Amplifier transistor substitution.

[Sylvia Else]

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.

[Kevin Aylward]

>"Sylvia Else" wrote in message news:ej4csu...@mid.individual.net...

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

[Phil Allison]

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

[Cursitor Doom]

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.

[Sylvia Else]

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.

[Benderthe.evilrobot]

"Sylvia Else" <syl...@not.at.this.address> wrote in message
news:ej4kq2...@mid.individual.net...

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.

[bitrex]

2SA1381?

http://www.mouser.com/ds/2/149/KSA1381-889316.pdf

[Sylvia Else]

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.

[Winfield Hill]

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

[Sylvia Else]

Yes, at the time there were high voltage transistors available that
could be used for other interesting purposes.

Thanks for the offer.

Sylvia.

[Sylvia Else]

Hmm....

They may have supplied a circuit diagram and board layouts in the user
manual, but this thing is clearly not designed to be repaired. There are
multiple wires soldered directly to the boards, including some of those
from the power transformer, and to the side of the board that's not
readily accessible until after they're been desoldered. How did they
assemble this thing? With some kind of right-angled soldering iron?

Sylvia.

[Cursitor Doom]

On Sat, 18 Mar 2017 19:03:54 -0700, Winfield Hill 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.

Win, could you possibly post a few part numbers for these devices here,
please?

[Cursitor Doom]

I'm x-posting this to a more appropriate forum, Sylvia, the guys there
will be better able to assist you with this...

[Sylvia Else]

On 18/03/2017 8:27 PM, Sylvia Else wrote:

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.

[Tauno Voipio]

Short in deflection plate or its wiring?

--

-TV

[Winfield Hill]

Cursitor Doom wrote...

What, you want to know what's in my secret stash?!!
200 to 300V video transistors are pretty amazing.
They made broadband high-voltage amplifiers, taking
advantage of the low-capacitance deflection-plate
load, only a few pF. Most are gone now, along with
the CRTs they powered. But a few are still here.
To find them I type TO-126 into my 'Everything'
search engine, maybe adding a voltage like 300V,
and check their availability on Octopart.

Whoa, I see we can still buy Toshiba's 2SC5460,
800V, 2.2pF. Quest has inventory, $0.39, qty 50.

KSA1381 pnp 300V 3.1pF 7W Fairchild $0.25 Digi-Key
KSC3503 npn 300V 2.6pF 7W Fairchild $0.26 Digi-Key
KSA1142 pnp 180V 4.5pF 8W Fairchild $0.25 Digi-Key
KSC2682 npn 180V 3.2pF 8W Fairchild $0.11 Rochester
KSC5026 npn 800V 35pF 20W Fairchild $0.45 Digi-Key
MJE350 pnp 300V 6.4pF(meas) 21W, multiple sources
MJE340 npn 300V 4.1pF(meas) 21W, multiple sources

The MJE340 and MJE350 datasheets don't mention
capacitance, so I had ignored those for years.
But eventually I measured two ON Semi parts,
Cob at 100V, and was pleasantly surprised.
There's also the MJD340 and MJD350 in DPak.


--
Thanks,
- Win

[Cursitor Doom]

On Mon, 20 Mar 2017 06:20:48 -0700, Winfield Hill wrote:

> KSA1381 pnp 300V 3.1pF 7W Fairchild $0.25 Digi-Key KSC3503 npn 300V
> 2.6pF 7W Fairchild $0.26 Digi-Key KSA1142 pnp 180V 4.5pF 8W Fairchild
> $0.25 Digi-Key KSC2682 npn 180V 3.2pF 8W Fairchild $0.11 Rochester
> KSC5026 npn 800V 35pF 20W Fairchild $0.45 Digi-Key MJE350 pnp 300V
> 6.4pF(meas) 21W, multiple sources MJE340 npn 300V 4.1pF(meas) 21W,
> multiple sources

Thanks, Win. I'm going to stock up while there's still some left out
there. I may never need to use any, but I find a comprehensive and
copious stash of discretes helps compensate for my unfortunate lack of
spirituality. :-)

[Sylvia Else]

That's worth checking. Thanks.

Sylvia.

[John Robertson]

You may be able to find your BF472s sourced from India:

http://www.diyaudio.com/forums/solid-state/269984-bf471-472-good-replacement-2sa1381-2sc3503.html

I did a search and no BF numbers showed up, however they did manufacture
the device recently according to the referenced PDF. You may have luck
tracing the part through CDLI's distribution network. It never hurts to ask!

I would also check T710 (BF458) and T707 (BF199) as well though before
ordering anything. A simple gain test should suffice. Try to find a
reliable source for these transistors, there are a lot of Chinese (etc.)
fakes out there!

Have you upgraded the electrolytic capacitors in your scope? They are
most likely well past their Best-Before date!

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

[Cursitor Doom]

On Tue, 21 Mar 2017 08:34:08 -0700, John Robertson wrote:

> You may be able to find your BF472s sourced from India:
>
> http://www.diyaudio.com/forums/solid-state/269984-bf471-472-good-
replacement-2sa1381-2sc3503.html
>
> I did a search and no BF numbers showed up, however they did manufacture
> the device recently according to the referenced PDF. You may have luck
> tracing the part through CDLI's distribution network. It never hurts to
> ask!

Is Indian stuff kosher? Like others here I've had big problems with fake
semis from the far east; voltage regs in particular.

>
> I would also check T710 (BF458) and T707 (BF199) as well though before
> ordering anything. A simple gain test should suffice. Try to find a
> reliable source for these transistors, there are a lot of Chinese (etc.)
> fakes out there!

Oh yes, see above!

> Have you upgraded the electrolytic capacitors in your scope? They are
> most likely well past their Best-Before date!

They were *all* totally fine! I mean I'm sure they're not as they were
when new, but none of us are. ;-) The fault with that scope (if it's the
one I'm thinking of) turned out to be a diode in the SMPS section that
someone had subbed with an inferior part. It needed a fast recovery diode
and someone had stuck in the first spare diode the could find I'd
imagine. It would have worked fine at 50Hz but this was 20kHz. One of the
guys who posts here spotted it from a photo I posted!

[Sylvia Else]

I replaced the damaged resistors and the transistor, and the
oscilloscope is working again. Thanks to those who made suggestions, and
a special thanks to Phil for pointing me to Wes, whose retail arm,
Wagner, were able to supply the BF472.

Sylvia.