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Fender Passort PD-250 stuck in protect mode

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  • #16
    That's one that I haven't seen before, I guess that anything can go bad. Let us know how it turns out.

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    • #17
      These are a pain! I rewound one in a 250 unit that burned up... You have to wind it bifilar... or trifilar... that is with all the wires at once so they lie parallel. WInding individual sections will not work well. The leakage inductance will kill the supply. These amps are a piece of junk and are very hard to work on. Mine burned up a second time and that was enough...

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      • #18
        This is not a PCF circuit - this amp does not have one - it is not needed with this low power. This is inductance that is part on the output voltage (+/-51V) filter. It is wound bifilar, which is difficult to do at home if you don't have proper tools. But you can easily buy it from mouser, or farnell, or newark (or maybe Fender). Just ask Fender what is the inductance, check the core material and measure the dimesions. It is something like 3-5$ in every shop.
        It was just shorted but I never seen this inductor shorted. It was just poor quality. Buy a new one, check output diodes and everything should be fine. Best luck if you want to wound it on your own. I personally wouldn't do it.

        PS: 52 Bill's first guess that this is a short circuit in the power supply was brilliant.

        Mark
        Last edited by MarkusBass; 01-28-2012, 10:02 AM.

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        • #19
          Just be careful as most two-winding chokes you find in the shops will be common-mode chokes with ferrite cores. They look the same but will be totally useless for the job. It has to be an energy storage choke with an iron powder core.

          Fender really stuffed a lot of wire onto that core. Home winding might be easier if you buy a larger core and wind until you get the same inductance as the small one. My top tip is to tape the core to the halfway point of the wires and then wind it in two goes using the two free ends.
          "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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          • #20
            Originally posted by MarkusBass View Post
            PS: 52 Bill's first guess that this is a short circuit in the power supply was brilliant.

            Mark
            Hey there's no reason to make fun of my ability to point out the obvious.

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            • #21
              Sorry, but, as you know, something obvious for one person is not always obvios for another person. Sometimes pointing out the obvious can help - it happened to me several times. Quite often when amp is fixed there is a thought: "it was so obvious".

              Mark

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              • #22
                I have seen one of thesee that overheated and shorted. This inductor serves as a balancing device in the switching power supply. It got so hot that it melted the solder as well as puncturing the winding insulation with a resulting short between a couple of the turns.

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                • #23
                  Post misfire..
                  Attached Files
                  Last edited by rrflorida; 01-29-2012, 06:16 PM.

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                  • #24
                    Lots of good input - thanks for that.

                    I would have preferred to purchase a new inductor, but didn't know the specs - schematic doesn't specify, nor does the parts list. So I opted to rewind. After filing and sanding the core, it was super smooth, so that went well. I hadn't heard of this "bifilar" concept, but it makes sense. Fortunately, I just copied what I saw from the unwind, and so unknowingly rewound it bifilar. Now there is clearly a necessary technique that I do not posess... If you thought the inductor looked cheesy before, have a look at it now:

                    Click image for larger version

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                    Ugh. So I throw it back in, fire up the PS, and no joy - still in protect mode. Can't stop thinking about it while not sleeping that night, I realized I didn't wire it correctly.. And I didn't even check before putting back in circuit. Not smart. So, now I'm faced with re-plumbing the wires and the result is even more ugly , and not a little embarrasing, as I didn't want to undo the epoxy and risk damaging the wire:

                    Click image for larger version

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                    Still optimistic, I throw the word's ugliest inductor back in, and fire up the PS. It comes on! In power mode and not protect mode! I figure this amp is now conquered, and I don't care if it's 1am, I'm putting it back together for the full reward. Once it's together, I connect a cheap microphone that I use in another PA, and turn on the Passport, and no sound comes out of the speakers. Play with all the adjustments, try different channels - no sound. As I'm sitting there contemplating, I hear two snapping noises, and the amp goes dead, and there is that odor...

                    I take it all back apart, open up the supply, and see some exploded diodes. Inventory of bad parts I found: R4, D5, D6, Q1, Q2. These parts are all on the "input" side of the big tansformer out of the rectifier.

