Groups > sci.electronics.design > #452804

Re: Amplifier transistor substitution.

From	Sylvia Else <sylvia@not.at.this.address>
Newsgroups	sci.electronics.design
Subject	Re: Amplifier transistor substitution.
Date	2017-03-21 10:57 +1100
Message-ID	<ejb8jsFq8nkU1@mid.individual.net> (permalink)
References	<ej4csuFe5h4U1@mid.individual.net> <ej8t1vFblegU1@mid.individual.net> <oao5fm$ds8$1@dont-email.me>

Show all headers | View raw

On 20/03/2017 7:59 PM, Tauno Voipio wrote:
> On 20.3.17 04:28, Sylvia Else wrote:
>> 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.
>
> Short in deflection plate or its wiring?
>

That's worth checking. Thanks.

Sylvia.

Thread

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

csiph-web