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5150ii burned resistor / possible arcing

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  • #1

    5150ii burned resistor / possible arcing

    5150ii with a burned r69, which I believe is in the heater supply.
    j59 and j60 look a little burned, or at least the plastic on the ribbon connector darkened.
    Cleaned most of the scoring, but it looks like I'm gonna have to cut the scoring out because it goes so deep.
    (side note, all burned areas need to be eradicated or just surface cleaned?)
    In my diagnosis haste I threw in a new r69 and fired it up, sans tubes. Burning.
    Unplugged the heater connector from the PT, fired it up, burning.
    I think I saw a spark around the power tubes board, but maybe not.

    No continuity between power tube pins 2/3
    Good continuity from pins 2/7, as well as the heater supply ribbon cable.

    I ordered new sockets, screen grid resistors, and new r69

    Any insight? How would one check for arcing? Why would the resistor continue to burn after heater voltage and power tubes are out. I would like to do some live amp voltage measuring but I only have a moment before the 3w r69 I threw in starts to go tits up.
    Attached Files
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  • #2
    You probably do have a arc going. Turn off the room lights and see if you spot a glow. That resistor elevates the heater string by connecting it to the +24v supply.

    You wouldn't measure continuity from 2 to 3, but that doesn't mean no arc is there.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Is the discoloration on the board at J59&60? This could very well be where your problem is, I have seen this before but that could be a bit of useless trivia in this case. That connector is suspect but the board where the connector pins are located is suspect as well. Burnt or discolored board material and or connector material can become conductive, especially when it blackens... carbonizes. If anything is pulling ACFIL1 down too much like an unexpected carbon resistor placed between it and ground composed of burnt or charred circuit-board for example, this would cause R69 to over-current and fail, tubes in or not.

      Unplug that connector and give it a go, if the burning stops then the connector is at fault, if not, well then it's on the mainboard at the pins probably.
      ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

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      • #4
        That's exactly where it is burnt. Im going to pull the connector tongith and see what's good. The damage is so extensive under r69 I think I'm going to Dremel out as much as possible as well. Will report back tonight. Thanks Enzo/Sowhat

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        • #5
          No burning with the connector pulled. The j59/j60 pins on the main board have discoloration at the board level. So, does this make you think the connector failed and the power tubes aren't arcing?
          As far as next steps, I still need to dremel out the burnt board material.

          Comment

          • #6
            Originally posted by wormdirt View Post
            No burning with the connector pulled. The j59/j60 pins on the main board have discoloration at the board level. So, does this make you think the connector failed and the power tubes aren't arcing?
            As far as next steps, I still need to dremel out the burnt board material.
            It's not the mainboard thank goodness, that has been eliminated and that R69 is safe for now.

            It can be in the connector or any way along the path to the tube sockets and even into the interior of the tubes. Fortunately there's not much to the circuit at this point.

            I would say that a heater to cathode short in one of the power tubes is the cause of all this. The tube socket could be suspect, you might see a bit of burn between the heater and cathode pins of the power tube socket as well as the solder connections and traces on the output tube board and all the way to the charred connector housing.

            With that connector disconnected and the tubes pulled a simple resistance check between the disconnected lines should reveal the fault... none of the connector pin sockets should be connected to each other in any way. If they are, even if just a slight amount of resistance is present then there's your culprit.
            ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

            Comment

            • #7
              As a side note: If you already have not pulled the output tube board from the chassis, now is probably a good time to do this. Your problem is isolated to this assembly and it's way easier working on it separately up on the workbench instead of poking around inside the chassis.
              ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

              Comment

              • #8
                J59 and j60 have continuity on the pins on the main board side. I haven't cleared out all the burn mess from the resistor explosion yet, so maybe that's contributing. Tube sockets are on the way so those are getting replaced either way. Next step is still to fully clear out the damage at work tomorrow on my bench.

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                • #9
                  Originally posted by wormdirt View Post
                  J59 and j60 have continuity on the pins on the main board side.
                  Yeah... those pins should have continuity on the main board because they are fundamentally leads coming from a coil on the power transformer. That's not where your focus should be, also I believe if you remove fuse F2 that continuity will disappear, but that's not your main problem right now. The problem is on the output tube board, right now probably it's that connector but maybe not, maybe it's some other part of the output board. You got to pull that thing out and go over it with a fine tooth comb. Lastly without access to a good lab grade tube tester that can put power tubes through their paces, I would replace all the output tubes with fresh brand new ones just for safety. A matched quad of 6L6GC's would do the trick nicely, but this is of course after you find the short and fix it.
                  Last edited by Sowhat; 03-29-2018, 05:33 AM.
                  ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                  Comment

                  • #10
                    I was browsing around the internet and found a solution addressing this problem pretty much dead on. Although I don't like the methodology much because it modifies the amp, it may be the less complicated choice in how to deal with this problem.

                    PEAVEY 6505+ | PCBA Removal

                    It's the burnt pin locations that make this troublesome, the cable assembly can be replace or made with regular Molex parts available at Mouser, Allied, Newark, Digikey, etc etc... but the mainboard pin location really needs isolation, once burnt you can't really fix it without cutting out the area and bonding in some new board and replacing the pins... that's a ridiculous amount of work.
                    ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                    Comment

                    • #11
                      Once the sockets show up, that should hopefully clear this up. The cable/connector looks like it only has minor discoloration, and no continuity where it shouldn't be. As a side note, I discovered what I think is a MOV or varistor cracked and broken, but I'm not sure what it is. The schemo has it labeled as TR1 and 70240200
                      Attached Files

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                      • #12
                        Massive carbon in the power tube sockets
                        Attached Files

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                        • #13
                          Ok
                          Thermistor ordered too. Sockets en route. I'll update when the sockets are installed

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                          • #14
                            Originally posted by wormdirt View Post
                            Massive carbon in the power tube sockets
                            Yeah, that board needs some TLC.
                            ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                            Comment

                            • #15
                              Ok, new tube socket is in, new 4.7k for r69
                              The NTC isn't going to be here for a few days.. Will I be OK firing it up, or is the current limiting that important to use every time.

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