                    I'm not sure what can cause this, but when I took the PS back apart, I noticed lots of small metal shavings on the trace side of the PCB. I'm thinking these are from my solder sucker, and I didn't clean the board after my surgery (remember I said I'm not a technician? No kidding..). So I can't say this caused the issue, but it couldn't have helped.

                    So I will replace these parts and test the PS with no load again, after cleaning up the PCB. If this works, then I suppose add the other boards to the PS and see? I'll take any suggestions on the best approach.

                    And why no sound I wonder? I double checked the power amp transistors, and the connectors/wiring from the pictures taken during disassembly..

                    Could my inductor have caused this?

                    Oh, and I noticed the PS fan didn't come on - should it be on as soon as the PS turns on, or is it thermally controlled and only comes on when needed?

                    My wife says "maybe it's time to give up" - to which I reply "and declare this inanimate object the winner??". Can't do it. Not yet anyway..

                    Thanks for all the input

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                    • #25
                      Sounds like you rewired it from "completely shorted" to "one winding backwards". That would cancel out the inductance and cause very high peak currents.

                      You probably blew the switching transistors and their antiparallel diodes. If you have an old ATX computer power supply, maybe you can scavenge some replacements from that.

                      edit: from looking at the schematic, not likely, but at least the IRF840 is easily available. I'd replace all four. Also when checking the coil wiring, note the little dots on the schematic that identify the starts of the two windings. Of one wire, the start should go to D10/D11 and the end should go to C11 positive.

                      Of the other wire, the start should go to the negative end of C12, and the end should go to D12/D13.

                      http://music-electronics-forum.com/a...-schematic.pdf

                      You could ditch the little plastic base of the coil if it's confusing things, and just work with the four wires sticking out of it.

                      Don't forget to check D10-D13 too!
                      Last edited by Steve Conner; 01-29-2012, 08:53 PM.
                      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                      • #26
                        One winding backwards?! Sigh... Good call Steve. I put the start for the coils on pins 1 & 3. Should be 10 & 3. These are some painful lessons. But at least since you explained it, we know the cause, and it's fixable. So that's good news for me. Thanks!

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                        • #27
                          Do you have the rest of the 250 schematic? I would like to have a file of it. That inductor seems to run hot. I wonder if the skin effect might be a problem. I would wind it with Litz wire if I could find that. When I rewound mine, the wires were about 30 feet long and it is helpful to have a helper to handle them while winding.

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                          • #28
                            I got the schematic from the thread that was referenced above. The other schems might be there too.

                            Litz wire would certainly help somewhat, at the cost of making it even more of a hassle to wind. And if the core was undersized, it wouldn't help with the core losses.

                            If it were me I'd get another core two sizes bigger, and use ordinary magnet wire one gauge heavier. If you post the core dimensions and number of turns, I can figure out a suitable part and calculate how many turns the new core would need. Probably the hardest part would be figuring out a mechanical way to mount the new heavier component.

                            PS: Don't ask how I know the symptoms of a backwards choke winding!
                            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                            • #29
                              I was digging through some documents from Fender and I came across one that discussed this choke. He's the quote...

                              "The large choke, L5 has been known to have bad connections to the PCB, or arcing / shorts between the windings. This toroid coil is not available through Fender. It has 2 windings that measure 360 uh. Pay attention to the phase of the windings. A replacement transformer manufacturer can supply this part."

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                              • #30
                                The current coil takes up about all the available real estate on the board. I would prefer to have one made to spec.. I went to a manufacturer website the other day and quickly got confused about all the parameters. SDS wants $50+parts each, 2 coil minimum, which isn't really econimically feasible. So I figured I'd have the best luck just working with the original one.

                                The replacement parts arrived today, so hopefully I'll get those in tonight, then rewire the ends of one of the coils to get the "phase" correct, then do a smoke test.

                                That quote from Fender sounds spot on. Too bad they didn't stock lots of extras, although if they are all this quality, I don't think I'd want one..

                                Thanks for the input,
                                randy

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