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Blown cap in a Speco signal tracer amp

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    Blown cap in a Speco signal tracer amp

    Well I was bored this afternoon so I hauled out the box of test gear I shipped home from Ontario this summer. My gramps had an electronics repair shop back in the days the pre-date color tv and he had tons of tubes and test gear all over the place. Pity though, he never owned an oscilloscope.
    Anyways, one of the items I brough home was a Speco signal tracer amp with it's probe and manual. Before I plugged it in I read the manual and learned that the unit needed to be plugged in a certain way or else you would blow the unit (this is also the days before polarized plugs) so after I properly plugged it in (or so I thought) I turned and to my delight it did work but I was getting an excess buzz from somewhere. I then tried it out with the signal marking generator (which also worked but did not need a polarized plug) and after a while the unit suddenly stopped responding to the probe and a few moments later I was greeted with a fizz and then a "pop" and at which point I promptly yanked the plug from the wall.

    I was expecting something very destructive but after opening the case it looks like a single cap that went to ground heated up, shorted and vented by blowing out one of the half-melted wax ends, spraying wax everywhere!
    It's a .25 MFD (.25uf?), 200 VDC cap (that's as far as I know) and once it's replaced the unit should run fine again.
    Now, I need to find a modern equivalent for this cap, will it be hard?
    Also, would it be okay to replace the current plug with a polarized one since electricians now must follow a code that follows proper polarization?
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    #2
    Re: Blown cap in a Speco signal tracer amp

    This is an old paper capacitor - replace ALL of them before powering up old valve/tube equipment! Also, replace any capacitors between Live and Neutral with X2-class units, and any between Live and Chassis, as well as Neutral and Chassis with Y2 (or Y1) class units.
    Measure insulation resistance between chassis and both pins on the plug (with the power switch on) with a high voltage insulation tester. There may be a resistor between Live or Neutral and chassis, so remove it if you get low readings.
    A 0.22uF unit will do fine (the original of 0.25uF is an old value)
    Be sure to reform all electrolytic capacitors in this unit with a series lamp limiter http://www.vintage-radio.com/projects/lamp-limiter.html with the rectifier valve (or diode) in place.
    Another method to reform the electrolytic capacitors out of circuit can be found on http://www.vintage-radio.com/project...-reformer.html . Use an 1:1 isolating transformer for this project, and note the location and values of all electrolytic capacitors before removing them for reforming.
    If any units get excessively warm, they should be replaced.
    My first choice in quality Japanese electrolytics is Nippon Chemi-Con, which has been in business since 1931... the quality of electronics is dependent on the quality of the electrolytics.

    Comment


      #3
      Re: Blown cap in a Speco signal tracer amp

      Originally posted by japlytic
      This is an old paper capacitor - replace ALL of them before powering up old valve/tube equipment! Also, replace any capacitors between Live and Neutral with X2-class units, and any between Live and Chassis, as well as Neutral and Chassis with Y2 (or Y1) class units.
      Measure insulation resistance between chassis and both pins on the plug (with the power switch on) with a high voltage insulation tester. There may be a resistor between Live or Neutral and chassis, so remove it if you get low readings.
      A 0.22uF unit will do fine (the original of 0.25uF is an old value)
      Be sure to reform all electrolytic capacitors in this unit with a series lamp limiter http://www.vintage-radio.com/projects/lamp-limiter.html with the rectifier valve (or diode) in place.
      Another method to reform the electrolytic capacitors out of circuit can be found on http://www.vintage-radio.com/project...-reformer.html . Use an 1:1 isolating transformer for this project, and note the location and values of all electrolytic capacitors before removing them for reforming.
      If any units get excessively warm, they should be replaced.

      Give me a few moments to digest all that.
      Find Nedry!


      Check the Vending machines!!

      <----Computer says I need more beer.

      Comment

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