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Inaccessible filter caps - draining query

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  • Inaccessible filter caps - draining query

    Hi folks

    so how do you suggest I go about draining filter caps that are sitting on the pcb with no wires showing above, legs trimmed below and sealed all round?

    cheers

    steve

  • #2
    Thus Click image for larger version

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    • #3
      Do you have a schematic?
      If so, use it to identify accessible points/components where the caps connect to.
      - Own Opinions Only -

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      • #4
        Agree^^^

        Those caps connect somewhere. In a tube amp, the plate pins of the power tubes are convenient. In a SS amp, the power transistors have legs. Though with values like 220uf 350v, I'd expect it is a tuber.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Originally posted by Enzo View Post
          I'd expect it is a tuber.
          Finally we find ourselves face to face with the elusive 'rock n roll potato'.

          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #6
            Originally posted by Enzo View Post
            Agree^^^

            Those caps connect somewhere. In a tube amp, the plate pins of the power tubes are convenient. In a SS amp, the power transistors have legs. Though with values like 220uf 350v, I'd expect it is a tuber.
            Sometimes the first set of filters caps are located before the standby switch so this may not work in all cases. Best to see the schematics first.
            Experience is something you get, just after you really needed it.

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            • #7
              Got a Peavey on the bench with same situation.

              The big old choke resistor is right there for draining the caps.

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              • #8
                Click image for larger version

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ID:	921167 Here’s the relevant bit of the schematic. I am wondering if c142 is faulty. I get 0.00vdc at the junction with the IN 4007 diode it joins up with, with or without power.

                At the moment I am getting sound, but whisper quiet. Before I lash out on some new tubes, I am seeing if anything else looks wrong.

                best

                steve

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                • #9
                  Originally posted by drewl View Post

                  The big old choke resistor is right there for draining the caps.
                  What is a choke resistor?

                  - Own Opinions Only -

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                  • #10
                    Originally posted by Steve Blackdog View Post
                    ............ I am wondering if c142 is faulty. I get 0.00vdc at the junction with the IN 4007 diode it joins up with, with or without power......
                    What is the resistance from that point to ground?

                    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                    • #11
                      Originally posted by Helmholtz View Post

                      What is a choke resistor?
                      R200 on your schematic, the big white power resistor in your picture.
                      for smoothing out high voltage ripple, some amps use a transformer "choke" other amps use a big resistor.
                      Check the resistance of R200, 203 and 204.
                      Check the rectifier diodes and if any filter caps are shorted.
                      And check continuity between all those parts and their solder connections.

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                      • #12
                        Originally posted by Steve Blackdog View Post
                        I am wondering if c142 is faulty. I get 0.00vdc at the junction with the IN 4007 diode it joins up with, with or without power.
                        steve
                        So you mean zero Volt at "HT PS"? If so, you have no PI and preamp supply. Check if R203 is open. Do the EL34s have screen voltage?

                        Why don't you post the complete schematic?
                        Not even clear what brand and model this amp is.
                        Last edited by Helmholtz; 12-28-2020, 11:42 PM.
                        - Own Opinions Only -

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                        • #13
                          Choke resistor is a description. Peavey tends to put a big 400 ohm resistor in the B+ string between first and second filter stages - where a choke would have been in amps that had chokes.
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #14
                            Originally posted by Helmholtz View Post

                            So you mean zero Volt at "HT PS"? If so, you have no PI and preamp supply. Check if R203 is open. Do the EL34s have screen voltage?

                            Why don't you post the complete schematic?
                            Not even clear what brand and model this amp is.

                            thanks helm holtz
                            I have been cautious about sharing the schematic in full, as Blackstar gave it to me on the basis that I use it for my personal use only. But here it is in full.

                            cheers

                            steve

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                            • #15
                              R201/2/5 & 6 already drain the capacitors as well as balance the voltages.
                              Give it time to discharge before those angry pixies try and bite you.
                              Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                              If you can't fix it, I probably can.

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