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Customer returned fixed Fender Princeton 112 reverb: Suddenly fuse keeps blowing

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  • Customer returned fixed Fender Princeton 112 reverb: Suddenly fuse keeps blowing

    I received this solid state combo 10 days ago. The 2 vol. pots were physically damaged by the owner. I replaced the pots (had to take the PCB out to solder pots in), tested amp 2-3 times for 5 min. each time and all worked fine. Customer came, tested his amp for a few minutes, all is well and he took the amp home. 1 week later he calls me that for the first time since he picked the amp up, he played it at home. After 10min. the 2A fuse blew and amp was dead. He said nothing like this ever happened in the 15 yrs he owned this amp. I told him to bring it back. I opened it up yesterday again and all looks good inside at first glance. I replaced the 2A fuse which right away blew after turning amp on.

    My question is, before I start trouble shooting this issue which is not easy to do with a dead amp, am I really responsible for this issue or is the customer responsible since he tested the amp at my place and accepted it. Who knows what he did in the 1 week after pick up.

    I would like to get some advice here what would be the proper thing to do.

    Thanks for any input.
    "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

  • #2
    It's probably a coincidence, but the best thing to do would be to fix it and chalk it up to good customer service/reputation.

    Comment


    • #3
      You probably should have tested the amp more than 5 or 10 minutes. If it is a head and the customer hooked it up to a bad load I would say that you might not be responsible but consider your reputation. If it is a combo and failed I would say that you should probably fix it. Many shops will say it's a different problem and the customer is out of luck. If a speaker failed and took an output device you might just cover the labor and have the customer pay for parts. Unfortunately catastrophic failure is part of the game, especially on units that old. I would suggest that you have a cookout area to test completed units in the future and run units at a reasonable operating situation for half an hour at least. Remember that reputation means business. If you are worried that you cannot affect a repair of a solid state output section you probably should not be repairing them. Personally, I would fix it. But I would not fix it a third time if it is a load issue. Make sure you ask if he used an extension cab. I used to tape cards on every amp repair stating that I was not responsible for damage done by a defective load condition such as speakers, wiring, and encouraged the customer to have it checked. Still, I gave a one time fix no matter what usually. If they take you to court they will win.

      Comment


      • #4
        I agree that you probably aren't responsible. I agree that you should probably just fix it.

        I specialize in ambiguity.
        "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

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment


        • #5
          Originally posted by Chuck H View Post
          I agree that you probably aren't responsible. I agree that you should probably just fix it.

          I specialize in ambiguity.
          That's not ambiguity, it's contradiction!

          (and that was pedantry)

          Getting back on topic, when you took the PCB out did you maybe disturb the insulating washers under the output transistors?
          "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

          Comment


          • #6
            Start by testing every transistor on the board and make sure there is not a shorted circuit anywhere. BTW what type of pots were used as replacements... values? What pots specifically? Clean, dirty channel or both? Are they making ground contact w/ the exact type of pot harness? Too many questions and not enough information.
            When the going gets weird... The weird turn pro!

            Comment


            • #7
              Mid sized (say, 50 to 100W) SS Fenders usually have:
              1) all in a single board construction, meaning you have to fully remove "everything" even for simple repairs such as replacing pots (your case)
              2) it means you are also removing the power section from the chassis.
              Typically the plastic power transistors (Tip142/147 or something similar) are mounted on the edge of the PCB, on a strip of aluminum which then gets bolted to a square aluminum bar which acts as a heat spreaded which then gets bolted to the aluminum chassis.
              *There is thermal silicon grease smeared between mating surfaces* which you must replace when re-mounting.
              You also need to make sure that there is good contact between surfaces (i. e. no dirt or debris there) and that bolts are reasonably tight (not so much that you strip their threads though).
              I think that when remounting you did not seat it properly as stated above, the aluminum strip absorbed the heat produced by a short 2/5 min. test, but after, say , 10/15 min at rehearsal levels it could not take it any more.
              So yes, you are responsible, sorry.
              Start by googling the Princeton 112 schematic and layout (they usually come together) or ask it from those great guys at Fender.
              They've always been helpful in my experience.
              It's also good that you did not "reinforce" or bypass the fuse, so the repair won't be that difficult or expensive.
              We'll help you through, of course.
              Good luck.
              PS: I usually subject all amps to at least 10 minutes of AC DC classics, full tilt.
              They have the advantage of sounding good , even distorting.
              Less boring than pink noise or sinewave sweeps.
              That's enough to get very hot heatsinks.
              30 minutes are enough to have very hot transformers.
              Juan Manuel Fahey

