210
    Location:
    Oakland, CA
    hmm. More testing, and unecessary action. I tried jumping negative pins of either C11 or C2 to chassis and that removed buzz. As I mentioned yesterday, jumping Pin 14 to chassis reduced buzz, but did not kill. Jumping the negative leads of either the 4 caps to Pin 14 or to each other didn't help. Thinking it might be a bad grounding wire, I removed the wire-wrapped grounding wire between Pin 14 and grounding pin( on a soldered and screwed on grounding mount) and replaced with new wire(no wire wrap tool so attempted to solder). Of course this didn't help, and probably made things a bit worse. Still get a reduction in buzz when jump Pin 14 to chassis, and negative capacitor leads to chassis now ALMOST gets rid of buzz. I'll try cleaning off the grounding mounts tomorrow and see if helps, seems that something is bad at the grounding mount. ARGGHHH
     

     

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