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Fender Stage 160 Instantly Blowing Fuses

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  • #1

    Fender Stage 160 Instantly Blowing Fuses

    Hey guys, I'm having an issue with a Fender Stage 160 that makes a loud buzz noise and blows the fuse almost instantly. Let me give you guys some back ground:

    I picked up the amp from a Garage sale for $25 that had a buzz to it. It looked like it was brand new so I figured, what the hell, for 25 bucks how could I go wrong. Got it home took the circuit board out and saw that C63 had a bad solder point and re-soldered it and to my surprise it worked perfectly. Excited and happy that I got a hell of a deal, I had a few buddies over to jam. After playing for 2 hours, amp shuts off and blows the fuse. It was late and I did not want to mess with it so I let it sit over night to cool off and plus I did not have any more fuses.

    Next day I popped a new fuse in and it blew almost instantly, making a buzz before it blew. I tested the power supply and it came out normal, so I know its in the board. I downloaded the schematic and looked it over but I'm sort of lost on where to start and/or what to check and test first. Is there a common problem or part that causes the fuse to blow? Could the bad solder point mean that the capacitor was going bad and it was giving a sign? Any help would be very appreciated.

    Thanks,
    Nick
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  • #2
    What do you mean the power supply checked out normal? The power supply is on the board with everything else.

    Are any of the output transistors shorted?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      I hooked the wires up for the power switch to the board and the black and white wires coming from the "power box"(I don't know the correct term for it sorry) to the board and tested the other wires coming out of the "power box".

      What numbers are the output transistors? Are they Q14-Q19? If so, they all looked like they had no damage to them, but I was not sure how to test them. I'm looking it up now how to test them but I wont be able to fool around with it tonight. I'll try and get those tested tomorrow and post the results.

      Also, sorry if I'm ignorant on testing and using the correct terms for the electrical components. I know the basic electrical stuff and my brother(An electrical engineer) is trying to help me as much as he can but he is a busy man. So try and put up with me sort of having a crash course on this lol.

      Oh and thanks for the reply Enzo!

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      • #4
        The big cube of iron where the mains wires go is the power transformer. The power supply consists of that plus a bunch of things on the board - rectifiers, filter capacitors, etc.

        You can;t tell by looking if a transistor is good. The larger ones screwed to the heat sink are the output transistors - if i recall without looking, they are types TIP142 and TIP147. Each has three legs conecting to the circuit board. With power off and the amp NOT plugged into the wall, yuo can measure resistance between the three legs on each part.

        If any show a low resistance - a couple ohms or less - then they are probably shorted.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Ok, tested the output transistors and one of the tip142's and one of the tip147's were a quarter of what all the other transistors resistance levels were. The transistors that I assumed were good are around 1.6-2.0 and the two that I think are bad were rated at .5. I went ahead and ordered 5 of each and was thinking that I should replace all of them while I'm at it. Is that a bad idea?

          Thanks again Enzo. I'm crossing my fingers that this fixes the issue!

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          • #6
            Why not, they are not expensive.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              That's what I figured. Well they are on their way so I'll post back in here once I get those soldered in and see if this solves the problem. Thanks again!

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              • #8
                Make sure the new ones are insulated from the heat sink. There are thin mica or silicone squares between the transistors and the heatsink now aren;t there? And the screw has a plastic shoulder washer so it doesn;t touch as it goes through the mounting hole, right?
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  Yes there was a film like heat sync compound between the transistor and heat sync and also the little plastic washers were on the screw. So your just saying just make sure that the transistor does not touch the metal heat sync in any way, correct?

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                  • #10
                    Exactly.

                    I always do that carefully, and even though I am confident in my techniques, I ALWAYS check with my ohm meter between the metal tab and the heat sink itself, just to be sure.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      I'll make sure to check with the multimeter before I plug anything in.

                      I just realized that I'm going to need to put the heat sync stuff back on again and I'm not sure what that is or what it needs. Is it like heat sync compound you would put on a CPU out of a computer (if you know what the stuff is)? Or what stuff would I need to do it?

                      Thanks

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                      • #12
                        You're correct.

                        It is heat sink compound, like you would put on a CPU of a computer. I usually clean all the old stuff off.

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                        • #13
                          Sweet! I think I have some Arctic silver laying around.

                          Thanks!

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                          • #14
                            Ok replaced the output transistors and the two the were blown before, blew again and blew the fuse again when I turned it on. What is the next step in testing the amp? Thanks in advanced!!

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                            • #15
                              If new ones are blowing, there was more wrong than just outputs. Look at the vertical string of resistors on the drawing R81-R86, and the parts associated with them, Q10,Q11 and D22,D23. Check all those parts.

                              Note there are +/-15 or 16v at top and bottom of that. With the blown power transistors removed, check that pair of 15v supplies. Present?

                              Every output transistor has a 5 watt .47 ohm resistor associated with it. R94 etc. Make sure none of those are open.

                              Work with the speakers disconnected until the amp is stable.
                              Education is what you're left with after you have forgotten what you have learned.

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