              Comment


              • #8
                Originally posted by J M Fahey View Post
                I think that when remounting you did not seat it properly as stated above, the aluminum strip absorbed the heat produced by a short 2/5 min. test, but after, say , 10/15 min at rehearsal levels it could not take it any more.
                So yes, you are responsible, sorry.
                This is a possible cause, as the thick aluminum bar can be mounted incorrectly so that the mating surfaces do not make full contact. If this is the case, then yes you are responsible for the repairs, probably a shorted output transistor.

                If this is not the case, then you need to find out exactly what the problem is and try and determine if something that you did in the earlier repair could have caused it or not. Every customer is different, some will be okay with the situation others will demand that you fix the problem at your expense. If you can make a case for the problem being coincidental then you can either stand your ground or meet him halfway.

                Comment


                • #9
                  Thanks everyone for all the valuable input. I did have to remove the PCB to solder in the 2 Vol. pots. I will test the 2 output transistors to see if they are dead. If, so they would not cost much to replace. But yes, I need to find out why this problem arose. If the transistors did get too hot then it must be a problem with me putting the aluminium bar back in between the chassis and alum. strip where the transistors are mounted on. I will put all new thermal silicon on (which I didn't, my fault) and see what happens.
                  I ordered the transistors already just in case, I've got a feeling that this is just the problem here.

                  Will keep you posted. I much appreciate all the advice.
                  "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

                  Comment


                  • #10
                    Originally posted by kka View Post
                    If the transistors did get too hot then it must be a problem with me putting the aluminium bar back in between the chassis and alum. strip where the transistors are mounted on.
                    Before you do anything else, check the fit of the heatsink assembly. If there is any space between the heavy bar and the mounting strip, then you can assume that the output transistors may have overheated and shorted out. That metal bar can be installed 180 degrees out of position, but the flat surfaces will not align and meet up. If this is the case, you will see a definite space between the two mating surfaces.

                    If not, then test the transistors with your meter and see if either is shorted. It could also be a shorted power supply diode that is causing the fuse to blow. Which may or may not have anything to do with the earlier repair.

                    Comment


                    • #11
                      I like to use the cheap "laser guided" IR thermometers to scan the various components after repairs are made. You can measure temperature rise and difference over time - hopefully before things start to run away. Also saves on burnt fingies

                      Comment


                      • #12
                        I found out that the heatsink assembly had a little gap to the aluminium block, therefore the 2 transistors overheated after 10 min. I'm now buying a new set of TIP142 and TIP147 . My question is, I could find those transistors at mouser.com, they have a TIP147TU (click link), which is the same package size "TO-3PN" as the original ones in the Fender amp.
                        However the TIP142 they only have in a package size "TO-220AB" . Could I use that one or does it HAVE to be the TO-3PN package?
                        "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

                        Comment


                        • #13
                          Originally posted by kka View Post
                          However the TIP142 they only have in a package size "TO-220AB" . Could I use that one or does it HAVE to be the TO-3PN package?
                          I checked and they have the TIP142 in the TO-3P in stock from On and ST. Just search TIP142. The TO-220 is too small and will not fit the board lead spacing and will not fit on the heatsink without some modification. I've had the best luck with the On parts in these amps.

                          As for the heatsink try flipping the spacer bar and see if the gap goes away.

                          Comment


                          • #14
                            You will find the old "TO3P"
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                            now is "TO218"
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                            or "TO247"
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                            Hole and leg dimensions and spacing are the same, of course.
                            I think there is also a 3rd name too.
                            I use tons of them.
                            Have found ON, Fairchild and ST are equally good.
                            Remember ordering proper micas (and insulating nipples if necessary).
                            Juan Manuel Fahey

                            Comment


                            • #15
                              Thanks for the info. I really can not find a TIP142 in TO-3 at mouser.com or anywhere else in the US. They all seem to be the TO-220 package or SOT-93. I have got a TIP142 in TO-220 in stock. The old broken on I can cut off leaving the legs in the PCB. I could connect the smaller TO-220 to those legs easily and could also mount it on the heat sink. I will keep looking for a TO-3, but if I can't locate one, is it possible to use the smaller TO-220 instead if I can mechanically install and connect it?
                              "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

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