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  • High Voltage Safety Manual.

    Found this document from the University of Colorado (Posted on the University of Tennessee website ) while looking at some data on capacitor discharging and wanted to share!

    High Voltage Safety Manual.pdf


    An interesting safety annecdote: I'm pretty diligent about discharging the HV caps when performing any service on an amplifier. Years ago, I built a discharge tool out of a set of heavy duty insulated clip leads I found in a box of TV repair equipment. It's been a reliable tool for years, and about a week and a half ago I went to use it like I normally do and happened to test the DC voltage on the caps to make sure that the voltage is bled off. I have to admit, I don't always do that. This particular time, however, I did, and read a high voltage still present in the caps! Sometimes it's a tight fit, so I thought the clips hadn't made good contact and I reattached the discharging leads. But I monitored the voltage on my DM, and the cap was not discharging. The current limiting resistor, had gone open with no visual indication of any heat damage or anything.
    I'm glad I took the extra 10-15 seconds and read the DC voltage. May have saved my life this this time. Definitely saved me further injury by indicating that a device I depend on for safety had become faulty and needed to be replaced or repaired.

    Here is a redundant copy of the above doc as well:
    High_Voltage_Safety_Manual.pdf
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  • #2
    Good reading, thanks!

    I repaired an amp a little while ago and found that if I removed the ground reference for the main PCB (I had to remove it as it connected to the leg of a big filter can-cap, and that's what I was replacing) it gave 300v potential on the guitar strings compared to the chassis, the whole circuit floated up to B+ basically. I measured the current and it was 1mA. Although I made the best connection I possibly could when I put it together it still makes me worry that one wire coming loose could cause that in this amp. Seeing the table of lethal currents makes me relax a little! Good to get a recap with real-world experiences of best practice too...

    Comment


    • #3
      Originally posted by OwenM View Post
      Good reading, thanks!

      I repaired an amp a little while ago and found that if I removed the ground reference for the main PCB (I had to remove it as it connected to the leg of a big filter can-cap, and that's what I was replacing) it gave 300v potential on the guitar strings compared to the chassis, the whole circuit floated up to B+ basically. I measured the current and it was 1mA. Although I made the best connection I possibly could when I put it together it still makes me worry that one wire coming loose could cause that in this amp. Seeing the table of lethal currents makes me relax a little! Good to get a recap with real-world experiences of best practice too...
      Doesn’t make me feel any better. Take a typical Fender amp, B+ around 460VDC; if you you bridge that voltage to ground with and part of your body, and if your body resistance is anywhere near... lets say 5k. You now have 92mA conducting through you. For a 200lb man, that’s well within the range of seizing your muscle control and causing them to clamp. Your stuck.
      That circuit is more than capable of delivering that current (and more), and still well out of the reach of that fuse blowing.
      If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

      Comment


      • #4
        Originally posted by SoulFetish View Post
        Found this document from the University of Colorado (Posted on the University of Tennessee website ) while looking at some data on capacitor discharging and wanted to share!

        High Voltage Safety Manual.pdf


        An interesting safety annecdote: I'm pretty diligent about discharging the HV caps when performing any service on an amplifier. Years ago, I built a discharge tool out of a set of heavy duty insulated clip leads I found in a box of TV repair equipment. It's been a reliable tool for years, and about a week and a half ago I went to use it like I normally do and happened to test the DC voltage on the caps to make sure that the voltage is bled off. I have to admit, I don't always do that. This particular time, however, I did, and read a high voltage still present in the caps! Sometimes it's a tight fit, so I thought the clips hadn't made good contact and I reattached the discharging leads. But I monitored the voltage on my DM, and the cap was not discharging. The current limiting resistor, had gone open with no visual indication of any heat damage or anything.
        I'm glad I took the extra 10-15 seconds and read the DC voltage. May have saved my life this this time. Definitely saved me further injury by indicating that a device I depend on for safety had become faulty and needed to be replaced or repaired.

        Here is a redundant copy of the above doc as well:
        [ATTACH]55852[/ATTACH]
        I drain caps through an old analog multi meter. I can set it on VDC and watch as the meter drops to 0
        Vote like your future depends on it.

        Comment


        • #5
          I wonder who were the subjects to gather the data on shock currents? Arts students perhaps?
          Experience is something you get, just after you really needed it.

