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Re: Amplifier transistor substitution.

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On 18/03/2017 8:27 PM, Sylvia Else wrote:
> The smoke escaped from my old oscilloscope today. I've determined that
> it came out of R740, to the bottom right of the circuit diagram.
>
> https://www.dropbox.com/s/9053dnyr0izv88r/XAMP.PDF?dl=0
>
> This is because T711 has shorted out.
>
> Not very surprisingly I cannot source a replacement BF472, so I need to
> find a substitute.
>
> This is the final X-axis amplifier, so we're not talking high
> frequencies. Is it likely to be sensitive to the particular transistor
> characteristics?
>
> Sylvia.

This is not proving as straight forward as I'd hoped. I've removed T711, 
and now R739, which is the counterpart of the failed R740, at the top 
right, is starting to emit smoke. The voltage across R732 (attached to 
the base of T708) is about 30 volts, which explains the pain experienced 
by R739, but is not otherwise easy to explain.

I saw a similar voltage across R737 (bottom right) until I removed T711, 
but then the voltage returned to normal.

I find it hard to believe that two transistors would fail randomly so 
close in time, and some kind of common mode failure seems more likely, 
but I'm stumped if I can see what it might be.

Any thoughts?

Sylvia.

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Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-18 20:27 +1100
  Re: Amplifier transistor substitution. "Kevin Aylward" <kevinRemovAT@kevinaylward.co.uk> - 2017-03-18 10:41 +0000
    Re: Amplifier transistor substitution. Cursitor Doom <curd@notformail.com> - 2017-03-18 11:13 +0000
    Re: Amplifier transistor substitution. bitrex <bitrex@de.lete.earthlink.net> - 2017-03-18 18:15 -0400
      Re: Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-19 12:44 +1100
        Re: Amplifier transistor substitution. Winfield Hill <hill@rowland.harvard.edu> - 2017-03-18 19:03 -0700
          Re: Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-19 13:14 +1100
          Re: Amplifier transistor substitution. Cursitor Doom <curd@notformail.com> - 2017-03-19 14:11 +0000
            Re: Amplifier transistor substitution. Winfield Hill <hill@rowland.harvard.edu> - 2017-03-20 06:20 -0700
              Re: Amplifier transistor substitution. Cursitor Doom <curd@notformail.com> - 2017-03-20 23:26 +0000
  Re: Amplifier transistor substitution. Phil Allison <pallison49@gmail.com> - 2017-03-18 04:08 -0700
    Re: Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-18 22:42 +1100
      Re: Amplifier transistor substitution. "Benderthe.evilrobot" <Benderthe.evilrobot@virginmedia.com> - 2017-03-18 22:01 +0000
  Re: Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-19 16:34 +1100
    Re: Amplifier transistor substitution. Cursitor Doom <curd@notformail.com> - 2017-03-19 14:13 +0000
      Re: Amplifier transistor substitution. John Robertson <spam@flippers.com> - 2017-03-21 08:34 -0700
        Re: Amplifier transistor substitution. Cursitor Doom <curd@notformail.com> - 2017-03-21 18:18 +0000
  Re: Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-20 13:28 +1100
    Re: Amplifier transistor substitution. Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2017-03-20 10:59 +0200
      Re: Amplifier transistor substitution. Sylvia Else <sylvia@not.at.this.address> - 2017-03-21 10:57 +1100
  Re: Amplifier transistor substitution - fixed. Sylvia Else <sylvia@not.at.this.address> - 2017-03-22 19:33 +1100

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