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Thread: Sunn Concert Bass wonky voltages

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    Sunn Concert Bass wonky voltages

    I bought a '74 silverface concert bass off a dude who had a video on youtube for $50. His video made me cringe, but I figured it'd be an easy fix.
    https://www.youtube.com/watch?v=op2bmb4XpTo&t=2424s

    According to the guy: "It has 45v going to the input board in the front of the unit instead of 25v, and some times this will result in receiving a shock through the guitar in to your hands. The cause is either a bad resister, or diode, but the guys at the shop swear it's a cap."

    So, when I got it, I checked the power transistors, they all checked fine. I noticed the volume pot isn't original, and they had a wire soldered from the case to c129. I changed all the electrolytic caps and took out the death cap circuit/polarity switch, replaced the bridge rectifier and most of the mylar caps, and a few resistors in the preamp that were out of spec (r126, r127). I swapped out Q101-103 with replacement since what was in the amp wasn't original, and they had 4 legs instead of 3 with leads from the 4th leg badly tacked on to c129. The amp works and sounds really good now, but when you stop playing and are unplugged, there's a static/thunderstorm/whooshing white noise. It doesn't get louder with any of the knobs, and you can't hear it when playing, only when you stop. It's there without an instrument plugged in. And there's still 45v instead of 25v going to the preamp board. Here are some voltage measurements:

    Here are some voltage measurements. -15v to preamp is good, ground between preamp and pa is good, but the +25v to preamp measures 45v.

    Q101 g = .6mv, d = 31v, s = .793;
    q102 g = 0mv, d = 17.34v, s = .795v;
    q103 g = 0mv, d = 17v, s = .850v.

    I'm suspecting the super high voltage at the preamp +25 rail is possibly responsible for the static crashing noise, but I'm kind of stumped about the best way to track down the culprit.

    Any tips or suggestions about were to look/what to do next?

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    Last edited by Gioanpj; 03-20-2020 at 03:39 AM.

  2. #2
    Supporting Member The Dude's Avatar
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    Yikes! The guy in that video has no business inside an amp!

    Uploading or linking a schematic would be a good start.

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    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Supporting Member loudthud's Avatar
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    The schematic is posted in this thread: https://music-electronics-forum.com/...ad.php?t=32812

    The layout for channel 1 preamp is in post 7 of this thread: https://music-electronics-forum.com/...ad.php?t=41401

    The two layouts I have (finally) uploaded successfully and should attach to this post. They say Concert Bass but they are the same as the Concert Lead. The preamp is channel 1.

    The way I read the schematic, the power supply goes into the preamp on Channel 1 pin 43 (+32V) and goes to the Reverb on pin 42 and the rest if channel 2 on pin 46 (+25V). The source for preamp Voltages is on the power amp board pin 30.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Sunn_Concert_Bass_Preamp_PCB_2.GIF 
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ID:	57472   Click image for larger version. 

Name:	Sunn_Concert_Bass_Pwramp_PCB_3.GIF 
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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    I have the schematic for the amp already. I checked some of the resistors from the power amp board out of circuit and several had drifted way past 10% tolerance and 3 or 4 were open/shorted. So, I decided I'm just going to rebuild the power supply from the ground up and go from there.

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    Lifetime Member Enzo's Avatar
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    I have the schematic for the amp already.
    yes, but the rest of us didn't. Always good form to post a schematic for any discussion you start.

    Isolate the problem. Do you have the power amp in jack? Plug a shorting plug into that to ground the power amp input. Does the noise remain?

    A handy shorting plug is your guitar. Plug into the power amp in jack and dial the guitar down to zero.

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    Education is what you're left with after you have forgotten what you have learned.

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