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Damn near started a fire in my Hot Rod Deluxe

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  • Damn near started a fire in my Hot Rod Deluxe

    Oh man. Was at Frys (remember them) and saw the still cary parts and thought I could buy the two power resisters that Fromel changes as part of the kit. Mine have blackened the PCB. Picked up a pair 5w4.7r and soldered them in with a little standoff as they are supposed to be. Fired it up, 10 seconds in, smoking. Turned it off.

    Two diodes got so hot they de-soldered themselves, one of the power resistors did the same, ruining the pad in the process. Nice work genius.

    So, the original are 5w470r. Replaced with 5w4.7r. I’m guessing I just let way to much current pass through? I’ve got some work to to put it back correctly but this is my “learn by mistake” project.

    I’ve got the diodes back in, ran out of time to reinstall the 5w470r’s that I removed.

    How big of a dumbass move was that?

  • #2
    Originally posted by Oldmactech View Post
    How big of a dumbass move was that?


    Enjoy. Every. Sandwich.

    Comment


    • #3
      If the diodes got so hot that the leads melted solder, say 190C, then the maximum junction temperature was likely exceeded. Therefore replace the diodes. Much easier than removing the whole board in two weeks time as the diodes failed.
      Experience is something you get, just after you really needed it.

      Comment


      • #4
        Also eye up the caps across those zeners for any bulging.
        "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


        • #5
          .
          If there is a lot of heat damage to your board, you might want to reinforce the traces in the LV supply. Using a small solder helper toolkit you can pick up for around $4-6, you can scrap away the solder mask from the copper traces pretty easily. - Like this:
          (keep in mind this board isn't all that bad, I was just using it as an illustration)
          Click image for larger version

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          Then I tin the traces by flowing solder along the exposed copper. I like to use a small amount of liquid flux over the traces. This really helps them take the flow of solder very quickly.

          Click image for larger version

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          I've done this on more blues/deville fender amps than I can count and it goes a long way in keeping those components and circuit board working reliably going forward.
          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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          • #6
            Thanks, I’ll look into that.
            I have an exact, plenty of flux and good solder. Not sure what wire to use but I have different sizes. I assumed braided is correct.

            Like to replace the zener diodes but can’t find them on the schematic. They function currently but likely headed for failure.

            Comment


            • #7
              Can't find on schematic? Look in power supply. Can you find the two 470 ohm dropping resistors R78,79? They are directly connected to those resistors. CR13,14.

              If all you need is the number: 1N5353

              The 5w resistors have like an inch or so of wire lead. We usually slip them through the hole and trim off the excess. I prefer to keep the wires. I do as Soul did, cleaning the coating off the traces back from the holes. I then insert the resistors, then bend the excess wire down along the bared traces. I then run solder the length of the wire. SO it looks like what SOul did, but with wire running through the solder too. That extra wire adds to the heat sinking of the solder and trace. Also you can mount the resistors up off the board a little to increase air cooling.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Thanks. Finally found them on the schematic.

                Oredered a pair and ZenerDiodes from Digikey.

                I will try the bend wire to meet the trace. Like the concept. Need to look and see how many traces need work.

                Comment


                • #9
                  How best to proceed?

                  I reinstalled the two power resistors and the two diodes that popped off due to heat. Ran temporary jumpers until my replacement parts arrive. I’ve spending time getting more educated on schematics so I’m operating at a C- grade level for that.

                  I could use some symptom troubleshooting advice:


                  When I turn on the power (standby) all tubes light- oddly the drive channel yellow light lights on standby, not supposed to. Switch off standby, 60 cycle hum, loud at first, dissipates after 1-2 seconds to light hiss. Guitar sounds surprisingly good, loud, seems stuck in drive mode. Tone controls function. Nothing else. Drive, master, reverb, presence not functioning. Going over the schematic with such widespread outing maybe a relay or more than 1 got toasted?

                  Thanks for any help.

                  Comment


                  • #10
                    Do you have the proper +/- 16V at those zeners?
                    "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


                    • #11
                      There might be other issues to look at involving the IC's on the board. If stuck in drive mode then there is a 4560 IC that would be a suspect. In fact, the other IC's would be suspect since they could have been damaged as well and that may help indicate why the reverb is not working. Do as G1 says and post your measurements of the -/+16v supplies. At this point post the version of the Hot Rod Deluxe schematic that you are referencing so everyone is on the same page.
                      When the going gets weird... The weird turn pro!

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                      • #12
                        The schematic is available at thetubestore.com I believe it’s REV A 1996.

                        I measured the Zeners and neither is +/-16.

                        C41 is -13.1
                        C40 is 3.1.

                        I measured these on DC Voltage 20 scale on standard DMM.
                        Last edited by Oldmactech; 03-05-2020, 11:12 PM.

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                        • #13
                          Schematic Here

                          I'm presuming you mean CR13 and CR14- C is the designation for a capacitor. Is that a typo, or do you actually have 3.1V on CR13 and not 13.1 like its counterpart? If so, you likely have a near short on that rail. What are your voltages on the other side of those 470 ohm resistors- the side right off of the rectifier diodes?
                          "Yeah, well, you know, that's just, like, your opinion, man."

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                          • #14
                            Yes thanks.

                            I’m reading 46.8 at R78 -46.8 at R79. R78 is -1.28 R79 out is .11. Nowhere near +/- 16v.

                            Cr 13 reads .10, CR14 reads -1.28 now.

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                            • #15
                              You had checked these zeners and they measured ok, yes?
                              Are any of the IC's feeling real hot?
                              "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

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