Forums

Amplifier transistor substitution.

Started by Sylvia Else March 18, 2017
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.
>"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
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. >
** A nice old Hameg CRO, worth keeping going in anyone's book. You likely did not check the WES catalogue. It lists the BF472 for $0.86 + gst. FYI: I'm not your pal, but nor am I a psycho. .... Phil
On Sat, 18 Mar 2017 10:41:40 +0000, 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
Whatever sub you choose, it would probably be a good idea to replace the other 472 with the same type as well. A matched pair would be nice.
On 18/03/2017 10:08 PM, Phil Allison wrote:
> 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. >> > > > ** A nice old Hameg CRO, worth keeping going in anyone's book. > > You likely did not check the WES catalogue. > > It lists the BF472 for $0.86 + gst. >
Thanks for that. Sylvia.
"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:ej4kq2FfmovU1@mid.individual.net...
> On 18/03/2017 10:08 PM, Phil Allison wrote: >> 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. >>> >> >> >> ** A nice old Hameg CRO, worth keeping going in anyone's book. >> >> You likely did not check the WES catalogue. >> >> It lists the BF472 for $0.86 + gst. >> > Thanks for that.
Another possibility is do a parametric search on the 2SC types. Archives of CRT VGA monitor schematics could provide some useful leads. Video output transistors commonly go up to 600MHz - the faster ones are less likely to have the voltage rating you need. A lot of monitors ran the horizontal driver from a 250V HT rail - plenty of driver transistors with enough voltage rating, but many not fast enough.
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
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.
Sylvia Else wrote...
>
Sylvia, as mentioned, those are low-capacitance high-voltage video transistors. All the rage when CRTs were in heavy use, but dropped like a rock thereafter. Most were in TO-126 packages. You should replace both transistors, and maybe the associated bypass cap, in case it was the problem. I've a good collection of TO-126 video. There's still a modest supply at various places. But if you don't easily obtain replacements, let me know and I'll mail you a few. -- Thanks, - Win
On 19/03/2017 1:03 PM, Winfield Hill wrote:
> Sylvia Else wrote... >> > > Sylvia, as mentioned, those are low-capacitance > high-voltage video transistors. All the rage > when CRTs were in heavy use, but dropped like a > rock thereafter. Most were in TO-126 packages. > You should replace both transistors, and maybe > the associated bypass cap, in case it was the > problem. I've a good collection of TO-126 video. > There's still a modest supply at various places. > But if you don't easily obtain replacements, > let me know and I'll mail you a few. > >
Yes, at the time there were high voltage transistors available that could be used for other interesting purposes. Thanks for the offer. Sylvia.