          Comment


          • #6
            Originally posted by nickb View Post
            I wonder who were the subjects to gather the data on shock currents? Arts students perhaps?
            Lol

            Well, one could hope
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

            Comment


            • #7
              Cue that scene from Ghostbusters!

              Had a bad ground in amp I was building and also had voltage on input jack rise up to a couple hundred and Sting me.

              I got accustomed to jerking my hand away from an amp when feeling the slightest tingle.

              I usually end up smacking it on something even after just static

              Comment


              • #8
                Originally posted by nickb View Post
                I wonder who were the subjects to gather the data on shock currents? Arts students perhaps?
                They compiled data during a psych experiment
                Vote like your future depends on it.

                Comment


                • #9
                  Originally posted by SoulFetish View Post
                  Doesn’t make me feel any better. Take a typical Fender amp, B+ around 460VDC; if you you bridge that voltage to ground with and part of your body, and if your body resistance is anywhere near... lets say 5k. You now have 92mA conducting through you. For a 200lb man, that’s well within the range of seizing your muscle control and causing them to clamp. Your stuck.
                  That circuit is more than capable of delivering that current (and more), and still well out of the reach of that fuse blowing.
                  Yeah.. I know what you mean...

                  With the amp I was working on, a Laney VC-30 for the record, I got some advice and was told it was a high voltage but incapable of giving out much current, so I shouldn't worry to much. I still DID worry, but what are we to do? It remains that this amp only has one wire between it and that potential on the strings. I wrapped it up real good and made sure it was as good a solid solder as I have ever made but still makes me uneasy...

                  Comment


                  • #10
                    Originally posted by nickb View Post
                    I wonder who were the subjects to gather the data on shock currents? Arts students perhaps?
                    Ah haaa ha...
                    I see. Its always open season on the art kids, til you need ‘em to “photoshop” the blackface costume out of that picture from a Halloween 30 years ago.


                    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                    Comment


                    • #11
                      I rebuilt my discharge leads. Decided to “beef“ ‘em up a little for the stable release of version 2.0
                      Cant have going open on me again.
                      Click image for larger version

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                      Overkill?
                      If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                      Comment


                      • #12
                        That looks good. Something like that, I like to clip one end to the metal shaft of a nice insulated screwdriver. I hate trying to clip to something while it's sparking.
                        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                        Comment


                        • #13
                          Originally posted by SoulFetish View Post
                          I rebuilt my discharge leads. Decided to beef em up a little for the stable release of version 2.0
                          Cant have going open on me again.
                          [ATTACH=CONFIG]55994[/ATTACH]
                          Overkill?
                          I'm just getting started in repairing amps and came across this post. In looking to build a cap discharge tool, I've seen that some use lightbulbs in the circuit. Perhaps that would help avoid the above case where a resistor failed open, as you'd see right away that the bulb wasn't lit? thoughts?

                          Comment


                          • #14
                            Originally posted by jy999 View Post

                            I'm just getting started in repairing amps and came across this post. In looking to build a cap discharge tool, I've seen that some use lightbulbs in the circuit. Perhaps that would help avoid the above case where a resistor failed open, as you'd see right away that the bulb wasn't lit? thoughts?
                            The light bulb limiter would only fail to light if there was an open circuit in the AC power supply inside the amplifier chassis. I've never seen this happen, but I'm not a repair tech. It's main purpose is to keep things from popping as they might under full current AND as a diagnostic tool because you can read some symptoms based on the brightness of the bulb (I won't cover it here though. Some searching here will reveal best practice and use.)
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                            Comment


                            • #15
                              On another note relative to this older thread...I don't have a "discharge tool". I just use a straight alligator clip lead connected to the plate end of a preamp tube and put the standby switch in the play position. It takes a little longer through a 100k resistor than it might otherwise, but I'm not usually in that much of a hurry. I just hook it up and let it do it's thing while I'm doing something else like firing up other bench gear and heating the iron, etc. No special unitask tool required. ALWAYS TEST VOLTAGE ON THE CAPS AFTER DISCHARGING BEFORE PROCEEDING!!!
                              Last edited by Chuck H; 09-03-2020, 01:22 AM.
                              "Take two placebos, works twice as well." Enzo

                              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                              Comment

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