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Groups > sci.electronics.design > #452527
| From | Sylvia Else <sylvia@not.at.this.address> |
|---|---|
| Newsgroups | sci.electronics.design |
| Subject | Re: Amplifier transistor substitution. |
| Date | 2017-03-19 12:44 +1100 |
| Message-ID | <ej663vFpjcqU1@mid.individual.net> (permalink) |
| References | <ej4csuFe5h4U1@mid.individual.net> <BPednSosMON4k1DFnZ2dnUU7-aHNnZ2d@giganews.com> <qkizA.83409$qD6.33723@fx34.iad> |
On 19/03/2017 9:15 AM, bitrex wrote: > On 03/18/2017 06:41 AM, Kevin Aylward wrote: >>> "Sylvia Else" wrote in message news:ej4csuFe5h4U1@mid.individual.net... >> >>> 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? >> >> The key bit about that part is high voltage with low cob, and relatively >> high ft. Its cob is around 2pf >> >> The BF472 has a voltage rating of 300V. It has a companion part BF470 >> with a voltage rating of 250V, its just possible that the design don't >> need 300V. You would have to check. >> >> So, look for a PNP 300V, 2 Watt, cob < 4pf ft > 50MHz, I > 50ma >> >> -- Kevin Aylward >> http://www.anasoft.co.uk - SuperSpice >> http://www.kevinaylward.co.uk/ee/index.html > > 2SA1381? > > http://www.mouser.com/ds/2/149/KSA1381-889316.pdf Yes, that does seem a possibility. I've simulated part of the circuit from T705 onwards, using 2N3904 and 2N3906 (ignoring the voltage limitations), and with capacitors between the collector and base. Even with 2pF (as with the BF472) the sweep is not very linear at the highest rate (80ns), and worse with the 3.1pf (as with the 2SA1381) but that's right at the limits of the scope's abilities anyway, with the Y amplifier bandwidth not really being up to that. At lower sweep rates, the capacitance has little effect. 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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