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  • #16
    Thank you Enzo.
    I am suspecting the power supply. The unit was given to me, and didn't have a fuse installed. I put in a fuse and upon powerup there was a 1/4 second of hum before the fuse blew.
    I deal with PS hum all the time in vintage radios, failed shorted caps or rectifier bridges bad. I'll start with the basics on this amp before attempting another powerup. The BX100 diagram shows a straightforward supply, and if it is similar to the BFX with the effects module) I should have no trouble.
    Generally I'd think if the trouble was in someplace other than the PS the unit would still power up but not work right. An immediate shutdown to me might indicate the filter caps, bridge or even the output transistors.
    An exact diagram would be great if possible, (all the sources online lead to no file) but I'll do the best I can with the other drawing.
    Thanks for your kind reply sir.

    Comment


    • #17
      Actually, it is WAY more likely you have a shorted output transistor than a power supply failure. Of course the PS can fail, but a lot less often than the outputs. If the PS is the problem, a shorted rectifier diode or bridge is WAY more likely than a shorted filter cap.

      And it takes mere seconds with an ohm meter to check all those for shorted condition.
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        Thanks Enzo,
        I will add that to the troubleshooting list before powerup. I do have a couple of transistor testers.

        I've has bridges fail before in 70s stereo systems,easy to test with the Ohmmeter. The 2 filter caps will probably get changed as a general rule while I have the board out. I didn't want to hijack this thread with repair and troubleshooting, and if I have other issues, I'll start a new thread on one of the repair columns. Thanks for your thoughts Enzo.

        Comment


        • #19
          An ohm meter is perfectly fine for detecting shorted transistors.

          Please do yourself a favor and fix what is wrong with the amp so it works, only THEN go back and do all the cap updates and stuff.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #20
            Hi Enzo,
            Ok, I can check the transistors first, then possibly bring it up on the Variac? Maybe I am 'cap shy' due to the hundreds of failed electrolytics I've come across over the years. I know that the other caps should be replaced later on, I was only worried about the 2 filter caps in the power supply for right now.
            What I was thinking, 'if' one of those filters is shorted, it might take out a new transistor if I had to replace one or more.
            If for example there is a shorted or super leaky (electrically) cap, the amp probably wouldn't work.
            It will be a couple of weeks before I start in this amp, having to finish up a couple of 30's radios right now.
            Appreciate the advice, and will report back when I get into the board. Take care.

            Comment


            • #21
              BFX100.zip

              The output section uses mosfet transistors.
              If they are damaged in any way, they may not take out the fuse immediately.

              Until you get the amp stable, Remove The Load on the output.
              (ie: no speaker or dummy load)

              Comment


              • #22
                Thank you sir, for the drawings.
                I will make a note of that, to remove the load. The fuse was popping almost immediately upon powerup, with a tiny fraction of a second of 60 cycle hum.
                I have an account at Mouser in case I need to order the MOSFETS.
                Some basic tests first and I will go from there. I'll definitely report back when I get the chassis on the bench. Thanks again Jazz for the file, and kind reply.
                Gary.

                Comment


                • #23
                  I have restored many jukebox amps from the 1930s on up to the present day. I can usually expect to replace most every cap in a 1950s Seeburg. After all, they are 60 years old. But your BX is not 50 years old. So it might need some caps, or not, but they won't be the first things on my list.

                  I use a variac, but this might be a good place for the well known "light bulb limiter".

                  If you think a main filter might be shorted, then measure it and find out, that is why god made ohm meters.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment


                  • #24
                    During the repairs, "coarse" setting and tests output stage, the best protection are resistors (or battery resistance) 100-200 Ohm / 100W in a positive and negative side of the power output stage.

                    For a voltage to +/- 60V they limit the current output transistors to a value 0.3 - 0.6A corresponding to dissipation of 20 - 40W, and this value submitted most of the output transistors.

                    When the output stage repaired and quiescent current is set to the desired value, remove protective resistors, readjust quiescent current.
                    Now output stage is ready to test on the oscilloscope and dummy load for rated power and symmetry sinus.

                    About knowledge language
                    https://translate.google.com/
                    sorry about that

                    vk
                    Who does not know and knows that he does not know - teach him Confucius)
                    Who knows and does not know that he knows - wake him Confucius)

                    Comment


                    • #25
                      Enzo, I do have a 'dim bulb ' tester for testing for shorts, saved me a lot of times. The first test I give a vintage radio.
                      On the older equipment, it's a given that all paper caps be replaced, such a the Seeburg.
                      I know the BX is not that old, and hopefully the filters are fine. I suspect all the other caps in the board are fine. I have no history on this amp, (only that is was presented to me without a fuse installed) and was not sure if it was abused or not. Once the power supply is verified as good, (and inspecting the output mosfet transistors) I can do as VintageTiki suggests.

                      I do have an older Tektronix scope, if it come to that point.
                      Vintagetiki, thanks for your tips, copied and saved.
                      Although I have been doing old radios since the mid 70's, the pro music equipment repair is a departure from what I know. Some basics remain, but I hope to widen my knowledge on this kind of equipment. Thanks to you all, and will report what I find in a couple of weeks.
                      Gary.

                      Comment


                      • #26
                        reminder for beginners and those who are not

                        http://antiqueradio.org/dimbulb.htm

                        http://www.cab1net.com/forum/viewtopic.php?f=59&t=1018
                        Who does not know and knows that he does not know - teach him Confucius)
                        Who knows and does not know that he knows - wake him Confucius)

                        Comment


                        • #27
                          Good links for those that have not used or heard of a Dim Bulb Tester. I've been a member of the ARF for many years, and the DBT is one of the first piece if equipment we recommend. Some get fancy with building one, others build it simple.
                          If your equipment has a short, the bulb will light brightly, protecting your equipment.

                          Comment


                          • #28
                            Hi guys,
                            Just an update on the BFX100. I finally got it on the bench. Recitifier bridge, caps and supporting parts tested ok in the power supply. I went right to the MOSFETS, both were shorted. Supporting parts on the small power amp board were fine.
                            A member here told me that he would have like to see Crate use a larger heat sink on them.
                            Parts ordered from Mouser should be here this week.
                            Upon powerup, I may use the dim bulb, in the rare case something else is shorted, but I am pretty confident the amp will work.

                            Another note, I found a few fractured solder pads on the main board, and the 10 pin connector to the main amp board. I've see these fractures on many home theater systems, especially on large resistors, and amp modules.
                            Thanks and will let you know what happens.
                            Click image for larger version

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                            Comment


                            • #29
                              Originally posted by gary rabbitt View Post
                              Upon powerup, I may use the dim bulb, in the rare case something else is shorted, but I am pretty confident the amp will work.
                              Edit that statement to say... "Upon powerup, I WILL use the dim bulb, in the rare case something else is shorted, but I am pretty confident the amp will work."

                              Variacs with ammeter will work too, but sometimes the amp will have a relay that kicks on after about 90VAC or so. I have a few of those confident moments where I started the amp up and the light bulb was still shinning bright. Better safe than sorry.
                              When the going gets weird... The weird turn pro!

                              Comment


                              • #30
                                Thanks Dr. Gonz,
                                Yes I WILL use the dim bulb tester (Standard practice on the vintage radios I do).
                                I have a Variac and Ammeter too. I don't see a relay in the power supply, so that's not an issue.
                                Parts should be here tomorrow or Saturday. Will keep you informed.
                                Thanks!

                                Comment

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