I thought that resistor was for snubbing any oscillation when a low
impedance is present at both the base and emitter since the 2N2207 is
not slow. 47 ohms in series with a base is not usually significant in
other respects for a signal transistor.
I do not have either of the high voltage transistors I suggested
immediately handy but if Ren¨¦ has problems, I can pick some up and
break out my 547 to run some tests and make some measurements. I
gather from his posts that the unblanking is happening quickly enough.
In his place with a 2N2207 that died of old age, I would probably go
through and change all of them while checking for proper operation.
I do like sockets for transistors. My favorite trick has been to use
flush mounted collet socket pins although mounting them flush takes
more time. They work great for TO-92 as well as signal diode,
resistor, and capacitor leads:
I did not have to flush mount those but did just for the heck of it.
The 723 regulators ran hot enough to cause the original plastic
sockets to become loose.
On Wed, 10 Oct 2012 13:57:38 -0700, David Wise
<david_wise@...> wrote:
You have more data than I. But consider the application. It has to
generate a 70V step, as fast as possible, but the most important part
is the last little bit, otherwise the trace will be dim at the start of
the fastest sweeps. Note that they put in a resistor that isolates the
anti-saturation clamp diode from the base - that could only be to
minimize C-B capacitance outside the transistor. The transistor is
driving a high-impedance load; I don't see current-handling ability
figuring very large. But the ultimate test is to pop in a candidate
and give it a spin. Yay sockets!
Dave Wise
-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...] On Behalf Of David
Sent: Wednesday, October 10, 2012 1:13 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: Bright dots at start of the trace on a 547
I found a bunch of different Cob values (up to 12pF!?) for the 2N2207
with unclear test conditions so I am not convinced 2.5pF Cob is a
magic value. Low Ic small junction transistors are not as common as
they used to be and I suspect the better gain characteristics of an Ic
200mA transistor might make up for the higher output capacitance of
the larger junction in this case.
The 547 I recently acquired works fine as far as I can tell except for
dirty switches and potentiometers but I have started taking notes on
it.
On Wed, 10 Oct 2012 09:39:47 -0700, David Wise
<david_wise@...> wrote:
Good catch, George. I only searched for "-300V" to "-70V",
forgetting that the staff is sloppy about voltage polarities.
Searching for positive breakdown voltages yields many many hits.
And good point, David Hess, but the 2N2207's 2.5pF Cob is
specified at 6V.
Dave Wise
David Hess