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Acoustic 470 fuse blower

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  • Acoustic 470 fuse blower

    I have an Acoustic 470 that is blowing main fuses. I had this sitting around for a while and decided to re-address some of the issues. The original problem was that it was blowing main fuses after a short hum through the speakers. I had found some output transistors that were shot so i replaced them with the proper NPN's and fired it up, the fuse blew again. Anyways, after a letting the poor amp sit in the dark corner of shop I decided today to test things further: I tested the rectifier (again) and it was ok, I tested the filter cap and it was reading a short, as well i replaced the output capacitor. I also replaced the rectifier and the filter cap. Well, I decided to bring it up very slow on the variac and noticed I was getting nothing on the DMM from the circuit or filter cap. I unplugged the PT from the rectifier and tested it out of circuit, it was getting no voltage to secondary. Well, the fuses had blown previously so I replaced it once again, tested the PT out of circuit again, now the 7amp fuse inline with the PT and the main fuse totally cooked!!! Bad PT??

  • #2
    You may have cooked the pwr xfmr from replacing the fuse so many times with a short downstream. And we'll assume you used the correct fuse value.

    The short hum heard through a speaker before the fuse blew was DC from a shorted output device.
    The farmer takes a wife, the barber takes a pole....

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    • #3
      Im sure you are right. Do you think it would worth the time to completely yank th PT from the amp and test it just on ac to see if there a chance I am overlooking something?or do you think I killed it with repeated attempts to get it to work?

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      • #4
        All you can do is disconnect all secondary windings from the circuitry leaving only the primary connected to the switch, fuse and line cord. If it blows the mains fuse that way its junk.

        If it doesn't, you need to look for shorts in the pwr supply and output section.....which takes all of about 30 seconds to ID.
        The farmer takes a wife, the barber takes a pole....

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        • #5
          Never think up reasons NOT to check something.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Hey I am probably really pushing the point here but I have one final question here and I will chill on this, but below is a portion of the schematic which I have a question about. The ground reversal switch has three connections. One being the wire that comes from ground and two others from which this switch flip-flops between. Well, there is a capacitor,C502, between the ground wire and the ground from the line cord, this capacitor is the bridge of these two wires at the terminal strip. Upon obsessing more on this, when I put a meter between the switch connection and the other side (line ground) I am getting no continuity, meaning I am getting no connection between these two wires, unless the cap is not allowing continuity? I realize they are ground but I also am wondering if something may be shorting at the switch?

            p.s. Ive also just realized something, I have been using two 5 amp fuses instead of a 5 and 7 amp. Maybe Ive lost it...

            Last edited by randomair; 01-28-2009, 06:17 AM.

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            • #7
              What?


              If there is no continuity between two points, then they can't be shorted. And a short somewhere else won;t remove continuity from around here.

              First, look at the polarity switch. The center terminal doesn;t go to ground, it goes to the grounded CAP. Caps do not show continuity on a resistance scale. Unless they are shorted. So unless C502 is shorted, there will be no continuity from the center of the polarity switch to chassis.

              Now if C502 is itself shorted, then flipping the polarity switch to the hot side will blow the main fuse.

              The 7 amp fuse is the idiot fuse. The amp is fused at 5 amps, but just uin case some moron decides a 30 amp fuse from his car is a good idea, they wired a 7 amp fuse in series with the 5 but hid it inside. Now if they replace a blown 5 with a 30, there will still be the 7 inside to prevent fires. Using two 5 amp fuses is the same as using the correct fuse.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Of course. Thanks for your input. On to a replacement.

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                • #9
                  Are you sure that S504/505 or the associated wiring for this voltage selector are not the issue?

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                  • #10
                    Acoustic Amplifier Parts and Schematics Here!

                    www.acousticcontrolcorporation.com

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                    • #11
                      I'm thinking SPAM

                      If you are going to take the time to type... why don't you participate instead of leaving your website URL?
                      How about just sharing some info like the rest of us.
                      Otherwise your just another annoying free commercial here and that is not in the true spirit of AMPAGE.
                      Bruce

                      Mission Amps
                      Denver, CO. 80022
                      www.missionamps.com
                      303-955-2412

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                      • #12
                        Information?

                        I'm happy to share information. What do you need?

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                        • #13
                          Being you have a parts link on your page, do you have a working 470 PT laying around, cuz that is what I need. I am located just north of you. Thanks!

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                          • #14
                            acoustic 470 Power Transformer

                            What is the exact number stamped on your transformer and I will check.

                            I've got pneumonia, give me a couple days!

                            Doug Forbes
                            DEF Audio Consulting

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                            • #15
                              560015
                              5247335

                              I appreciate it. Again, I am just up the road, so I can hop on I-5 and be there when the time is right. Thanks again!

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