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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?
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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